- Email: [email protected]
Resetting baroreceptors to a lower arterial pressure level by enalapril avoids baroreflex mediated activation of sympathetic nervous system by nifedipine
Life Sciences 68 (2001) 2769–2779
Resetting baroreceptors to a lower arterial pressure level by enalapril avoids baroreflex mediated activation of sympathetic nervous system by nifedipine Weiguo Zhanga,*, Zhongyun Wangb a
Cardiovascular Institute and Fu Wai Hospital, Peking Union Medical College and Chinese Academy of Medical Sciences, Beijing, China b Liaoning Institute of Traditional Chinese Medicine, Shenyang, Liaoning, China Received 8 September 2000; accepted 9 November 2000
Abstract Baroreceptor-unloading-mediated activation of sympathetic nervous system (SNS) by antihypertensive agents, such as dihydropyridine calcium channel blockers (CCB), has been considered to compromise the beneficial effects of the therapy and lead to unsatisfying clinical outcome. The present study was aimed at finding a novel way of using CCB without activating SNS. In anaesthetized Wistar rats, baroreceptor-unloading-mediated reflex activation of SNS, as indicated by tachycardia and increase of plasma catecholamines, was observed after mean arterial pressure (MAP) was decreased by 15 mmHg during 4-h administration of nifedipine, a CCB. However an angiotensin-converting enzyme inhibitor (ACEI), enalapril did not cause tachycardia or increase plasma catecholamine levels when it decreased MAP by 15 mmHg. After 100 min (supposedly baroreceptor resetting or adaptation to hypotension had occurred), enalapril infusion was gradually replaced by nifedipine infusion in 40 min. Nifedipine was infused for another 100 min, which kept the lowered MAP unchanged and did not activate SNS. In anaesthetized spontaneously hypertensive rats (SHR), baroreceptor-mediated reflex activation of SNS was observed after MAP was decreased by 25 mmHg during 4-h nifedipine administration. However enalapril did not cause tachycardia or increase plasma catecholamine levels when it decreased MAP by 25 mmHg. After 100 min, enalapril infusion was gradually replaced by nifedipine infusion in 40 min. Nifedipine was then infused for another 100 min, which kept the lowered MAP unchanged and did not activate SNS. The present study indicated that reflex activation of SNS caused by antihypertensive effect of CCB could be avoided if, prior to CCB administration, baroreceptors have been reset to a lower MAP by a drug that does not activate baroreceptor reflex. © 2001 Elsevier Science Inc. All rights reserved. Keywords: Angiotensin-converting enzyme inhibitor (ACEI); Baroreceptor reflex; Baroreceptor resetting; Calcium channel blocker (CCB); Hypertension; Spontaneously hypertensive rats (SHR); Sympathetic nervous system (SNS)
* Corresponding author. Dept of Internal Medicine, The University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd/J4.132, Dallas, TX 75390-8586, USA. Tel.: 1-214-6487944; fax: 1-214-6487902. E-mail address: [email protected] (W. Zhang) 0024-3205/01/$ – see front matter © 2001 Elsevier Science Inc. All rights reserved. PII: S 0 0 2 4 - 3 2 0 5 ( 0 1 )0 1 0 7 8 -5
2770
W. Zhang, Z. Wang / Life Sciences 68 (2001) 2769–2779
Introduction Dihydropyridine calcium channel blockers (CCB) or calcium antagonists have been thought to be superior to other classes of antihypertensive drugs, because (1) they reduce arterial blood pressure physiologically by decreasing the total peripheral resistance that is increased in essential hypertension [1]; (2) they have a neutral or beneficial impact on blood lipid profile whose imbalance frequently coexists with hypertension, which may be further impaired by some other antihypertensive drugs [1–3]; (3) they have a diuretic effect and protect or improve renal function [4]; and (4) very importantly, they may retard the development and progression of atherosclerosis [5–8]. Despite the recent controversies surrounding the safety and long-term ability of calcium antagonists to alter the course of cardiovascular disease [9,10], the main concern in using calcium antagonists for antihypertensive treatment is that when the dihydropyridine lowers arterial pressure, it activates the sympathetic nervous system (SNS) through arterial baroreceptor unloading-mediated autonomic reflex, which leads to unsatisfying clinical outcomes. It has been documented that cardiovascular system has its circadian rhythm [11–13]. The clinical outcome of antihypertensive therapy may be unsatisfying if the activation of SNS triggered by extrinsic stimulus (CCB) coincidentally overlaps with the activation set by intrinsic circadian rhythm, the results of which may additively enhance sympathetic nerve outflow. Some therapeutic strategies have been suggested in order to reduce the activation of SNS in response to dihydropyridine. Such strategies include limiting the speed of antihypertensive action, maintaining stable plasma level and effect of antihypertensive drug over the dosing interval, combining two drugs with different mechanisms of action [14–16]. Based on cardiovascular circadian rhythm, a novel chronotherapy has been suggested [13]. The main principle for this new chronotherapy is to choose two classes of drugs with different mechanism of action and use them at different times during the day. By appropriately choosing the drugs and their acting time, not only the adverse effects, but also the risks of sympathetic traffic could be reduced. If a drug that does not cause a profound activation of SNS when decreasing arterial pressure is administered first, and a CCB is added on to maintain the antihypertensive effect, then CCB may not activate SNS. The neurophysiological support for this hypothesis comes from the evidence that arterial baroreceptors resetting to hypotension can take place very quickly [17–19]. If baroreceptors can be reset to the lower level of arterial pressure by the first drug that does not cause baroreflex-mediated activation of SNS, then keeping arterial pressure at this level with calcium antagonist may not stimulate the baroreceptors because CCB just maintains the lowered arterial pressure level which baroreceptors have adapted to. The purpose of the present study was to test this novel hypothesis in animal models. In the present experiment, enalapril, an angiotensin-converting enzyme inhibitor (ACEI) was used first to lower arterial pressure and nifedipine, a CCB was added later on to maintain the level of the arterial pressure that enalapril had created, to see whether nifedipine activates SNS after ACEI. This was first performed in normotensive rats and then extended to hypertensive rats, which have been a widely used animal model for essential hypertension in humans. The present results indicate that reflex activation of SNS caused by antihypertensive effect of CCB could be avoided if, prior to CCB administration, baroreceptors have been reset to a lower MAP by a drug that does not activate baroreceptor reflex.
