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Effect of isoproterenol on the blood vessels of the spontaneously hypertensive rat
MI(‘ROVASCULAR KESEARC’H9, 101-106 (1975)
Effect
of lsoproterenol Spontaneously
on the Blood Hypertensive
Vessels Rat
of the
PHILLIP M. HUTCHINS, A. WAYNE GREENE, AND THOMAS D. RAINS Department qfphysiology, Bowman Gray School of Medicine, Wake Forest University, Winston-Salem, North Carolina 27103 Receiced June 5, 1974 The reactivity of skeletal muscle arterioles to isoproterenol injections was compared in the normal control rat (NCR) and spontaneously hypertensive rat (SHR). The response to one second injections of 10-s-10-4g/ml of isoproterenol was dose dependent and of greatest magnitude in the vessel category with the smallest control diameter. SHR had lower threshold in small arteriolar vessels, but NCR exhibited greater maximum percent increase in vessel diameter.
INTRODUCTION The involvement of the sympathetic nervous system and circulating catecholamines in the initiation and maintenance of essential hypertension is not well understood (Engleman et al., 1970; Julius et al., 1971; Okamoto, 1969; Nosaka and Wang, 1972). There are several reports of increased cardiac output and a beta hyperkinetic state early in the hypertensive process (Conway, 1970; Fujiwara et al., 1972; Folkow et al., 1971). There is, likewise, ample evidence that the level of sympathetic activity including the adrenal medulla is elevated during all stages of the disease (Yamori et al., 1972; Clark, 1971; Nakamura et al., 1971; Folkow et al., 1971). Various investigators have demonstrated an increased reactivity to norepinephrine in hypertensive animals (Haeusler and Haefely, 1970; Bohr et al., 1971: Armstrong, (1972), while others have observed a decrease in the response to norepinephrine (Shibata et al., 1973; Spector et al., 1969). Still other researchers have found that the reactivity to norepinephrine is the same in aortic strip preparations (Hallback et al., 1971) and also in intact, in vivo vessels (Hutchins et al., 1973b) of the spontaneously hypertensive rate (SHR) and the normal control rat (NCR). Although the cardiac beta effects have been investigated by chronic pharmacological blockade in the SHR (Vavra et al., 1973; Frohlich, 1971; Folkow et al., 1972) the peripheral beta receptors have been little studied. Spector et al. (1969) have shown that aortic strips from SHR exhibit greater relaxation to isoproterenol. The possibility exists that, by virtue of the prolonged elevation in sympathetic activity and a concomitant increase in adrenal medullary hormones, the peripheral beta receptors have been altered in reactivity. Indeed this could be one of the many factors acting in concert to produce the rise in total peripheral resistance seen late in the hypertensive process. This investigation was undertaken to determine the reactivity of intact microvessels to isoproterenol in the skeletal muscle bed of spontaneously hypertensive and normal control rats. Copyright 8 1975by Academic Press,Inc. All rights of reproduction in any form reserved. Printed in Great Britain
101
102
HUT(‘HINS,GREENE AND RAINS
METHODS The experimental model used for this investigation was the cremaster muscle of the 100 g rat (5-6 wk). Two groups of rats were used: the spontaneously hypertensive rat (Okamoto and Aoki, 1963) and normal Wistar rats (CFN-Carworth Farms). The Kyoto control Wistar strain was not available at the time of this study. A total of 38 animals were used in this study, 21-NCR and 17-SHR. They were anesthetized with a warm solution of 10% urethane and 2% chloralose (6 ml/kg, ip). The animals were maintained between 36 and 38°C by thermostatically controlled heating mats. The trachea was cannulated to provide an adequate airway. The cremaster in a 100 gratis approximately 250 pm thick with two layers of skeletal muscle surrounding the blood vessels. The surgical approach is through a longitudinal incision in the ventral surface of the scrotal skin. Using a heat cautery and blunt dissection the cremaster and enclosed testis are freed of the scrotal skin and connective tissue. In a 100 g rat there is very little connective tissue, a finding not seen in larger animals. A ligature is placed at the caudal tip of the cremaster and an incision is made by cautery in the ventral surface of the cremaster. The testis is then freed from the cremaster and extirpated. Additional ligatures are placed around the cremaster and it is stretched over an electrically heated pedestal, 1.8 cm in diameter and 1.5 cm in height. The cremaster is irrigated with a pH-adjusted, balanced salt solution and covered with a 1.8 cm, round, no. 1 thickness, microscope slide cover slip The covered nature of the preparation prevents equilibration of the cremaster with atmospheric gases and eliminates the need for irrigation solutions The cremaster temperature is maintained at approximately 34.5”, the normal in situ temperature of the cremaster. The ipsilateral femoral artery is cannulated in a retrograde manner with a silastic and polyethylene microinjection chamber. Injections