- Email: [email protected]
Relationship of Immunoassayable Clonidine Plasma Levels to its Pharmacologic Action in Clinical and Experimental Hypertension
Session 7
Relationship of Immunoassayable Clonldlne Plasma Levels to Its Pharmacologic Action in Clinical and Experimental Hypertenslon* William]. Louis, M.D.; S. N. Anavekar, M.B., Ph.D.; Elizabeth L. Comeau, Ph.D.; and Bevyn]arrott, Ph.D., B. Pharm.
C
lonidine is a potent hypotensive drug in man, reducing blood pressure in doses of 0.3 to 1.5 mg daily,':" It produces no postural hypotension, and its hypotensive effect is associated with mild bradycardia. Pharmacologic studies in animal models both in vitro and in vivo have shown that clonidine is a selective Q2-adrenoceptor agonist, and its hypotensive action is believed tobe due to activation of vasomotor centers in the brain, resulting in a suppression of sympathetic outflow to the cardiovascular system and also in bradycardia through an increased parasympathetic activity to the heart." However, it should be stressed that clonidine does not only lower blood pressure: it also causes sedation," reduces conditioned salivation," antagonizes pentylene-tetrazol-induced convulsions," and attenuates the morphine-withdrawal syndrome. -: Thus, clonidine is acting at other CN S sites unrelated to blood pressure control, but is probably activating Q2-adrenoceptors at these sites. Unfortunately, the high potency of clonidine has hampered studies of its pharmacokinetics and distribution, because very sensitive assays are required for such investigations. Having developed a highly sensitive radioimmunoassay capable of quantitating picograms of clonidine, ~ we investigated the pharmacokinetics of the drug in man. We also examined the temporal relationship between tissue levels and blood pressure response in rats. METHODS
Pharmacokinetic Studies in Man
Eighteen normotensiveadults (aged 21to 23 years)were studied after informed consent had been obtained. After fastingovernight, the subjects were givenan oral dose of75 ....g of clonidine. anettimed venous blood samples were taken for analysis of elonidine" and plasma norepinephrine levels.9 Pulse rate and blood pressures were also recorded at the same intervals. Approximately two weeks later, some of the same subjects were given oral doses of 150 JAog, and the investigation was repeated exactlyto examinethe dose-dependency of the pharmacokinetics. Plasma concentration data were subjected to multiexponential regression analysis using the computer program AUlOMOD lo to compute the various pharmacokineticparameters.
Table I-Pharmacoanetic Parameter' (Mean ± SE) for Clonidirae after Oral Administration in Human Dose
150 JAog
Cmu
Tmu
AVC
(ng/ml)
(hours)
(ng-hr-ml')
0.29 ±O.OOI 0.61 ±0.04
2.6 ±0.8 2.4 ±0.64
10.9 ± 1.7 9.1 ±0.95
3.4 ±0.31 5.7 ±0.53
Brain was dissected into regions;some peripheral tissues also were taken for analysis. To study the steady-state tissue distribution of clonidine, Alza mini-pumps containing clonidine (2 mglml) were implanted subcutaneously, releasingthe drug at approximately 2 ....g/hour, Aftersix days, rats were killed and tissues dissected for analysis. RESULTS
Pharmacokinetic Studies in Man After oral administration of clonidine at doses of75 and 150 fJ.g, clonidine appeared in the plasma after a lag time of approximately 20 minutes, and the peak levels (Cmax) were attained approximately at 2.5 hours. Thereafter, plasma declined in a monoexponential manner, with a half-life of nine to 11 hours. The Cmax and area under the plasma concentration curve increased approximately linearly with \ increase in oral dose (Table 1).