W. Zhang, Z. Wang / Life Sciences 68 (2001) 2769–2779
2771
Material and methods Experimental rats were from the Institutional Animal Center where the animals were housed in a controlled environment with free access to standard rat chow and tap water. The experimental protocols were in accordance to NIH guidelines for laboratory animal use and approved by the Institutional Animal Care and Use Committee. Protocol 1 (in normotensive rats) Wistar rats (male, 300–330 g) were initially anesthetized with methohexital sodium (60 mg kg21 i.p.). The right jugular vein, left carotid artery and right femoral artery were catheterized with PE-50 tubing filled with physiological saline for drug delivery, arterial blood pressure measurement and blood sampling respectively. Tracheotomy was performed with PE-240 tubing inserted into the trachea. After the surgery, a-chloralose was injected (50 mg kg21 in 2 min, i.v.) and then continuously infused (30 mg kg21 h21) through a side branch of the carotid catheter. The rectal temperature was maintained at 37.5 8C by an external heating pad and lamp. The experiment started 30 min after above procedure was finished. Mean arterial pressure (MAP) and heart rate (HR) was recorded for 20 min as baseline values. The rats then received one of the following treatments: Vehicle (control) group (n57): This group of rats received vehicle injection and infusion in the same way as the other 2 experimental groups received. Three rats received physiological saline (vehicle for enalapril) and four received 40% polyethylene glycol (PEG. MW 400) in physiological saline (vehicle for nifedipine). Because there was no detectable difference between saline and PEG treated rats, all animals were pooled into one control group. Nifedipine group (n57): This group of rats received a nifedipine injection (2 mg kg21 over 2 min) which was followed by continuous nifedipine infusion. The infusion rate was adjusted (between 0.5–1.0 mg h21) to keep the MAP 15 mmHg lower than the baseline level throughout the 4-h experiment. Enalapril 1 nifedipine group (n57): This group of rats received an enalapril injection (5 mg kg21 over 2 min) which was followed by continuous enalapril infusion. The infusion rate was adjusted (0.9–1.3 mg h21) to keep the MAP about 15 mmHg lower than the baseline level. The infusion rate of enalapril was gradually reduced after 100 min and completely stopped after 140 min, whereas nifedipine infusion was started after 100 min and its infusion rate (between 0.1–1.0 mg h21) was increased to keep antihypertensive effect from enalapril unchanged during the rest time of the 4-h experiment. The blood sample was taken with heparinized chilled syringe from femoral artery before the infusion and every 80 min after infusion. The loss of blood due to sampling (about 1 ml each time) was immediately restored with same amount of blood from an anesthetized donor rat that received the exactly same treatment, but was excluded from data analysis. Figure 1 illustrates the experimental protocol. Protocol 2 (in hypertensive rats) Spontaneously hypertensive rat (male, 260–280 g. SHR) received the same anesthetizing and surgical preparation as in protocol 1. The experiment started 45 min after above procedure was finished. The basal MAP and HR were recorded for 20 min. The rats then received one of the following treatments: Vehicle (control) group (n56): This group of rats
2772
W. Zhang, Z. Wang / Life Sciences 68 (2001) 2769–2779
Fig. 1. Illustration of the experimental procedure for both protocol 1 and 2. In the group that received calcium channel blocker (CCB), nifedipine was infused for 4-h after a bolus injection of nifedipine. The infusion speed was controlled in order to lower the mean arterial pressure to a certain level. In the group that received angiotensinconverting enzyme inhibitor enalapril plus nifedipine (ACEI1CCB), enalapril was injected first and its infusion was followed to achieve the similar reduction of MAP to CCB group. Enalapril infusion was gradually withdrawn after 100 min and stopped after 140 min; in the meantime nifedipine was incrementally added on to keep the level of blood pressure created by enalapril unchanged for the rest of the experiment. For the dosage of each drug, see respective protocols.
received vehicle injection and infusion as in protocol 1. Nifedipine group (n57): This group of rats received a nifedipine injection (4 mg kg21 over 2 min) which was followed by continuous nifedipine infusion. The infusion rate was adjusted (between 0.7–1.2 mg h21) to keep the MAP 25 mmHg lower than the baseline level throughout 4-h experiment. Enalapril 1 nifedipine group (n57): This group of rats received an enalapril injection (10 mg kg21 over 2 min) which was followed by continuous enalapril infusion. The infusion rate was adjusted (1.2–1.6 mg h21) to keep the MAP about 25 mmHg lower than the baseline level. The infusion rate of enalapril was gradually reduced after 100 min and completely stopped after 140 min, whereas nifedipine infusion was started after 100 min and its infusion rate (between 0.1–1.4 mg h21) was adjusted to keep the antihypertensive effect from enalapril unchanged during the rest time of the 4-h experiment. The time course of the experiment and the blood sampling method were as same as in protocol 1 (Figure 1 illustrates the experimental protocol). Plasma catecholamine assay The blood samples were transferred to chilled tube and centrifuged at 3000 rpm and 4 8C for 10 min to separate the plasma from blood. The plasma samples were frozen at 270 8C until catecholamine measurement. The measurement was performed with high performance liquid chromatography with electrical detection [20]. Data analysis The statistical analysis was performed by two-way analysis of variance (ANOVA) followed by the Newman-Keuls test. All data are presented as mean6s.e.m. A probability (P) of less that 0.05 was considered to be statistically significant.
W. Zhang, Z. Wang / Life Sciences 68 (2001) 2769–2779
2773
Table 1 Mean arterial blood pressure (MAP), heart rate (HR), plasma norepinephrine (NE) and epinephrine (E) levels in 3 groups of anaesthetized Wistar rats before calcium channel blocker (CCB) nifedipine, angiotensin-converting enzyme inhibitor (ACEI) enalapril plus nifedipine (ACEI1CCB) or vehicle infusions. There is no statistical difference between any two groups (p5ns). MAP (mmHg) HR (beats min21) NE (nmol l21) E (nmol l21)
Vehicle (n57)
CCB (n57)
ACEI1CCB (n57)
94.064.0 37168 2.8560.14 2.7560.10
95.062.7 36766 2.7960.32 2.8460.22
96.363.0 36967 2.5760.21 2.8760.20
Results Protocol 1 (in normotensive rats) The basal MAP, HR and plasma norepinephrine (NE) and epinephrine (E) in anesthetized Wistar rats before drug or vehicle infusions are shown in table 1. There is no statistical difference among vehicle (control), nifedipine and enalapril 1 nifedipine groups. The effects of vehicle, nifedipine and enalapril 1 nifedipine on MAP were shown in figure 2. There is no substantial change in MAP over the entire course of the experiment in control group. Nifedipine and enalapril reduced MAP by 15 mmHg each. In enalapril 1 nifedipine group, the infusion rate of enalapril was gradually decreased after 100 min; in the meantime, nifedipine was incrementally added on. Enalapril infusion was stopped after 140 min and the antihypertensive effect of enalapril was then taken over by nifedipine, which kept the MAP unchanged for the rest time of the experiment. Figure 3 shows the changes of HR and the plasma catecholamine levels after the infusion of vehicle or different drugs. In nifedipine group,
Fig. 2. The changes in mean arterial pressure (MAP) in Wistar rats that received vehicle (control), calcium channel blocker nifedipine (CCB) and angiotensin-converting enzyme inhibitor enalapril plus nifedipine (ACEI1CCB) (n57 each). Nifedipine (2 mg kg21 injection and 0.5–1.0 mg h21 infusion) and enalapril (5 mg kg21 injection and 0.9–1.3 mg h21 infusion) reduced MAP by 15 mmHg each from baseline. In ACEI1CCB group, enalapril was gradually withdrawn after 100 min; in the meantime, nifedipine was incrementally added on (0.1–1.0 mg h21). Enalapril infusion was stopped after 140 min and the antihypertensive effect of enalapril was then taken over by that of nifedipine, which kept the MAP unchanged for the rest of the experiment. * p , 0.05 vs. control.
2774
W. Zhang, Z. Wang / Life Sciences 68 (2001) 2769–2779
Fig. 3. The changes in heart rate (HR) and plasma norepinephrine (NE) and epinephrine (E) in Wistar rats received vehicle (control), calcium channel blocker nifedipine (CCB) and angiotensin-converting enzyme inhibitor enalapril plus nifedipine (ACEI1CCB) (n57 each). Nifedipine (2 mg kg21 injection and 0.5–1.0 mg h21 infusion) and enalapril (5 mg kg21 injection and 0.9–1.3 mg h21 infusion) reduced MAP by 15 mmHg each from baseline. The hypotensive effect of nifedipine provoked tachycardia and elevated plasma NE and E levels in the CCB group. However the similar effect from enalapril did not cause tachycardia and change plasma NE and E. When antihypertensive effect of enalapril was taken over by that of nifedipine, HR, NE and E were unchanged in ACEI1CCB group. * P , 0.05 vs. control.
the reduction of MAP by nifedipine caused a significant increase in HR and elevation of plasma NE and E levels, which represent the activation of SNS. In enalapril 1 nifedipine group, the initial reduction of MAP by enalapril did not cause an increase in HR or elevation of plasma NE and E levels. HR and plasma NE and E levels were kept unchanged after the antihypertensive effect of enalapril was taken over by that of nifedipine. Protocol 2 (in hypertensive rats) The basal mean MAP, HR and plasma NE and E in anesthetized SHR before drug or vehicle infusions are shown in table 2. There is no statistical difference among vehicle, nifedipine and enalapril 1 nifedipine groups. Those parameters were comparably elevated as reported before when SHR were compared with normotensive rats [21,22].