of isoproterenol were made through this chamber into the femoral, the inferior epigastric and finally, the cremasteric artery. One carotide artery was cannulated for the recording of systemic blood pressure and heart rate on a Brush recorder. The microvessels were observed through Zeiss optics at magnifications of 80-l 60x. The microscope image was projected onto a Sony AVC-3210 video camera which was connected to a Conrac monitor and Sony AV3600 video recorder which allowed subsequent analysis of vessel diameter. A 3 ft optical fiber and Quartz-Iodide light source provided illumination through a long working distance condenser. Heating effects were minimized by the use of fiber optics to remove infrared and near-infrared wavelengths. The injections were via the femoral artery microinjection chamber connected to a constant rate infusion pump. The time duration for the injection was 1 sec. At a pump rate of 2.93 ml/min, the total volume for one injection was approximately 50 ~1. The vessels under observation were video-recorded for a control period of 1 min prior to the injection and an experimental period of 1 min following the injection. The diameter for each period was integrated and an average diameter was obtained for each of the pre- and postinjection periods. The response to isoproterenol was determined as the ratio of the average postinjection diameter (experimental) to the
BETA
RECEPTORS
IN HYPERTENSIOY
103
average preinjection diameter (control). In this way the artifacts of vasomotion and spurious vessel diameter changes are averaged out. Vessel categorization was according to their order of branching in the vascular tree (Hutchins et al., 1973a). The cremasteric artery was designated as a first-order arteriole (Al). Branches off this first-order arteriole were denoted as second-order arterioles (A2). Branches off A2s were A3s and off A3s were A&. RESULTS A total of 888 observations were made in 17 SHRs and 21 NCRs. The dose-response curves for the two groups of animals and the four vessel categories are shown in Fig. 1. The control injection was the injection of the vehicle (Normosol-R, Abbott Laboratories) and is labeled by “Norm.” The abscissa corresponds to the concentration of active base in grams per milliliter and is plotted logarithmically. The ordinate presents a normalized averaged diameter. A value of 1.0 indicates that the diameter remained unchanged after the isoproterenol injection. A value of 1.6 indicates that the vessel dilated to 160% of its control diameter (e.g., from a control diameter of 10 pm to an average postinjection diameter of 16 pm). The values represent the mean of all observations in this category + SEM. The average control diameter for Al vessels was 106 pm; A2, 59 pm; A3,25 pm; and A4, 12 pm. The p values were computed from Student’s two-tailed t test. The total observations are shown in parentheses above the probability values. It is seen from Fig. 1 that there are statistically significant differences at the higher concentrations of isoproterenol in the Al and A2 vessel categories. In these vessels there seemed to be a slight reduction in vessel size with the higher concentrations of isoproterenol. The third- and fourth-order arterioles of the SHR exhibited a lower threshold than did similar vessels in the NCR. However, at the highest concentrations of isoproterenol the NCR fourth-order arterioles demonstrated a greater dilatation than did A4s from SHR. Since these doses gave a maximum response, it may be concluded that the maximum dilatation of the SHR was less than that of the NCR in the smallest arterioles. The mean blood pressure for the SHR was 122.0 -t 2.3 mmHg; while NCR mean blood pressure averaged 107.9 & 2.6 mmHg. DISCUSSION It appears from Fig. 1 that there are two apparent differences between the arteriolar microvasculature of the SHR and that of the NCR. These are (1) a lower threshold for the SHR, and (2) a greater dilation by the NCR to isoproterenol. The lowered threshold for the SHR appears at the 10m8(A3) and 10e7 (A3 and A4) concentrations of isoproterenol. The threshold for the NCR was observed to be at 10e6 g/ml. Assuming that p agonists work through c-AMP, this reduced threshold for the SHR is in agreement with the data of Triner et al. (1972) who showed that in lo-12 wk SHR the basal adenylate cyclase activity was elevated. In addition, they observed an increase in c-AMP in the SHR. However, this finding was not confirmed by Amer (1973), who found significantly lower levels of c-AMP in SHR and stress
104
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Norm
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10m7
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ISOPROTERENOL FIG. 1. Response of the four arteriolar categories to different concentrations of isoproterenol base. Values represent the arithmetic mean k 1 SEM. The numbers just above the standard error bars represent p values computed from a two-tailed t test. The numbers in parentheses just above the p values are the number of observations. The dashed lines represent the SHR and the solid lines, NCR.