Pharmacodynamic Studies in Man The two doses ofclonidine produced significant reductions in mean arterial blood pressure (MAP) as well as in pulse rate. The time for maximal changes in these cardiovascular parameters closely mirrored the time for Cmax and increased with the higher dose of clonidine. Side effects, such as sedation and dry mouth, also were maximal at the time of Cmax, and their frequency and severity appeared to be doserelated. Plasma norepinephrine concentrations were also studied longitudinally in subjects given 75 and 150 fJ.g of clonidine. There was no significant change in mean plasma norepinephrine levels with time after 75 fJ.g of clonidine while after 150 ug, plasma norepinephrine levels fell significantly one hour after administration of clonidine and then returned to normal after six hours (Table 2). Table i-Time Course ojChange. in Plasma Norepinephrine Concentration (ngIml) in Man after Oral Administration ofClonidine TIme, hr
Dose
Tissue Distribution in Rats
Ratswere injected intravenously (IV) with clonidine (20JAoglkg) via a lateral tail vein and decapitated at varying times after injection. *From the Clinical Pharmacology and Therapeutics Unit, Department of Medicine, University of Melbourne, Austin Hospital, Heidelberg, Victoria, Australia. Supported by grants from the National Health and Medical Research Council of Australia. Reprint requests: Dr: Louis, Clinical Pharmacologq and Therapeutics, Austin Hospital, Heidelberg, Victoria, Australia 2084 352
0 75~g
0.39 ± 0.08 150 ....g 0.43 ± 0.05
0.35 :t
2
3
4
6
0.39
0.45
0.44
0.40
:t
:t
:t
:t
8
12
24
0.44 0.40 0.44 :t
:t
±
0.05 0.08 0.07 0.08 0.05 0.09 0.07 0.09 0.29· 0.34· 0.27· 0.34· 0.33· 0.36 0.40 0.44 ± ± ± ± ± ± ± ± 0.03 0.06 0.03 0.05 0.05 0.05 0.04 0.04
*p
Table 3-ClonitUne Concentrationa in eNS Regiona at the Tame ofPeak HfIPOtenBive BaponBe (10 min) After an IV DOle of~O fLg/1cg and Subsequent Half-Ufe ofEUminatiOn From TheBe Begiona* Clonidine, nglg wet wt
Region Striatum Hippocampus Cerebral cortex Hypothalamus Pons Midbrain Spinal cord Medulla oblongata Cerebellum
Half-life, min
25.8t
loot
24.6 23.9 20.5 20.3
58
18.7 17.3 16.6
8.6
49 45 42 39
64 43 25
*Modified from Reference Il, tMean of 5 rats.
Uptake and Elimination ofClonidine in Rat CNS Previous studies in our laboratory" have shown that a dose of 20 fLglkg of clonidine injected IV into anesthetized rats produces a submaximal change in MAP and heart rate. The maximal fall in MAP occurs at ten minutes and returns to normal at 120 minutes. We determined the levels ofclonidine in regions ofeNS at the peak of the hypotensiveresponse and then measured the half-life (tY2) for disappearance of clonidine after the peak level (Table 3).
Tissue Concentrations ofC lonidine in Rat Tissues Under Steady-state Conditions Since the half-life of elimination of clonidine in rat blood and the hypotensive response is much shorter than in humans, we measured the tissue concentrations of clonidine under steady-state conditions. We believed that this would approximate more closely the tissue distribution in man. It was found that the highest concentration of clonidine was in kidney, followed by spleen, lung, intestine, stomach, liver, brain, heart, muscle, and adipose tissue." These tissue-plasma concentration ratios were similar to those seen in 'rats at the time of the maximal hypotensive response after an acute injection of clonidine (20 fLglkg). DISCUSSION
Earlier studies of the pharmacokinetics ofclonidine in man used high doses of clonidine (150to 300 fLg) and complex biochemical assay methods.'?" We have developed a sensitive and specific radioimmunoassay for clonidine which allowed us to determine the pharmacokinetics in humans of a 75-fLg oral dose of clonidine for the first time to our knowledge. We also examined the pharmacokinetics of clonidine after a 15O-fLg dose, and this suggested that linear pharmacokinetics were applicable at least over this dose range. These studies showed that clonidine has desirable phannacokinetic properties in man, since' the drug was well absorbed and had an elimination half-life of approximately ten hours. It is generally believed that clonidine acts in the CNS to reduce the sympathetic nervous activity to heart and blood vessels." However; when we monitored the plasma norepinephrine levels in subjects at various times after administration of 75 fLg of clonidine, there was no significant fall in