W. Zhang, Z. Wang / Life Sciences 68 (2001) 2769–2779
2775
Table 2 Mean arterial blood pressure (MAP), heart rate (HR), plasma norepinephrine (NE) and epinephrine (E) levels in 3 groups of anaesthetized spontaneously hypertensive rats before calcium channel blocker (CCB) nifedipine, angiotensin-converting enzyme inhibitor (ACEI) enalapril plus nifedipine (ACEI1CCB) or vehicle infusions. There is no statistical difference between any two groups (p5ns). MAP (mmHg) HR (beats min21) NE (nmol l21) E (nmol l21)
Vehicle (n56)
CCB (n57)
ACEI1CCB (n57)
153.766.6 39768 3.3360.18 4.0060.33
148.266.2 409610 3.2260.22 3.7360.18
151.065.9 415615 3.0860.18 4.0460.29
The effect of vehicle, nifedipine and enalapril 1 nifedipine on MAP was shown in figure 4. There is no substantial change in MAP over the entire course of the experiment in control group. Nifedipine and enalapril reduced MAP by 25 mmHg each. In enalapril 1 nifedipine group, the infusion rate of enalapril was gradually decreased after 100 min; in the meantime, nifedipine was incrementally added on. Enalapril infusion was stopped after 140 min and the antihypertensive effect of enalapril was then taken over by nifedipine, which kept the MAP unchanged for the rest time of the experiment. Figure 5 shows the changes of HR and the plasma catecholamine levels after the infusion vehicle or different drugs. In nifedipine group, the reduction of MAP by nifedipine caused a significant increase in HR and elevation of plasma NE and E levels, which represent the activation of SNS. In enalapril 1 nifedipine group, the initial reduction of MAP by enalapril did not cause an increase in HR or elevation of plasma NE and E levels. HR and plasma NE and E levels were kept unchanged after the antihypertensive effect of enalapril was taken over by nifedipine.
Fig. 4. The changes in mean arterial pressure (MAP) in spontaneously hypertensive rats (SHR) that received vehicle (control, n56), calcium channel blocker nifedipine (CCB, n17) and angiotensin-converting enzyme inhibitor enalapril plus nifedipine (ACEI1CCB, n57). Nifedipine (4 mg kg21 injection and 0.7–1.2 mg h21) and enalapril (10 mg kg21 injection and 1.2–1.6 mg h21) reduced MAP by 25 mmHg each from baseline. In ACEI1CCB group, enalapril was gradually withdrawn after 110 min; in the meantime, nifedipine was incrementally added on (0.1–1.4 mg h21). Enalapril infusion was stopped after 140 min and the antihypertensive effect of enalapril was then taken over by nifedipine, which kept the MAP unchanged for the rest of the experiment. * P , 0.05 vs. control.
2776
W. Zhang, Z. Wang / Life Sciences 68 (2001) 2769–2779
Fig. 5. The changes in heart rate (HR) and plasma norepinephrine (NE) and epinephrine (E) in spontaneously hypertensive rats received vehicle (control, n56), calcium channel blocker nifedipine (CCB) and angiotensinconverting enzyme inhibitor enalapril plus nifedipine (ACEI1CCB) (n57, respectively). Nifedipine (4 mg kg21 injection and 0.7–1.2 mg h21 infusion) and enalapril (10 mg kg21 injection and 1.2–1.6 mg h21 infusion) reduced MAP by 25 mmHg from baseline. Hypotensive effect of nifedipine provoked tachycardia and elevated plasma NE and E in CCB group. However the similar effect from enalapril did not cause tachycardia and increase plasma NE and E. When antihypertensive effect of enalapril was taken over by that of nifedipine, HR, NE and E were unchanged in ACEI1CCB group. * P , 0.05 vs. control.
Discussion The present study in both normotensive and hypertensive rat models demonstrates that the baroreceptor unloading-mediated activation of sympathetic nervous system by antihypertensive action of nifedipine, a dihydropyridine calcium channel blocker, can be avoided by pretreatment with enalapril, an angiotensin-converting enzyme inhibitor. Enalapril was used to lower arterial pressure and reset arterial baroreceptors to the hypotensive level. Nifedipine was used after the baroreceptors had adapted to the hypotensive level established by enalapril to keep the antihypertensive effect unchanged. The study provides experimental support for the clinical application of the novel chronotherapy for hypertension [13], which is to choose two classes of drugs with different mechanism of action and use them at different times during the day in order to reduce reflex activation of SNS and other side effects associated with acute antihypertensive action of CCB.
W. Zhang, Z. Wang / Life Sciences 68 (2001) 2769–2779
2777
The initiation of antihypertensive treatment with CCB is associated with activation of SNS. Accelerated HR, elevated plasma catecholamines and increased renal sympathetic nerve activity in response to acute hypotensive effect of nifedipine all represent the activation of SNS secondary to direct vasodilation and reduction of total peripheral resistance [23–26]. The acute activation of SNS has been widely documented by previous investigations in both human and animal subjects and is considered as a result of baroreceptor unloading [25,27,28]. Although the activation of SNS is a physiological response that aims to prevent the established hemodynamic homeostasis from disturbances, it is clinically detrimental in hypertension treatment because the higher heart rate and elevated plasma catecholamines are powerful predictor of the poor prognosis [26,29,30]. Investigations during the past decade have shown that the onset of episodes of transit cardiac ischemia as well as the myocardial infarction, ventricular tachycardia, thrombolic stroke and sudden cardiac death occurs more frequently in the morning (6:00–12:00 AM) than the rest of the day (so that morning is refereed as vulnerable period for those who have normal sleep-wake cycles) [11]. The underlying mechanism behind these is believed to be a series of neurohumoral activation, of which sympathetic activation is the most important [13]. If the extrinsic activation of SNS provoked by CCB overlaps coincidentally with the intrinsic activation set by circadian rhythm (bio-clock), then the activation of SNS would be additively enhanced, which could trigger more cardiovascular events [31,32]. Angiotensin-converting-enzyme inhibitors are a group of heterogeneous drugs that lower arterial pressure mainly by reducing angiotensin II production in plasma and tissue. In the present study, enalapril did not activate SNS as assessed by heart rate and plasma catecholamine levels either in normotensive rats when arterial pressure was reduced or in hypertensive rats when their arterial pressure was normalized. The administration of nifedipine two hours after enalapril maintained the antihypertensive effect of enalapril but indeed did not activate SNS. This chrono-combination of the 2 drugs provided an ideal control of arterial pressure. The purpose of enalapril administration is to initiate antihypertension effect and to reset arterial baroreceptors to the hypotensive level. The process of baroreceptor unloading and resetting in response to hypotensive effect of ACEI does not result in reflex activation of SNS. This is because that angiotensin II plays a pivotal role in modulating the process of sympathetic activation [33–35]. The removal of angiotensin II by inhibiting its generation causes sympathetic inhibition [35–38]. Thus during acute ACEI administration the sympathoexcitatory effect from baroreceptor unloading may not outweigh the sympathoinhibitory effect. This is frequently demonstrated by the fact that antihypertensive effect of ACEI is not associated with sympathetic activation in vivo [35–38]. As baroreceptor resetting to lower pressure could take place very quickly [17–19], the administration of CCB after ACEI, as shown in the present study, did not cause sympathetic activation. The possibility that dihydropyridine CCB can be used with other classes of antihypertensive drugs also exist. As demonstrated in the present study, it is required to initiate antihypertensive effect by using a drug that does not activate SNS, once baroreceptors are reset to the new lower level of arterial pressure, the antihypertensive effect from the first selected drug should be maintained by CCB. Because the activation of SNS evidently compromises the beneficial effect of CCB or even worsens the outcome of the antihypertensive therapy, the better use of CCB without reflex activation of SNS, as outlined in the present study, would be
2778
W. Zhang, Z. Wang / Life Sciences 68 (2001) 2769–2779
clinically interesting. Further investigation is required to determine if such a chronotherapy is beneficial in terms of reducing cardiovascular morbidity and mortality. Limitation of the study. First, the short-term baroreceptor resetting is usually not complete [17–19], hence in enalapril 1 nifedipine group the lack of significant sympathetic activation in response to nifedipine infusion may involve unknown mechanisms other than discussed here, which was not the focus of the present study but is worth further investigation. Second, although the infusion of enalapril was stopped after 140 min in enalapril 1 nifedipine group of normotensive and hypertensive rats, the residue effect of angiotensin-converting enzyme inhibition might still be affecting SNS and therefore could modulate the effect of nifedipine on SNS. This needs to be considered in future studies. In summary, the initial antihypertensive effect of dihydropyridine calcium channel blocker is associated with activation of sympathetic nervous system, whereas that of angiotensinconverting enzyme is not. CCB can be used in antihypertensive therapy without activation of sympathetic nervous system if it is administered after baroreceptors unloading and resetting have been established by an angiotensin-converting enzyme inhibitor.