hypertensive rats. He also noted that adenylate cyclase activity was normal but less responsive to stimulation in the hypertensive group. Spector et al. (1969) also report that aortic strips from SHR demonstrate a greater relaxation to IO-’ g/ml of isoproterenol, a finding which was confirmed in the smaller arterioles of this study. The smaller dilatory capacity in response to isoproterenol may be due to the SHR arterioles being partially dilated. We have previously reported (Hutchins and Darnell, 1974) that the small arterioles of the SHR have larger diameters than their NCR counterparts but are fewer in number. The effect of a prior existing, partial dilation
BETA RECEPTORSIN HYPERTENSION
IO5
would be that the maximum dilation would be reduced when computed on a percentof-control basis. Evidence for a functionally reduced maximum dilation was reported by Triner et ul. (1972) who found that the concentration of isoproterenol needed to increase the c-AMP level a set amount was five to ten times higher in the SHR than the NCR. As stated above, Amer (I 973) also found that adenylate cyclase activity was less responsive to stimulation. The differences between the SHR and NCR Al and A2 vessels at 10m6(Al), lo-’ (Al, and 10e8 (A2) g/ml of isoproterenol is most probably related to the observed change in threshold in the A3 and A4 arterioles. However, if one assumes a longitudinal pressure drop similar to that reported by Fronek and Zweifach (1973) these changes may be somewhat inconsequential in terms of the vascular bed as a whole. In addition to the documented alterations in beta activity of the heart, these results suggest that there is a modification of the beta receptor in the skeletal muscle vascular bed of the spontaneously hypertensive rat. ACKNOWLEDGMENTS This study was supported in part by Public Service Grant HL 13936 and a Grant from the North Carolina Heart Association. The assistance of Mrs. Lib Stewart in the preparation of this manuscript is gratefully acknowledged. REFERENCES AMER, M. (1973). Cyclic adenosine monophosphate and hypertension in rats. Science 179, 807-809. ARMSTRONG,J. (1972). Vascular reactivity to noradrenaline of 5-hydroxytryptamine in hypertensive rats. Brit. J. Pharmol. 45, 183P-184P. BOHR,D., SITRIN, M., AND HANSEN,T. (1971). Vascular smooth muscle in experimental hypertension. Clin. Sci. 40, 1P. CLARK, D. (1971). Effects of immunosympathectomy on development of high blood pressure in genetically hypertensive rats. Circ. Res. 28, 330-336. CONWAY,J. (1970). Hemodynamics aspects of hypertension. Circ. Res. Suppl. 126,27,143-148. ENGLEMAN,K., PORTNOY,C., AND SJOERDSMA, A. (1970). Plasma catecholamine concentrations in patients with hypertension. Circ. Res. Suppl. I. 26,27,1141-1146. FOLKOW,B., HALLBACK, M., LUNDGREN,Y., AND WEISS,L. (1971). The effects of “Immunosympathectomy” on blood pressure and vascular “reactivity” in normal and spontaneously hypertensive rats. Acta Physiol. Stand. 82, 27A. FOLKOW, B., HALLBACK, M., LUNDGREN,Y., AND WEISS,L. (1971). The effect of intense treatment with hypotensive drugs on structural design of the resistance vessels in spontaneously hypertensive rats. Acta Physiol. Stand. 83,280-282. FOLKOW, B., LUNDGREN,Y., AND WEISS,L. (1972). Effect of prolonged propranolol treatment on blood pressure and structural design of the resistance vesselsin young spontaneously hypertensive rats (SH R). Acta Physiol. Stand. 84,8A-9A. FROHLICH,E. (1971). Beta adrenergic blockage in the circulatory regulation of hyperkinetic states. Amer. J. Cardiol. 27, 195-199. FRONEK,K., ANDZWEIFACH,B. (1973). Comparison of the effect of papaverine on terminal vasculature of the cat mesentery and skeletal muscle. Microvas. Res. 6, 121-122. FUJIWARA,M., KUCHII, M., AND SHIBATA,S. (1972). Differences of cardiac reactivity between spontaneously and normotensive rats. Eur. J. Pharmol. 19, l-l 1. HAEULSER,G., AND HAEFELY,W. (1970). Pre- and postjunctional supersensitivity of the mesenteric artery preparation from normotensive and hypertensive rats. Naunyn Schmied. Arch. Pharmak. 266, 18-23.