norepinephrine levels in plasma. After the 15O-fLg dose, plasma norepinephrine levels fell significantly after one hour, but returned to normal in the next five hours; however, blood pressure was significantly reduced until 24 hours after clonidine administration. While this discrepancy might question only the reliability of plasma norepinephrine levels as an index of sympathetic function in man, it is pertinent to consider our results in relation to the recent electrophysiologic data obtained by Wallin and Frisk-Holmberg," They recorded multiunit sympathetic activity in branches of the peroneal nerve in hypertensive subjects during IV administration of clonidine (100 to 275 fLg). They concluded from their data that the hypotensive effect" of clonidine was not always reflected in a decrease in sympathetic nerve activity. Thus, they questioned the hypothesis that the mechanism of action of clonidine was solely a reduction in central sympathetic outflow and raised the possibility that clonidine may also act peripherally. It is well known that clonidine can act on isolated peripheral organ preparations in vitro to reduce sympathetic neurotransmission and also in brain slice preparation," TISsue distribution studies could give insight into possible sites of action of clonidine by establishing which tissues accumulate the drug and whether the accumulation and disappearance correlate with the hypotensive response. Obviously such studies cannot be done on man, so we have examined the tissue pharmacokinetics of clonidine in rats given a submaximal hypotensive dose of clonidine. It was found that at the peak' of the hypotensive response less than 2 percent of injected drug was present in the brain and that at least equal concentrations of clonidine were found in peripheral tissues, such as spleen, heart, and intestine. Since these tissues are innervated by the sympathetic nervous system, clonidine could well be acting locally to reduce sympathetic nervous activity. The highest level of clonidine was in the kidney (approximately eightfold higherthan in the brain), strengthening the hypothesis that clonidine may act in this organ by reducing secretion of renin. 18 Since the hypotensive response to elonidine in the rat is shorter than in man, as is the elimination half-life in rat blood, the rat may not be a relevant model for studying tissue kinetics after a single injection. Thus we infused clonidine continuously for six days and then measured tissue levels of clonidine. The accumulation of clonidine in various tissues was again similar to that seen after a single' injection. With regard to the regional distribution of clonidine in rat CNS, there was no selective accumulation of drug in regions such as pons, medulla oblongata, or hypothalamus, which are believed to control cardiovascular function. However, it should be stressed that clonidine exerts other CNS actions apart from reducing central sympathetic outfl~ and the present data on the regional distribution of clonidine in the CNS provide evidence for the wide-ranging neuropharmacologic actions of clonidine. · In conclusion, our studies on man and rats have strengthened the-view that clonidine may have a peripheral as well as a central hypotensive action. In view of clinical studies indicating that the central sedative action of clonidine is" an unacceptable. side effect in some patients, the development of an u.-aqrenoceptor agonist which enters the brain to a lesser extent than clonidine is desirable. CHEST I 83 I 2 I February, 1983 I SUpplement
353
REFERENCES 1 Ng Jt Phelan EL, McGregor DD, Laverty R, Taylor KM, Smirk H. Properties ofCatapres, a new hypotensive drug: a preliminary report. NZ Med J 1967; 66:864-70 2 Raftos J, Bauer GE, Lewis RG. Clonidine in the treatment of severe hypertension. Med J Aust 1973; 1:786-93 3 Langer SZ. Presynaptic regulation of the release of catecholamines. Pharmacol Rev 1980; 32:337-62 4 Schmitt H. The pharmacology of clonidine and related products. Handbook Exp Pharmacol 1977; 39:299-96 5 Rand MJ, Rush M, Wilson J. Some observations on the inhibition of salivation by St 155 (2-(2,6-dichlorophenyl-amine)-2-imidazoline). Eur J Pharmacoll969; 5:168-72 6 Papanicolaou J, Summers RJ, Vajda FJE, Louis WJ. Anticonvulsant effects of clonidine mediated through central at-adrenoceptors. Eur J Pharmacoll982; 77:163-66 7 Fielding S, Wilker Jt Hynes M, Szewczak M. A comparison of clonidine with morphine for antinociceptive and antiwithdrawal actions. J Pharmacol Exp Ther 1978; 207:899-905 8 Jarrott B, Spector S. Disposition of clonidine in rats as determined by radioimmunoassay. J Pharmacol Exp Ther 1978; 207:195-202 9 Da Prada M, Zurcher