References 1. van Zwieten PA. Calcium entry blockers (calcium antagonists). In: Van Zwieten PA, editor. Handbook of Hypertension. Amsterdam: Elsevier, 1984, pp. 331–42. 2. Hansson L. Calcium antagonists: an overview. Am Heart J 1991;122:308–11. 3. Lender D, Arauz-Pacheco C, Breen L, Mora-Mora P, Ramirez LC, Raskin P. A double blind comparison of the effects of amlodipine and enalapril on insulin sensitivity in hypertensive patients. Am J Hypertens 1999; 12:298–303. 4. Zanchi A, Brunner HR, Waeber B, Burnier M. Renal haemodynamic and protective effects of calcium antagonists in hypertension. J Hypertens 1995;13:1363–75. 5. Lichtlen PR, Hugenholtz PG, Rafflenbeul W, Hecker H, Jost S, Deckers JW. Retardation of angiographic progression of coronary artery disease by nifedipine. Results of the international nifedipine trial on antiatherosclerotic Therapy (INTACT). Lancet 1990;335:1109–13. 6. Hof RP, Tapparelli C, Weinstein DB. Hemodynamic, antivasoconstrictor, and antiatherosclerotic effects of calcium antagonists in animal models of atherosclerosis. J Cardiovasc Pharmacol 1990;15:S7–12. 7. Zanchetti A. Antiatherosclerotic effects of antihypertensive drugs: recent evidence and ongoing trials. Clin Exp Hypertens 1996;18:489–99. 8. Waters D, Lesperance J, Francetich M, et al. A controlled clinical trial to assess the effect of a calcium channel blocker on the progression of coronary atherosclerosis. Circulation 1990;82:1940–53. 9. Messerli FH, Grossman E. The calcium antagonist controversy: a posthumous commentary. Am J Cardiol 1998;82:35R–9R. 10. Epstein M. The calcium antagonist controversy: the emerging importance of drug formulation as a determinant of risk. Am J Cardiol 1997;79:9–19. 11. Muller JE, Tofler GH, Stone PH. Circadian variation and triggers of onset of acute cardiovascular disease. Circulation 1989;79:733–43. 12. Cooke HM, Lynch A. Biorhythms and chronotherapy in cardiovascular disease. Am J Hosp Pharm 1994;51: 2569–80. 13. Zhang W. Can we use calcium antagonist better in antihypertensive therapy? Circadian consideration. Pharmacol Res 1996;34:187–91. 14. Zanchetti A, Chalmers JP, Arakawa K, et al. The 1993 guidelines for the management of mild hypertension: memorandum from a WHO/ISH meeting. Blood Press 1993;2:86–100. 15. Luscher TF, Waeber B. Efficacy and safety of various combination therapies based on a calcium antagonist in
W. Zhang, Z. Wang / Life Sciences 68 (2001) 2769–2779
16. 17. 18. 19. 20. 21.
22.
23. 24. 25. 26. 27. 28. 29.
30. 31. 32. 33. 34. 35. 36.
37. 38.
2779
essential hypertension: results of a placebo-controlled randomized trial. J Cardiovasc Pharmacol 1993;21: 305–9. Anonym. The sixth report of the joint national committee on prevention, detection, evaluation and treatment of high blood pressure (JNC VI). NIH Publication 1998:3–64. Salgado HC, Krieger EM. Extent of baroreceptor resetting in response to sodium nitroprusside and verapamil. Hypertension 1988;11:I121–5. de Abreu GR, Salgado HC. Antihypertensive drugs distinctly modulate the rapid resetting of the baroreceptors. Hypertension 1990;15:I63–7. Xavier-Neto J, Moreira ED, Krieger EM. Viscoelastic mechanisms of aortic baroreceptor resetting to hypotension and to hypertension. Am J Physiol 1996;271:H1407–15. Zhang W, Liu L, Zheng D, et al. Effects of atrial natriuretic factor on patients with renovascular hypertension. Chin Med J 1992;105:732–7. Zhang W, Pettersson A, Thoren P, Hedner T. Effects of 1,1-dimethylguanidine administration on blood pressure, heart rate and renal sympathetic nerve activity in normotensive and spontaneously hypertensive rats. Acta Physiol Scand 1997;159:1–6. Zhang W, Thoren P. Hyper-responsiveness of adrenal sympathetic nerve activity in spontaneously hypertensive rats to ganglionic blockade, mental stress and neuronglucopenia. Pflugers Arch - Eur J Phsiol 1998; 437:56–60. Packer M. Calcium channel blockers in chronic heart failure. The risks of “physiologically rational” therapy. Circulation 1990;82:2254–7. Lindqvist M, Kahan T, Melcher A, Hjemdahl P. Acute and chronic calcium antagonist treatment elevates sympathetic activity in primary hypertension. Hypertension 1994;24:287–96. Zhang W, Sun X, Zhao X, Thoren P, Hedner T. Effects of calcium antagonist felodipine on renal sympathetic nerve activity in rats with congestive heart failure. Cardiovasc Drugs Ther 1997;11:645–50. Palatini P. Heart rate as a risk factor for atherosclerosis and cardiovascular mortality: the effect of antihypertensive drugs. Drugs 1999;57:713–24. Leenen FH, Holliwell DL. Antihypertensive effect of felodipine associated with persistent sympathetic activation and minimal regression of left ventricular hypertrophy. Am J Cardiol 1992;69:639–45. Frishman WH. Impact of calcium antagonists on early morning ischaemia in the patient with coronary disease. Eur Heart J 1999;1:B34–40. Francis GS, Cohn JN, Johnson G, Rector TS, Goldman S, Simon A. Plasma norepinephrine, plasma renin activity, and congestive heart failure. Relations to survival and the effects of therapy in V-HeFT II. The V-HeFT VA Cooperative Studies Group. Circulation 1993;87:VI40–8. Benedict CR, Shelton B, Johnstone DE, et al. Prognostic significance of plasma norepinephrine in patients with asymptomatic left ventricular dysfunction. SOLVD Investigators. Circulation 1996;94:690–7. Pahor M, Guralnik JM, Corti MC, Foley DJ, Carbonin P, Havlik RJ. Long-term survival and use of antihypertensive medications in older persons. J Am Geriatr Soc 1995;43:1191–7. Borhani NO, Mercuri M, Borhani PA, et al. Final outcome results of the Multicenter Isradipine Diuretic Atherosclerosis Study (MIDAS). A randomized controlled trial. JAMA 1996;276:785–91. Luft FC, Wilcox CS, Unger T, et al. Angiotensin-induced hypertension in the rat. Sympathetic nerve activity and prostaglandins. Hypertension 1989;14:396–403. Reid IA. Interactions between ANG II, sympathetic nervous system, and baroreceptor reflexes in regulation of blood pressure. Am J Physiol 1992;262:E763–78. Bernier NJ, Perry SF. Cardiovascular effects of angiotensin-II-mediated adrenaline release in rainbow trout Oncorhynchus mykiss. J Exp Biol 1999;202:55–66. Schofer J, Hobuss M, Aschenberg W, Tews A. Acute and long-term haemodynamic and neurohumoral response to nisoldipine vs captopril in patients with heart failure: a randomized double-blind study. Eur Heart J 1990;11:712–21. DiBona GF, Jones SY, Brooks VL. ANG II receptor blockade and arterial baroreflex regulation of renal nerve activity in cardiac failure. Am J Physiol 1995;269:R1189–96. Zhang W, Huang BS, Leenen FHH. Brain renin-angiotensin system and sympathetic hyperactivity in rats after myocardial infarction. Am J Physiol (Heart Cir Physiol) 1999;45:H1608–15.