106
HUTCHINS. GREENE AND RAINS
M., Lumcmh, Y., AND WEISS.L. (1971). Reactivity ofnoradrenaline of aortic strips and portal veins from spontaneously hypertensive and normotensive rats. Acra PhJsiol. .%a&. 81. 176-181. HUTCHINS,P., GOLDSTONE,J., AND WELLS, R. t1973a). Effects of hemorrhagic shock on the microvasculature of skeletal muscle. Microcasc. Rrs. 5, 131-140. HUTCHINS, P., RAINS, T., AND GREENE,A. (1973b). Microvascular reactivity to norepinephrine in the spontaneously hypertensive rat. Microcasc. Res. 6,123-124. HUTCHINS, P., AND DARNELL, A. (1974). Observation of a decreased number of small arterioles in spontaneously hypertensive rats. Circ. Res. 34, I-161-I-165(Supp. I). JULIUS,S., PASCUAL,A., AND MITCHELL,C. (1971). Relationship between cardiac output and peripheral resistance in borderline hypertension. Circulation 43, 382-390. NAKAMURA, K., GEROLD,M., AND THEONEN,J. (1971). Genetically hypertensive rats: Relationship between the development of hypertension and the changes in norepinephrine turnover and peripheral and central adrenergic neurons. Naunyn Schmied. Arch. Pharmak. 271,157-169. NOSAKA, S., AND WANG, S. (1972). Carotid sinus baroceptor functions in the spontaneously hypertensive rat. Amer. J. Physiol. 222,1079-1084. OKAMOTO,K., AND AOKI, K. (1963). Development of a strain of spontaneously hypertensive rats. HALLBACK,
Jap. Circ. J. 27,282-293.
OKAMOTO,K. (1969). Spontaneous hypertension in rats. hf. Rec. Exp. Pafhol. 7,227-270. SHIBATA,S., KURAHASKI,K., AND KUCHII, M. (1973). A possible etiology of contractility impairment of vascular smooth muscle from spontaneously hypertensive rats. J. Pharmacol. Exp. Ther. 185, 4066417. SPECTOR,S., FLEISCH,J. H., MALING, H. M., AND BRODIE,B. B. (1969). Vascular smooth muscle reactivity in normotensive and hypertensive rats. Science 166,1300-1301. TRINER, L., VULLIEMOZ,Y., VEROSKY,N., MANEGER,W., AND NAHAS, G. (1972). Cyclic AMP and vascular reactivity in spontaneous hypertensive rats. Fed. Prod. 32, 1929. VAVRA, I., TOM, H., AND GRESELIN,E. (1973). Chronic propranolol treatment in young spontaneously hypertensive and normotensive rats. Can. J. Physiol. Pharmacol. 51, 727-732. YAMORI, Y., YAMBE, H., DEJONG,W., LOVENBERG,W., AND SJOERDSMA, A. (1972). Effect of tissue norepinephrine depletion by 6-hydroxydopamine on blood pressure in spontaneously hypertensive rats. Eur. J. Pharmacol. 17, 135-140.
Effect
of lsoproterenol Spontaneously
on the Blood Hypertensive
Vessels Rat
of the
PHILLIP M. HUTCHINS, A. WAYNE GREENE, AND THOMAS D. RAINS Department qfphysiology, Bowman Gray School of Medicine, Wake Forest University, Winston-Salem, North Carolina 27103 Receiced June 5, 1974 The reactivity of skeletal muscle arterioles to isoproterenol injections was compared in the normal control rat (NCR) and spontaneously hypertensive rat (SHR). The response to one second injections of 10-s-10-4g/ml of isoproterenol was dose dependent and of greatest magnitude in the vessel category with the smallest control diameter. SHR had lower threshold in small arteriolar vessels, but NCR exhibited greater maximum percent increase in vessel diameter.