G. Simultaneous radioenzymatic determination of plasma and tissue adrenaline, noradrenaline and dopamine within the fentomole range. Life Sci 1976; 19:1161-74 10 Gomeri R, Gomeri C. AUTOMOD, a polyalgorithm for an integrated analysis of linear pharmacokinetic models. Com put BioI Med 1979; 9:39-48 11 Conway EL, Janott B. Clonidine distribution in the rat: temporal relationship between tissue levels and blood pressure response. Br J Pharmacoll980; 71:473-78 12 Conway EL, Janott B. TIssue pharmacokinetics of clonidine in rats. J Pharmacokinet Biopharm (in press) 13 Davies DS, Wing LMH, Reid JL, Neill E, TIppett ~ Dollery cr Pharmacokinetics and concentration-effect relationships of intravenous and oral clonidine. Clin Pharmacal Ther 1977; 21:593-601 14 Frisk-Holmberg M, Edlund PO, Paalzow L. Pharmacokinetics of clonidine and its relation to the hypotensive effects in patients. Br J Clin Pharmaco11978; 6:227-32 15 Wallin BG, Frisk-Holmberg M. The antihypertensive mechanism of clonidine in man: evidence against a generalized reduction of sympathetic activity. Hypertension 1981; 3:340-46 16 Pettinger WA, Keeton TIC, Campbell WB, Harer DC. Evidence for a renal a-adrenergic receptor inhibiting renin release. Cire Res 1976; 38:338-46
Comparative Quantitative Studies on Central and Peripheral a-Adrenoceptors*
mm Hg
200
i
5 min
1 sec
l •.~ .......
100~
_v
~\\I\f\f • • • • • • • •
•
lclonidiM 12Jl9/kg. 1v I
FIGURE 1. Typical effect on heart rate and arterial pressure of a pentobarbitone-anesthetized normotensive rat following intravenous injection with clonidine (2 JLglkg). Transient increase in pressure due to vasoconstriction brought about by stimulation of vascular and Ot-adrenoceptors. Longer-lasting decrease in arterial pressure is result of activation of central Ot-adrenoceptors. Peripheral as well as central mechanisms may account for reduction in cardiac frequency accompanying hypotension.
a.-
(Fig 1). The following proposed effects should be distinguished: (1) The stimulation of both 01- and at-adrenoceptor subtypes at postsynaptic sites in vascular smooth muscle causes vasoconstriction and, hence, a pressor response. (2) The stimulation ofcentral ~-adrenoceptors in the brain stem induces hypotension and bradycardia mediated via central mechanisms. (3) The stimulation of presynaptically located ~-adrenoceptors in the heart may contribute to the overall bradycardia. Figure 2 summarizes the various mechanisms possibly playing a part in the bradycardia induced by clonidine and related drugs. For detailed information on these basic pharmacologic principles, the reader is referred to the review articles by Schmitt.P Berthelsen and Pettinger," Kobtnger," VanZwieten,5.8 VanZwieten and Tlmmermans, 7.8 De jonge et aI,9.10 and Timmermans et al." Todiscriminate between the various a-adrenergic mechanisms underlying the cardiovascular profile of a-adrenoceptor agonists and to further characterize the a-adrenoceptor populations involved, quantitative relationships were derived between various cardiovascular activities of clonidine and a series of structurally related imidazolidines. VAGAL
SYMPATHO-ADRE NERGIC
---~-----------
cardiac ,- -- presynapt ic , ~2-adrenoceptors
P. B. M. W M. nmmennans, Ph.D.; A de Jonge, Ph.D.; and P. A van Zwieten, M.D., Ph.D.
and related drugs are known to be a-adrenocepClonidine tor agonists. This agonism may occur at the level of at least four distinctly different populations ofa-adrenoceptors, leading to the overall effects of hypotension and bradycardia *From the Division of Pharmacotherapy, Department of Pharmacy, University of Amsterdam, Amsterdam, The Netherlands. Reprint requests: Dr. Thnmermans, Department of Phaf'm!JCY, University ofAmsterdam, 1018 tv Amsterdam, the Netherlands 354
FIGURE 2. Mechanisms via which clonidine can induce bradycardia. Stimulation of central Ot-adrenoceptors causes reduction of peripheral sympathetic tone; hence, bradycardia (and hypotension). Moreover, enhancement of vagal re8ex bradycardia is also initiated within the CNS. A third, peripheral action may also be of significance. Activation of cardiac presynaptic Ot-adrenoceptors decreases amount of transmitter noradrenaline released from postganglionic sympathetic nerve endings, so that cardiac frequency is reduced. (Modified after Kobingen")
Relationship of Immunoassayable Clonldlne Plasma Levels to Its Pharmacologic Action in Clinical and Experimental Hypertenslon* William]. Louis, M.D.; S. N. Anavekar, M.B., Ph.D.; Elizabeth L. Comeau, Ph.D.; and Bevyn]arrott, Ph.D., B. Pharm.