Resetting baroreceptors to a lower arterial pressure level by enalapril avoids baroreflex mediated activation of sympathetic nervous system by nifedipine Weiguo Zhanga,*, Zhongyun Wangb a
Cardiovascular Institute and Fu Wai Hospital, Peking Union Medical College and Chinese Academy of Medical Sciences, Beijing, China b Liaoning Institute of Traditional Chinese Medicine, Shenyang, Liaoning, China Received 8 September 2000; accepted 9 November 2000
Abstract Baroreceptor-unloading-mediated activation of sympathetic nervous system (SNS) by antihypertensive agents, such as dihydropyridine calcium channel blockers (CCB), has been considered to compromise the beneficial effects of the therapy and lead to unsatisfying clinical outcome. The present study was aimed at finding a novel way of using CCB without activating SNS. In anaesthetized Wistar rats, baroreceptor-unloading-mediated reflex activation of SNS, as indicated by tachycardia and increase of plasma catecholamines, was observed after mean arterial pressure (MAP) was decreased by 15 mmHg during 4-h administration of nifedipine, a CCB. However an angiotensin-converting enzyme inhibitor (ACEI), enalapril did not cause tachycardia or increase plasma catecholamine levels when it decreased MAP by 15 mmHg. After 100 min (supposedly baroreceptor resetting or adaptation to hypotension had occurred), enalapril infusion was gradually replaced by nifedipine infusion in 40 min. Nifedipine was infused for another 100 min, which kept the lowered MAP unchanged and did not activate SNS. In anaesthetized spontaneously hypertensive rats (SHR), baroreceptor-mediated reflex activation of SNS was observed after MAP was decreased by 25 mmHg during 4-h nifedipine administration. However enalapril did not cause tachycardia or increase plasma catecholamine levels when it decreased MAP by 25 mmHg. After 100 min, enalapril infusion was gradually replaced by nifedipine infusion in 40 min. Nifedipine was then infused for another 100 min, which kept the lowered MAP unchanged and did not activate SNS. The present study indicated that reflex activation of SNS caused by antihypertensive effect of CCB could be avoided if, prior to CCB administration, baroreceptors have been reset to a lower MAP by a drug that does not activate baroreceptor reflex. © 2001 Elsevier Science Inc. All rights reserved. Keywords: Angiotensin-converting enzyme inhibitor (ACEI); Baroreceptor reflex; Baroreceptor resetting; Calcium channel blocker (CCB); Hypertension; Spontaneously hypertensive rats (SHR); Sympathetic nervous system (SNS)
* Corresponding author. Dept of Internal Medicine, The University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd/J4.132, Dallas, TX 75390-8586, USA. Tel.: 1-214-6487944; fax: 1-214-6487902. E-mail address: [email protected] (W. Zhang) 0024-3205/01/$ – see front matter © 2001 Elsevier Science Inc. All rights reserved. PII: S 0 0 2 4 - 3 2 0 5 ( 0 1 )0 1 0 7 8 -5
2770
W. Zhang, Z. Wang / Life Sciences 68 (2001) 2769–2779
Introduction Dihydropyridine calcium channel blockers (CCB) or calcium antagonists have been thought to be superior to other classes of antihypertensive drugs, because (1) they reduce arterial blood pressure physiologically by decreasing the total peripheral resistance that is increased in essential hypertension [1]; (2) they have a neutral or beneficial impact on blood lipid profile whose imbalance frequently coexists with hypertension, which may be further impaired by some other antihypertensive drugs [1–3]; (3) they have a diuretic effect and protect or improve renal function [4]; and (4) very importantly, they may retard the development and progression of atherosclerosis [5–8]. Despite the recent controversies surrounding the safety and long-term ability of calcium antagonists to alter the course of cardiovascular disease [9,10], the main concern in using calcium antagonists for antihypertensive treatment is that when the dihydropyridine lowers arterial pressure, it activates the sympathetic nervous system (SNS) through arterial baroreceptor unloading-mediated autonomic reflex, which leads to unsatisfying clinical outcomes. It has been documented that cardiovascular system has its circadian rhythm [11–13]. The clinical outcome of antihypertensive therapy may be unsatisfying if the activation of SNS triggered by extrinsic stimulus (CCB) coincidentally overlaps with the activation set by intrinsic circadian rhythm, the results of which may additively enhance sympathetic nerve outflow. Some therapeutic strategies have been suggested in order to reduce the activation of SNS in response to dihydropyridine. Such strategies include limiting the speed of antihypertensive action, maintaining stable plasma level and effect of antihypertensive drug over the dosing interval, combining two drugs with different mechanisms of action [14–16]. Based on cardiovascular circadian rhythm, a novel chronotherapy has been suggested [13]. The main principle for this new chronotherapy is to choose two classes of drugs with different mechanism of action and use them at different times during the day. By appropriately choosing the drugs and their acting time, not only the adverse effects, but also the risks of sympathetic traffic could be reduced. If a drug that does not cause a profound activation of SNS when decreasing arterial pressure is administered first, and a CCB is added on to maintain the antihypertensive effect, then CCB may not activate SNS. The neurophysiological support for this hypothesis comes from the evidence that arterial baroreceptors resetting to hypotension can take place very quickly [17–19]. If baroreceptors can be reset to the lower level of arterial pressure by the first drug that does not cause baroreflex-mediated activation of SNS, then keeping arterial pressure at this level with calcium antagonist may not stimulate the baroreceptors because CCB just maintains the lowered arterial pressure level which baroreceptors have adapted to. The purpose of the present study was to test this novel hypothesis in animal models. In the present experiment, enalapril, an angiotensin-converting enzyme inhibitor (ACEI) was used first to lower arterial pressure and nifedipine, a CCB was added later on to maintain the level of the arterial pressure that enalapril had created, to see whether nifedipine activates SNS after ACEI. This was first performed in normotensive rats and then extended to hypertensive rats, which have been a widely used animal model for essential hypertension in humans. The present results indicate that reflex activation of SNS caused by antihypertensive effect of CCB could be avoided if, prior to CCB administration, baroreceptors have been reset to a lower MAP by a drug that does not activate baroreceptor reflex.
W. Zhang, Z. Wang / Life Sciences 68 (2001) 2769–2779
2771
Material and methods Experimental rats were from the Institutional Animal Center where the animals were housed in a controlled environment with free access to standard rat chow and tap water. The experimental protocols were in accordance to NIH guidelines for laboratory animal use and approved by the Institutional Animal Care and Use Committee. Protocol 1 (in normotensive rats) Wistar rats (male, 300–330 g) were initially anesthetized with methohexital sodium (60 mg kg21 i.p.). The right jugular vein, left carotid artery and right femoral artery were catheterized with PE-50 tubing filled with physiological saline for drug delivery, arterial blood pressure measurement and blood sampling respectively. Tracheotomy was performed with PE-240 tubing inserted into the trachea. After the surgery, a-chloralose was injected (50 mg kg21 in 2 min, i.v.) and then continuously infused (30 mg kg21 h21) through a side branch of the carotid catheter. The rectal temperature was maintained at 37.5 8C by an external heating pad and lamp. The experiment started 30 min after above procedure was finished. Mean arterial pressure (MAP) and heart rate (HR) was recorded for 20 min as baseline values. The rats then received one of the following treatments: Vehicle (control) group (n57): This group of rats received vehicle injection and infusion in the same way as the other 2 experimental groups received. Three rats received physiological saline (vehicle for enalapril) and four received 40% polyethylene glycol (PEG. MW 400) in physiological saline (vehicle for nifedipine). Because there was no detectable difference between saline and PEG treated rats, all animals were pooled into one control group. Nifedipine group (n57): This group of rats received a nifedipine injection (2 mg kg21 over 2 min) which was followed by continuous nifedipine infusion. The infusion rate was adjusted (between 0.5–1.0 mg h21) to keep the MAP 15 mmHg lower than the baseline level throughout the 4-h experiment. Enalapril 1 nifedipine group (n57): This group of rats received an enalapril injection (5 mg kg21 over 2 min) which was followed by continuous enalapril infusion. The infusion rate was adjusted (0.9–1.3 mg h21) to keep the MAP about 15 mmHg lower than the baseline level. The infusion rate of enalapril was gradually reduced after 100 min and completely stopped after 140 min, whereas nifedipine infusion was started after 100 min and its infusion rate (between 0.1–1.0 mg h21) was increased to keep antihypertensive effect from enalapril unchanged during the rest time of the 4-h experiment. The blood sample was taken with heparinized chilled syringe from femoral artery before the infusion and every 80 min after infusion. The loss of blood due to sampling (about 1 ml each time) was immediately restored with same amount of blood from an anesthetized donor rat that received the exactly same treatment, but was excluded from data analysis. Figure 1 illustrates the experimental protocol. Protocol 2 (in hypertensive rats) Spontaneously hypertensive rat (male, 260–280 g. SHR) received the same anesthetizing and surgical preparation as in protocol 1. The experiment started 45 min after above procedure was finished. The basal MAP and HR were recorded for 20 min. The rats then received one of the following treatments: Vehicle (control) group (n56): This group of rats
2772
W. Zhang, Z. Wang / Life Sciences 68 (2001) 2769–2779
Fig. 1. Illustration of the experimental procedure for both protocol 1 and 2. In the group that received calcium channel blocker (CCB), nifedipine was infused for 4-h after a bolus injection of nifedipine. The infusion speed was controlled in order to lower the mean arterial pressure to a certain level. In the group that received angiotensinconverting enzyme inhibitor enalapril plus nifedipine (ACEI1CCB), enalapril was injected first and its infusion was followed to achieve the similar reduction of MAP to CCB group. Enalapril infusion was gradually withdrawn after 100 min and stopped after 140 min; in the meantime nifedipine was incrementally added on to keep the level of blood pressure created by enalapril unchanged for the rest of the experiment. For the dosage of each drug, see respective protocols.
received vehicle injection and infusion as in protocol 1. Nifedipine group (n57): This group of rats received a nifedipine injection (4 mg kg21 over 2 min) which was followed by continuous nifedipine infusion. The infusion rate was adjusted (between 0.7–1.2 mg h21) to keep the MAP 25 mmHg lower than the baseline level throughout 4-h experiment. Enalapril 1 nifedipine group (n57): This group of rats received an enalapril injection (10 mg kg21 over 2 min) which was followed by continuous enalapril infusion. The infusion rate was adjusted (1.2–1.6 mg h21) to keep the MAP about 25 mmHg lower than the baseline level. The infusion rate of enalapril was gradually reduced after 100 min and completely stopped after 140 min, whereas nifedipine infusion was started after 100 min and its infusion rate (between 0.1–1.4 mg h21) was adjusted to keep the antihypertensive effect from enalapril unchanged during the rest time of the 4-h experiment. The time course of the experiment and the blood sampling method were as same as in protocol 1 (Figure 1 illustrates the experimental protocol). Plasma catecholamine assay The blood samples were transferred to chilled tube and centrifuged at 3000 rpm and 4 8C for 10 min to separate the plasma from blood. The plasma samples were frozen at 270 8C until catecholamine measurement. The measurement was performed with high performance liquid chromatography with electrical detection [20]. Data analysis The statistical analysis was performed by two-way analysis of variance (ANOVA) followed by the Newman-Keuls test. All data are presented as mean6s.e.m. A probability (P) of less that 0.05 was considered to be statistically significant.
W. Zhang, Z. Wang / Life Sciences 68 (2001) 2769–2779
2773
Table 1 Mean arterial blood pressure (MAP), heart rate (HR), plasma norepinephrine (NE) and epinephrine (E) levels in 3 groups of anaesthetized Wistar rats before calcium channel blocker (CCB) nifedipine, angiotensin-converting enzyme inhibitor (ACEI) enalapril plus nifedipine (ACEI1CCB) or vehicle infusions. There is no statistical difference between any two groups (p5ns). MAP (mmHg) HR (beats min21) NE (nmol l21) E (nmol l21)
Vehicle (n57)
CCB (n57)
ACEI1CCB (n57)
94.064.0 37168 2.8560.14 2.7560.10
95.062.7 36766 2.7960.32 2.8460.22
96.363.0 36967 2.5760.21 2.8760.20
Results Protocol 1 (in normotensive rats) The basal MAP, HR and plasma norepinephrine (NE) and epinephrine (E) in anesthetized Wistar rats before drug or vehicle infusions are shown in table 1. There is no statistical difference among vehicle (control), nifedipine and enalapril 1 nifedipine groups. The effects of vehicle, nifedipine and enalapril 1 nifedipine on MAP were shown in figure 2. There is no substantial change in MAP over the entire course of the experiment in control group. Nifedipine and enalapril reduced MAP by 15 mmHg each. In enalapril 1 nifedipine group, the infusion rate of enalapril was gradually decreased after 100 min; in the meantime, nifedipine was incrementally added on. Enalapril infusion was stopped after 140 min and the antihypertensive effect of enalapril was then taken over by nifedipine, which kept the MAP unchanged for the rest time of the experiment. Figure 3 shows the changes of HR and the plasma catecholamine levels after the infusion of vehicle or different drugs. In nifedipine group,
Fig. 2. The changes in mean arterial pressure (MAP) in Wistar rats that received vehicle (control), calcium channel blocker nifedipine (CCB) and angiotensin-converting enzyme inhibitor enalapril plus nifedipine (ACEI1CCB) (n57 each). Nifedipine (2 mg kg21 injection and 0.5–1.0 mg h21 infusion) and enalapril (5 mg kg21 injection and 0.9–1.3 mg h21 infusion) reduced MAP by 15 mmHg each from baseline. In ACEI1CCB group, enalapril was gradually withdrawn after 100 min; in the meantime, nifedipine was incrementally added on (0.1–1.0 mg h21). Enalapril infusion was stopped after 140 min and the antihypertensive effect of enalapril was then taken over by that of nifedipine, which kept the MAP unchanged for the rest of the experiment. * p , 0.05 vs. control.
2774
W. Zhang, Z. Wang / Life Sciences 68 (2001) 2769–2779
Fig. 3. The changes in heart rate (HR) and plasma norepinephrine (NE) and epinephrine (E) in Wistar rats received vehicle (control), calcium channel blocker nifedipine (CCB) and angiotensin-converting enzyme inhibitor enalapril plus nifedipine (ACEI1CCB) (n57 each). Nifedipine (2 mg kg21 injection and 0.5–1.0 mg h21 infusion) and enalapril (5 mg kg21 injection and 0.9–1.3 mg h21 infusion) reduced MAP by 15 mmHg each from baseline. The hypotensive effect of nifedipine provoked tachycardia and elevated plasma NE and E levels in the CCB group. However the similar effect from enalapril did not cause tachycardia and change plasma NE and E. When antihypertensive effect of enalapril was taken over by that of nifedipine, HR, NE and E were unchanged in ACEI1CCB group. * P , 0.05 vs. control.
the reduction of MAP by nifedipine caused a significant increase in HR and elevation of plasma NE and E levels, which represent the activation of SNS. In enalapril 1 nifedipine group, the initial reduction of MAP by enalapril did not cause an increase in HR or elevation of plasma NE and E levels. HR and plasma NE and E levels were kept unchanged after the antihypertensive effect of enalapril was taken over by that of nifedipine. Protocol 2 (in hypertensive rats) The basal mean MAP, HR and plasma NE and E in anesthetized SHR before drug or vehicle infusions are shown in table 2. There is no statistical difference among vehicle, nifedipine and enalapril 1 nifedipine groups. Those parameters were comparably elevated as reported before when SHR were compared with normotensive rats [21,22].