C
lonidine is a potent hypotensive drug in man, reducing blood pressure in doses of 0.3 to 1.5 mg daily,':" It produces no postural hypotension, and its hypotensive effect is associated with mild bradycardia. Pharmacologic studies in animal models both in vitro and in vivo have shown that clonidine is a selective Q2-adrenoceptor agonist, and its hypotensive action is believed tobe due to activation of vasomotor centers in the brain, resulting in a suppression of sympathetic outflow to the cardiovascular system and also in bradycardia through an increased parasympathetic activity to the heart." However, it should be stressed that clonidine does not only lower blood pressure: it also causes sedation," reduces conditioned salivation," antagonizes pentylene-tetrazol-induced convulsions," and attenuates the morphine-withdrawal syndrome. -: Thus, clonidine is acting at other CN S sites unrelated to blood pressure control, but is probably activating Q2-adrenoceptors at these sites. Unfortunately, the high potency of clonidine has hampered studies of its pharmacokinetics and distribution, because very sensitive assays are required for such investigations. Having developed a highly sensitive radioimmunoassay capable of quantitating picograms of clonidine, ~ we investigated the pharmacokinetics of the drug in man. We also examined the temporal relationship between tissue levels and blood pressure response in rats. METHODS
Pharmacokinetic Studies in Man
Eighteen normotensiveadults (aged 21to 23 years)were studied after informed consent had been obtained. After fastingovernight, the subjects were givenan oral dose of75 ....g of clonidine. anettimed venous blood samples were taken for analysis of elonidine" and plasma norepinephrine levels.9 Pulse rate and blood pressures were also recorded at the same intervals. Approximately two weeks later, some of the same subjects were given oral doses of 150 JAog, and the investigation was repeated exactlyto examinethe dose-dependency of the pharmacokinetics. Plasma concentration data were subjected to multiexponential regression analysis using the computer program AUlOMOD lo to compute the various pharmacokineticparameters.
Table I-Pharmacoanetic Parameter' (Mean ± SE) for Clonidirae after Oral Administration in Human Dose
150 JAog
Cmu
Tmu
AVC
(ng/ml)
(hours)
(ng-hr-ml')
0.29 ±O.OOI 0.61 ±0.04
2.6 ±0.8 2.4 ±0.64
10.9 ± 1.7 9.1 ±0.95
3.4 ±0.31 5.7 ±0.53
Brain was dissected into regions;some peripheral tissues also were taken for analysis. To study the steady-state tissue distribution of clonidine, Alza mini-pumps containing clonidine (2 mglml) were implanted subcutaneously, releasingthe drug at approximately 2 ....g/hour, Aftersix days, rats were killed and tissues dissected for analysis. RESULTS
Pharmacokinetic Studies in Man After oral administration of clonidine at doses of75 and 150 fJ.g, clonidine appeared in the plasma after a lag time of approximately 20 minutes, and the peak levels (Cmax) were attained approximately at 2.5 hours. Thereafter, plasma declined in a monoexponential manner, with a half-life of nine to 11 hours. The Cmax and area under the plasma concentration curve increased approximately linearly with \ increase in oral dose (Table 1).