W. Zhang, Z. Wang / Life Sciences 68 (2001) 2769–2779
2775
Table 2 Mean arterial blood pressure (MAP), heart rate (HR), plasma norepinephrine (NE) and epinephrine (E) levels in 3 groups of anaesthetized spontaneously hypertensive rats before calcium channel blocker (CCB) nifedipine, angiotensin-converting enzyme inhibitor (ACEI) enalapril plus nifedipine (ACEI1CCB) or vehicle infusions. There is no statistical difference between any two groups (p5ns). MAP (mmHg) HR (beats min21) NE (nmol l21) E (nmol l21)
Vehicle (n56)
CCB (n57)
ACEI1CCB (n57)
153.766.6 39768 3.3360.18 4.0060.33
148.266.2 409610 3.2260.22 3.7360.18
151.065.9 415615 3.0860.18 4.0460.29
The effect of vehicle, nifedipine and enalapril 1 nifedipine on MAP was shown in figure 4. There is no substantial change in MAP over the entire course of the experiment in control group. Nifedipine and enalapril reduced MAP by 25 mmHg each. In enalapril 1 nifedipine group, the infusion rate of enalapril was gradually decreased after 100 min; in the meantime, nifedipine was incrementally added on. Enalapril infusion was stopped after 140 min and the antihypertensive effect of enalapril was then taken over by nifedipine, which kept the MAP unchanged for the rest time of the experiment. Figure 5 shows the changes of HR and the plasma catecholamine levels after the infusion vehicle or different drugs. In nifedipine group, the reduction of MAP by nifedipine caused a significant increase in HR and elevation of plasma NE and E levels, which represent the activation of SNS. In enalapril 1 nifedipine group, the initial reduction of MAP by enalapril did not cause an increase in HR or elevation of plasma NE and E levels. HR and plasma NE and E levels were kept unchanged after the antihypertensive effect of enalapril was taken over by nifedipine.
Fig. 4. The changes in mean arterial pressure (MAP) in spontaneously hypertensive rats (SHR) that received vehicle (control, n56), calcium channel blocker nifedipine (CCB, n17) and angiotensin-converting enzyme inhibitor enalapril plus nifedipine (ACEI1CCB, n57). Nifedipine (4 mg kg21 injection and 0.7–1.2 mg h21) and enalapril (10 mg kg21 injection and 1.2–1.6 mg h21) reduced MAP by 25 mmHg each from baseline. In ACEI1CCB group, enalapril was gradually withdrawn after 110 min; in the meantime, nifedipine was incrementally added on (0.1–1.4 mg h21). Enalapril infusion was stopped after 140 min and the antihypertensive effect of enalapril was then taken over by nifedipine, which kept the MAP unchanged for the rest of the experiment. * P , 0.05 vs. control.
2776
W. Zhang, Z. Wang / Life Sciences 68 (2001) 2769–2779
Fig. 5. The changes in heart rate (HR) and plasma norepinephrine (NE) and epinephrine (E) in spontaneously hypertensive rats received vehicle (control, n56), calcium channel blocker nifedipine (CCB) and angiotensinconverting enzyme inhibitor enalapril plus nifedipine (ACEI1CCB) (n57, respectively). Nifedipine (4 mg kg21 injection and 0.7–1.2 mg h21 infusion) and enalapril (10 mg kg21 injection and 1.2–1.6 mg h21 infusion) reduced MAP by 25 mmHg from baseline. Hypotensive effect of nifedipine provoked tachycardia and elevated plasma NE and E in CCB group. However the similar effect from enalapril did not cause tachycardia and increase plasma NE and E. When antihypertensive effect of enalapril was taken over by that of nifedipine, HR, NE and E were unchanged in ACEI1CCB group. * P , 0.05 vs. control.
Discussion The present study in both normotensive and hypertensive rat models demonstrates that the baroreceptor unloading-mediated activation of sympathetic nervous system by antihypertensive action of nifedipine, a dihydropyridine calcium channel blocker, can be avoided by pretreatment with enalapril, an angiotensin-converting enzyme inhibitor. Enalapril was used to lower arterial pressure and reset arterial baroreceptors to the hypotensive level. Nifedipine was used after the baroreceptors had adapted to the hypotensive level established by enalapril to keep the antihypertensive effect unchanged. The study provides experimental support for the clinical application of the novel chronotherapy for hypertension [13], which is to choose two classes of drugs with different mechanism of action and use them at different times during the day in order to reduce reflex activation of SNS and other side effects associated with acute antihypertensive action of CCB.
W. Zhang, Z. Wang / Life Sciences 68 (2001) 2769–2779
2777
The initiation of antihypertensive treatment with CCB is associated with activation of SNS. Accelerated HR, elevated plasma catecholamines and increased renal sympathetic nerve activity in response to acute hypotensive effect of nifedipine all represent the activation of SNS secondary to direct vasodilation and reduction of total peripheral resistance [23–26]. The acute activation of SNS has been widely documented by previous investigations in both human and animal subjects and is considered as a result of baroreceptor unloading [25,27,28]. Although the activation of SNS is a physiological response that aims to prevent the established hemodynamic homeostasis from disturbances, it is clinically detrimental in hypertension treatment because the higher heart rate and elevated plasma catecholamines are powerful predictor of the poor prognosis [26,29,30]. Investigations during the past decade have shown that the onset of episodes of transit cardiac ischemia as well as the myocardial infarction, ventricular tachycardia, thrombolic stroke and sudden cardiac death occurs more frequently in the morning (6:00–12:00 AM) than the rest of the day (so that morning is refereed as vulnerable period for those who have normal sleep-wake cycles) [11]. The underlying mechanism behind these is believed to be a series of neurohumoral activation, of which sympathetic activation is the most important [13]. If the extrinsic activation of SNS provoked by CCB overlaps coincidentally with the intrinsic activation set by circadian rhythm (bio-clock), then the activation of SNS would be additively enhanced, which could trigger more cardiovascular events [31,32]. Angiotensin-converting-enzyme inhibitors are a group of heterogeneous drugs that lower arterial pressure mainly by reducing angiotensin II production in plasma and tissue. In the present study, enalapril did not activate SNS as assessed by heart rate and plasma catecholamine levels either in normotensive rats when arterial pressure was reduced or in hypertensive rats when their arterial pressure was normalized. The administration of nifedipine two hours after enalapril maintained the antihypertensive effect of enalapril but indeed did not activate SNS. This chrono-combination of the 2 drugs provided an ideal control of arterial pressure. The purpose of enalapril administration is to initiate antihypertension effect and to reset arterial baroreceptors to the hypotensive level. The process of baroreceptor unloading and resetting in response to hypotensive effect of ACEI does not result in reflex activation of SNS. This is because that angiotensin II plays a pivotal role in modulating the process of sympathetic activation [33–35]. The removal of angiotensin II by inhibiting its generation causes sympathetic inhibition [35–38]. Thus during acute ACEI administration the sympathoexcitatory effect from baroreceptor unloading may not outweigh the sympathoinhibitory effect. This is frequently demonstrated by the fact that antihypertensive effect of ACEI is not associated with sympathetic activation in vivo [35–38]. As baroreceptor resetting to lower pressure could take place very quickly [17–19], the administration of CCB after ACEI, as shown in the present study, did not cause sympathetic activation. The possibility that dihydropyridine CCB can be used with other classes of antihypertensive drugs also exist. As demonstrated in the present study, it is required to initiate antihypertensive effect by using a drug that does not activate SNS, once baroreceptors are reset to the new lower level of arterial pressure, the antihypertensive effect from the first selected drug should be maintained by CCB. Because the activation of SNS evidently compromises the beneficial effect of CCB or even worsens the outcome of the antihypertensive therapy, the better use of CCB without reflex activation of SNS, as outlined in the present study, would be
2778
W. Zhang, Z. Wang / Life Sciences 68 (2001) 2769–2779
clinically interesting. Further investigation is required to determine if such a chronotherapy is beneficial in terms of reducing cardiovascular morbidity and mortality. Limitation of the study. First, the short-term baroreceptor resetting is usually not complete [17–19], hence in enalapril 1 nifedipine group the lack of significant sympathetic activation in response to nifedipine infusion may involve unknown mechanisms other than discussed here, which was not the focus of the present study but is worth further investigation. Second, although the infusion of enalapril was stopped after 140 min in enalapril 1 nifedipine group of normotensive and hypertensive rats, the residue effect of angiotensin-converting enzyme inhibition might still be affecting SNS and therefore could modulate the effect of nifedipine on SNS. This needs to be considered in future studies. In summary, the initial antihypertensive effect of dihydropyridine calcium channel blocker is associated with activation of sympathetic nervous system, whereas that of angiotensinconverting enzyme is not. CCB can be used in antihypertensive therapy without activation of sympathetic nervous system if it is administered after baroreceptors unloading and resetting have been established by an angiotensin-converting enzyme inhibitor.