Pharmacodynamic Studies in Man The two doses ofclonidine produced significant reductions in mean arterial blood pressure (MAP) as well as in pulse rate. The time for maximal changes in these cardiovascular parameters closely mirrored the time for Cmax and increased with the higher dose of clonidine. Side effects, such as sedation and dry mouth, also were maximal at the time of Cmax, and their frequency and severity appeared to be doserelated. Plasma norepinephrine concentrations were also studied longitudinally in subjects given 75 and 150 fJ.g of clonidine. There was no significant change in mean plasma norepinephrine levels with time after 75 fJ.g of clonidine while after 150 ug, plasma norepinephrine levels fell significantly one hour after administration of clonidine and then returned to normal after six hours (Table 2). Table i-Time Course ojChange. in Plasma Norepinephrine Concentration (ngIml) in Man after Oral Administration ofClonidine TIme, hr
Dose
Tissue Distribution in Rats
Ratswere injected intravenously (IV) with clonidine (20JAoglkg) via a lateral tail vein and decapitated at varying times after injection. *From the Clinical Pharmacology and Therapeutics Unit, Department of Medicine, University of Melbourne, Austin Hospital, Heidelberg, Victoria, Australia. Supported by grants from the National Health and Medical Research Council of Australia. Reprint requests: Dr: Louis, Clinical Pharmacologq and Therapeutics, Austin Hospital, Heidelberg, Victoria, Australia 2084 352
0 75~g
0.39 ± 0.08 150 ....g 0.43 ± 0.05
0.35 :t
2
3
4
6
0.39
0.45
0.44
0.40
:t
:t
:t
:t
8
12
24
0.44 0.40 0.44 :t
:t
±
0.05 0.08 0.07 0.08 0.05 0.09 0.07 0.09 0.29· 0.34· 0.27· 0.34· 0.33· 0.36 0.40 0.44 ± ± ± ± ± ± ± ± 0.03 0.06 0.03 0.05 0.05 0.05 0.04 0.04
*p
Table 3-ClonitUne Concentrationa in eNS Regiona at the Tame ofPeak HfIPOtenBive BaponBe (10 min) After an IV DOle of~O fLg/1cg and Subsequent Half-Ufe ofEUminatiOn From TheBe Begiona* Clonidine, nglg wet wt
Region Striatum Hippocampus Cerebral cortex Hypothalamus Pons Midbrain Spinal cord Medulla oblongata Cerebellum
Half-life, min
25.8t
loot
24.6 23.9 20.5 20.3
58
18.7 17.3 16.6
8.6
49 45 42 39
64 43 25
*Modified from Reference Il, tMean of 5 rats.
Uptake and Elimination ofClonidine in Rat CNS Previous studies in our laboratory" have shown that a dose of 20 fLglkg of clonidine injected IV into anesthetized rats produces a submaximal change in MAP and heart rate. The maximal fall in MAP occurs at ten minutes and returns to normal at 120 minutes. We determined the levels ofclonidine in regions ofeNS at the peak of the hypotensiveresponse and then measured the half-life (tY2) for disappearance of clonidine after the peak level (Table 3).
Tissue Concentrations ofC lonidine in Rat Tissues Under Steady-state Conditions Since the half-life of elimination of clonidine in rat blood and the hypotensive response is much shorter than in humans, we measured the tissue concentrations of clonidine under steady-state conditions. We believed that this would approximate more closely the tissue distribution in man. It was found that the highest concentration of clonidine was in kidney, followed by spleen, lung, intestine, stomach, liver, brain, heart, muscle, and adipose tissue." These tissue-plasma concentration ratios were similar to those seen in 'rats at the time of the maximal hypotensive response after an acute injection of clonidine (20 fLglkg). DISCUSSION
Earlier studies of the pharmacokinetics ofclonidine in man used high doses of clonidine (150to 300 fLg) and complex biochemical assay methods.'?" We have developed a sensitive and specific radioimmunoassay for clonidine which allowed us to determine the pharmacokinetics in humans of a 75-fLg oral dose of clonidine for the first time to our knowledge. We also examined the pharmacokinetics of clonidine after a 15O-fLg dose, and this suggested that linear pharmacokinetics were applicable at least over this dose range. These studies showed that clonidine has desirable phannacokinetic properties in man, since' the drug was well absorbed and had an elimination half-life of approximately ten hours. It is generally believed that clonidine acts in the CNS to reduce the sympathetic nervous activity to heart and blood vessels." However; when we monitored the plasma norepinephrine levels in subjects at various times after administration of 75 fLg of clonidine, there was no significant fall in
norepinephrine levels in plasma. After the 15O-fLg dose, plasma norepinephrine levels fell significantly after one hour, but returned to normal in the next five hours; however, blood pressure was significantly reduced until 24 hours after clonidine administration. While this discrepancy might question only the reliability of plasma norepinephrine levels as an index of sympathetic function in man, it is pertinent to consider our results in relation to the recent electrophysiologic data obtained by Wallin and Frisk-Holmberg," They recorded multiunit sympathetic activity in branches of the peroneal nerve in hypertensive subjects during IV administration of clonidine (100 to 275 fLg). They concluded from their data that the hypotensive effect" of clonidine was not always reflected in a decrease in sympathetic nerve activity. Thus, they questioned the hypothesis that the mechanism of action of clonidine was solely a reduction in central sympathetic outflow and raised the possibility that clonidine may also act peripherally. It is well known that clonidine can act on isolated peripheral organ preparations in vitro to reduce sympathetic neurotransmission and also in brain slice preparation," TISsue distribution studies could give insight into possible sites of action of clonidine by establishing which tissues accumulate the drug and whether the accumulation and disappearance correlate with the hypotensive response. Obviously such studies cannot be done on man, so we have examined the tissue pharmacokinetics of clonidine in rats given a submaximal hypotensive dose of clonidine. It was found that at the peak' of the hypotensive response less than 2 percent of injected drug was present in the brain and that at least equal concentrations of clonidine were found in peripheral tissues, such as spleen, heart, and intestine. Since these tissues are innervated by the sympathetic nervous system, clonidine could well be acting locally to reduce sympathetic nervous activity. The highest level of clonidine was in the kidney (approximately eightfold higherthan in the brain), strengthening the hypothesis that clonidine may act in this organ by reducing secretion of renin. 18 Since the hypotensive response to elonidine in the rat is shorter than in man, as is the elimination half-life in rat blood, the rat may not be a relevant model for studying tissue kinetics after a single injection. Thus we infused clonidine continuously for six days and then measured tissue levels of clonidine. The accumulation of clonidine in various tissues was again similar to that seen after a single' injection. With regard to the regional distribution of clonidine in rat CNS, there was no selective accumulation of drug in regions such as pons, medulla oblongata, or hypothalamus, which are believed to control cardiovascular function. However, it should be stressed that clonidine exerts other CNS actions apart from reducing central sympathetic outfl~ and the present data on the regional distribution of clonidine in the CNS provide evidence for the wide-ranging neuropharmacologic actions of clonidine. · In conclusion, our studies on man and rats have strengthened the-view that clonidine may have a peripheral as well as a central hypotensive action. In view of clinical studies indicating that the central sedative action of clonidine is" an unacceptable. side effect in some patients, the development of an u.-aqrenoceptor agonist which enters the brain to a lesser extent than clonidine is desirable. CHEST I 83 I 2 I February, 1983 I SUpplement
353
REFERENCES 1 Ng Jt Phelan EL, McGregor DD, Laverty R, Taylor KM, Smirk H. Properties ofCatapres, a new hypotensive drug: a preliminary report. NZ Med J 1967; 66:864-70 2 Raftos J, Bauer GE, Lewis RG. Clonidine in the treatment of severe hypertension. Med J Aust 1973; 1:786-93 3 Langer SZ. Presynaptic regulation of the release of catecholamines. Pharmacol Rev 1980; 32:337-62 4 Schmitt H. The pharmacology of clonidine and related products. Handbook Exp Pharmacol 1977; 39:299-96 5 Rand MJ, Rush M, Wilson J. Some observations on the inhibition of salivation by St 155 (2-(2,6-dichlorophenyl-amine)-2-imidazoline). Eur J Pharmacoll969; 5:168-72 6 Papanicolaou J, Summers RJ, Vajda FJE, Louis WJ. Anticonvulsant effects of clonidine mediated through central at-adrenoceptors. Eur J Pharmacoll982; 77:163-66 7 Fielding S, Wilker Jt Hynes M, Szewczak M. A comparison of clonidine with morphine for antinociceptive and antiwithdrawal actions. J Pharmacol Exp Ther 1978; 207:899-905 8 Jarrott B, Spector S. Disposition of clonidine in rats as determined by radioimmunoassay. J Pharmacol Exp Ther 1978; 207:195-202 9 Da Prada M, Zurcher G. Simultaneous radioenzymatic determination of plasma and tissue adrenaline, noradrenaline and dopamine within the fentomole range. Life Sci 1976; 19:1161-74 10 Gomeri R, Gomeri C. AUTOMOD, a polyalgorithm for an integrated analysis of linear pharmacokinetic models. Com put BioI Med 1979; 9:39-48 11 Conway EL, Janott B. Clonidine distribution in the rat: temporal relationship between tissue levels and blood pressure response. Br J Pharmacoll980; 71:473-78 12 Conway EL, Janott B. TIssue pharmacokinetics of clonidine in rats. J Pharmacokinet Biopharm (in press) 13 Davies DS, Wing LMH, Reid JL, Neill E, TIppett ~ Dollery cr Pharmacokinetics and concentration-effect relationships of intravenous and oral clonidine. Clin Pharmacal Ther 1977; 21:593-601 14 Frisk-Holmberg M, Edlund PO, Paalzow L. Pharmacokinetics of clonidine and its relation to the hypotensive effects in patients. Br J Clin Pharmaco11978; 6:227-32 15 Wallin BG, Frisk-Holmberg M. The antihypertensive mechanism of clonidine in man: evidence against a generalized reduction of sympathetic activity. Hypertension 1981; 3:340-46 16 Pettinger WA, Keeton TIC, Campbell WB, Harer DC. Evidence for a renal a-adrenergic receptor inhibiting renin release. Cire Res 1976; 38:338-46
Comparative Quantitative Studies on Central and Peripheral a-Adrenoceptors*
mm Hg
200
i
5 min
1 sec
l •.~ .......