References 1. van Zwieten PA. Calcium entry blockers (calcium antagonists). In: Van Zwieten PA, editor. Handbook of Hypertension. Amsterdam: Elsevier, 1984, pp. 331–42. 2. Hansson L. Calcium antagonists: an overview. Am Heart J 1991;122:308–11. 3. Lender D, Arauz-Pacheco C, Breen L, Mora-Mora P, Ramirez LC, Raskin P. A double blind comparison of the effects of amlodipine and enalapril on insulin sensitivity in hypertensive patients. Am J Hypertens 1999; 12:298–303. 4. Zanchi A, Brunner HR, Waeber B, Burnier M. Renal haemodynamic and protective effects of calcium antagonists in hypertension. J Hypertens 1995;13:1363–75. 5. Lichtlen PR, Hugenholtz PG, Rafflenbeul W, Hecker H, Jost S, Deckers JW. Retardation of angiographic progression of coronary artery disease by nifedipine. Results of the international nifedipine trial on antiatherosclerotic Therapy (INTACT). Lancet 1990;335:1109–13. 6. Hof RP, Tapparelli C, Weinstein DB. Hemodynamic, antivasoconstrictor, and antiatherosclerotic effects of calcium antagonists in animal models of atherosclerosis. J Cardiovasc Pharmacol 1990;15:S7–12. 7. Zanchetti A. Antiatherosclerotic effects of antihypertensive drugs: recent evidence and ongoing trials. Clin Exp Hypertens 1996;18:489–99. 8. Waters D, Lesperance J, Francetich M, et al. A controlled clinical trial to assess the effect of a calcium channel blocker on the progression of coronary atherosclerosis. Circulation 1990;82:1940–53. 9. Messerli FH, Grossman E. The calcium antagonist controversy: a posthumous commentary. Am J Cardiol 1998;82:35R–9R. 10. Epstein M. The calcium antagonist controversy: the emerging importance of drug formulation as a determinant of risk. Am J Cardiol 1997;79:9–19. 11. Muller JE, Tofler GH, Stone PH. Circadian variation and triggers of onset of acute cardiovascular disease. Circulation 1989;79:733–43. 12. Cooke HM, Lynch A. Biorhythms and chronotherapy in cardiovascular disease. Am J Hosp Pharm 1994;51: 2569–80. 13. Zhang W. Can we use calcium antagonist better in antihypertensive therapy? Circadian consideration. Pharmacol Res 1996;34:187–91. 14. Zanchetti A, Chalmers JP, Arakawa K, et al. The 1993 guidelines for the management of mild hypertension: memorandum from a WHO/ISH meeting. Blood Press 1993;2:86–100. 15. Luscher TF, Waeber B. Efficacy and safety of various combination therapies based on a calcium antagonist in
W. Zhang, Z. Wang / Life Sciences 68 (2001) 2769–2779
16. 17. 18. 19. 20. 21.
22.
23. 24. 25. 26. 27. 28. 29.
30. 31. 32. 33. 34. 35. 36.
37. 38.
2779
essential hypertension: results of a placebo-controlled randomized trial. J Cardiovasc Pharmacol 1993;21: 305–9. Anonym. The sixth report of the joint national committee on prevention, detection, evaluation and treatment of high blood pressure (JNC VI). NIH Publication 1998:3–64. Salgado HC, Krieger EM. Extent of baroreceptor resetting in response to sodium nitroprusside and verapamil. Hypertension 1988;11:I121–5. de Abreu GR, Salgado HC. Antihypertensive drugs distinctly modulate the rapid resetting of the baroreceptors. Hypertension 1990;15:I63–7. Xavier-Neto J, Moreira ED, Krieger EM. Viscoelastic mechanisms of aortic baroreceptor resetting to hypotension and to hypertension. Am J Physiol 1996;271:H1407–15. Zhang W, Liu L, Zheng D, et al. Effects of atrial natriuretic factor on patients with renovascular hypertension. Chin Med J 1992;105:732–7. Zhang W, Pettersson A, Thoren P, Hedner T. Effects of 1,1-dimethylguanidine administration on blood pressure, heart rate and renal sympathetic nerve activity in normotensive and spontaneously hypertensive rats. Acta Physiol Scand 1997;159:1–6. Zhang W, Thoren P. Hyper-responsiveness of adrenal sympathetic nerve activity in spontaneously hypertensive rats to ganglionic blockade, mental stress and neuronglucopenia. Pflugers Arch - Eur J Phsiol 1998; 437:56–60. Packer M. Calcium channel blockers in chronic heart failure. The risks of “physiologically rational” therapy. Circulation 1990;82:2254–7. Lindqvist M, Kahan T, Melcher A, Hjemdahl P. Acute and chronic calcium antagonist treatment elevates sympathetic activity in primary hypertension. Hypertension 1994;24:287–96. Zhang W, Sun X, Zhao X, Thoren P, Hedner T. Effects of calcium antagonist felodipine on renal sympathetic nerve activity in rats with congestive heart failure. Cardiovasc Drugs Ther 1997;11:645–50. Palatini P. Heart rate as a risk factor for atherosclerosis and cardiovascular mortality: the effect of antihypertensive drugs. Drugs 1999;57:713–24. Leenen FH, Holliwell DL. Antihypertensive effect of felodipine associated with persistent sympathetic activation and minimal regression of left ventricular hypertrophy. Am J Cardiol 1992;69:639–45. Frishman WH. Impact of calcium antagonists on early morning ischaemia in the patient with coronary disease. Eur Heart J 1999;1:B34–40. Francis GS, Cohn JN, Johnson G, Rector TS, Goldman S, Simon A. Plasma norepinephrine, plasma renin activity, and congestive heart failure. Relations to survival and the effects of therapy in V-HeFT II. The V-HeFT VA Cooperative Studies Group. Circulation 1993;87:VI40–8. Benedict CR, Shelton B, Johnstone DE, et al. Prognostic significance of plasma norepinephrine in patients with asymptomatic left ventricular dysfunction. SOLVD Investigators. Circulation 1996;94:690–7. Pahor M, Guralnik JM, Corti MC, Foley DJ, Carbonin P, Havlik RJ. Long-term survival and use of antihypertensive medications in older persons. J Am Geriatr Soc 1995;43:1191–7. Borhani NO, Mercuri M, Borhani PA, et al. Final outcome results of the Multicenter Isradipine Diuretic Atherosclerosis Study (MIDAS). A randomized controlled trial. JAMA 1996;276:785–91. Luft FC, Wilcox CS, Unger T, et al. Angiotensin-induced hypertension in the rat. Sympathetic nerve activity and prostaglandins. Hypertension 1989;14:396–403. Reid IA. Interactions between ANG II, sympathetic nervous system, and baroreceptor reflexes in regulation of blood pressure. Am J Physiol 1992;262:E763–78. Bernier NJ, Perry SF. Cardiovascular effects of angiotensin-II-mediated adrenaline release in rainbow trout Oncorhynchus mykiss. J Exp Biol 1999;202:55–66. Schofer J, Hobuss M, Aschenberg W, Tews A. Acute and long-term haemodynamic and neurohumoral response to nisoldipine vs captopril in patients with heart failure: a randomized double-blind study. Eur Heart J 1990;11:712–21. DiBona GF, Jones SY, Brooks VL. ANG II receptor blockade and arterial baroreflex regulation of renal nerve activity in cardiac failure. Am J Physiol 1995;269:R1189–96. Zhang W, Huang BS, Leenen FHH. Brain renin-angiotensin system and sympathetic hyperactivity in rats after myocardial infarction. Am J Physiol (Heart Cir Physiol) 1999;45:H1608–15.