100~
_v
~\\I\f\f • • • • • • • •
•
lclonidiM 12Jl9/kg. 1v I
FIGURE 1. Typical effect on heart rate and arterial pressure of a pentobarbitone-anesthetized normotensive rat following intravenous injection with clonidine (2 JLglkg). Transient increase in pressure due to vasoconstriction brought about by stimulation of vascular and Ot-adrenoceptors. Longer-lasting decrease in arterial pressure is result of activation of central Ot-adrenoceptors. Peripheral as well as central mechanisms may account for reduction in cardiac frequency accompanying hypotension.
a.-
(Fig 1). The following proposed effects should be distinguished: (1) The stimulation of both 01- and at-adrenoceptor subtypes at postsynaptic sites in vascular smooth muscle causes vasoconstriction and, hence, a pressor response. (2) The stimulation ofcentral ~-adrenoceptors in the brain stem induces hypotension and bradycardia mediated via central mechanisms. (3) The stimulation of presynaptically located ~-adrenoceptors in the heart may contribute to the overall bradycardia. Figure 2 summarizes the various mechanisms possibly playing a part in the bradycardia induced by clonidine and related drugs. For detailed information on these basic pharmacologic principles, the reader is referred to the review articles by Schmitt.P Berthelsen and Pettinger," Kobtnger," VanZwieten,5.8 VanZwieten and Tlmmermans, 7.8 De jonge et aI,9.10 and Timmermans et al." Todiscriminate between the various a-adrenergic mechanisms underlying the cardiovascular profile of a-adrenoceptor agonists and to further characterize the a-adrenoceptor populations involved, quantitative relationships were derived between various cardiovascular activities of clonidine and a series of structurally related imidazolidines. VAGAL
SYMPATHO-ADRE NERGIC
---~-----------
cardiac ,- -- presynapt ic , ~2-adrenoceptors
P. B. M. W M. nmmennans, Ph.D.; A de Jonge, Ph.D.; and P. A van Zwieten, M.D., Ph.D.
and related drugs are known to be a-adrenocepClonidine tor agonists. This agonism may occur at the level of at least four distinctly different populations ofa-adrenoceptors, leading to the overall effects of hypotension and bradycardia *From the Division of Pharmacotherapy, Department of Pharmacy, University of Amsterdam, Amsterdam, The Netherlands. Reprint requests: Dr. Thnmermans, Department of Phaf'm!JCY, University ofAmsterdam, 1018 tv Amsterdam, the Netherlands 354
FIGURE 2. Mechanisms via which clonidine can induce bradycardia. Stimulation of central Ot-adrenoceptors causes reduction of peripheral sympathetic tone; hence, bradycardia (and hypotension). Moreover, enhancement of vagal re8ex bradycardia is also initiated within the CNS. A third, peripheral action may also be of significance. Activation of cardiac presynaptic Ot-adrenoceptors decreases amount of transmitter noradrenaline released from postganglionic sympathetic nerve endings, so that cardiac frequency is reduced. (Modified after Kobingen")