INTRODUCTION The involvement of the sympathetic nervous system and circulating catecholamines in the initiation and maintenance of essential hypertension is not well understood (Engleman et al., 1970; Julius et al., 1971; Okamoto, 1969; Nosaka and Wang, 1972). There are several reports of increased cardiac output and a beta hyperkinetic state early in the hypertensive process (Conway, 1970; Fujiwara et al., 1972; Folkow et al., 1971). There is, likewise, ample evidence that the level of sympathetic activity including the adrenal medulla is elevated during all stages of the disease (Yamori et al., 1972; Clark, 1971; Nakamura et al., 1971; Folkow et al., 1971). Various investigators have demonstrated an increased reactivity to norepinephrine in hypertensive animals (Haeusler and Haefely, 1970; Bohr et al., 1971: Armstrong, (1972), while others have observed a decrease in the response to norepinephrine (Shibata et al., 1973; Spector et al., 1969). Still other researchers have found that the reactivity to norepinephrine is the same in aortic strip preparations (Hallback et al., 1971) and also in intact, in vivo vessels (Hutchins et al., 1973b) of the spontaneously hypertensive rate (SHR) and the normal control rat (NCR). Although the cardiac beta effects have been investigated by chronic pharmacological blockade in the SHR (Vavra et al., 1973; Frohlich, 1971; Folkow et al., 1972) the peripheral beta receptors have been little studied. Spector et al. (1969) have shown that aortic strips from SHR exhibit greater relaxation to isoproterenol. The possibility exists that, by virtue of the prolonged elevation in sympathetic activity and a concomitant increase in adrenal medullary hormones, the peripheral beta receptors have been altered in reactivity. Indeed this could be one of the many factors acting in concert to produce the rise in total peripheral resistance seen late in the hypertensive process. This investigation was undertaken to determine the reactivity of intact microvessels to isoproterenol in the skeletal muscle bed of spontaneously hypertensive and normal control rats. Copyright 8 1975by Academic Press,Inc. All rights of reproduction in any form reserved. Printed in Great Britain
101
102
HUT(‘HINS,GREENE AND RAINS
METHODS The experimental model used for this investigation was the cremaster muscle of the 100 g rat (5-6 wk). Two groups of rats were used: the spontaneously hypertensive rat (Okamoto and Aoki, 1963) and normal Wistar rats (CFN-Carworth Farms). The Kyoto control Wistar strain was not available at the time of this study. A total of 38 animals were used in this study, 21-NCR and 17-SHR. They were anesthetized with a warm solution of 10% urethane and 2% chloralose (6 ml/kg, ip). The animals were maintained between 36 and 38°C by thermostatically controlled heating mats. The trachea was cannulated to provide an adequate airway. The cremaster in a 100 gratis approximately 250 pm thick with two layers of skeletal muscle surrounding the blood vessels. The surgical approach is through a longitudinal incision in the ventral surface of the scrotal skin. Using a heat cautery and blunt dissection the cremaster and enclosed testis are freed of the scrotal skin and connective tissue. In a 100 g rat there is very little connective tissue, a finding not seen in larger animals. A ligature is placed at the caudal tip of the cremaster and an incision is made by cautery in the ventral surface of the cremaster. The testis is then freed from the cremaster and extirpated. Additional ligatures are placed around the cremaster and it is stretched over an electrically heated pedestal, 1.8 cm in diameter and 1.5 cm in height. The cremaster is irrigated with a pH-adjusted, balanced salt solution and covered with a 1.8 cm, round, no. 1 thickness, microscope slide cover slip The covered nature of the preparation prevents equilibration of the cremaster with atmospheric gases and eliminates the need for irrigation solutions The cremaster temperature is maintained at approximately 34.5”, the normal in situ temperature of the cremaster. The ipsilateral femoral artery is cannulated in a retrograde manner with a silastic and polyethylene microinjection chamber. Injections of isoproterenol were made through this chamber into the femoral, the inferior epigastric and finally, the cremasteric artery. One carotide artery was cannulated for the recording of systemic blood pressure and heart rate on a Brush recorder. The microvessels were observed through Zeiss optics at magnifications of 80-l 60x. The microscope image was projected onto a Sony AVC-3210 video camera which was connected to a Conrac monitor and Sony AV3600 video recorder which allowed subsequent analysis of vessel diameter. A 3 ft optical fiber and Quartz-Iodide light source provided illumination through a long working distance condenser. Heating effects were minimized by the use of fiber optics to remove infrared and near-infrared wavelengths. The injections were via the femoral artery microinjection chamber connected to a constant rate infusion pump. The time duration for the injection was 1 sec. At a pump rate of 2.93 ml/min, the total volume for one injection was approximately 50 ~1. The vessels under observation were video-recorded for a control period of 1 min prior to the injection and an experimental period of 1 min following the injection. The diameter for each period was integrated and an average diameter was obtained for each of the pre- and postinjection periods. The response to isoproterenol was determined as the ratio of the average postinjection diameter (experimental) to the
BETA
RECEPTORS
IN HYPERTENSIOY
103
average preinjection diameter (control). In this way the artifacts of vasomotion and spurious vessel diameter changes are averaged out. Vessel categorization was according to their order of branching in the vascular tree (Hutchins et al., 1973a). The cremasteric artery was designated as a first-order arteriole (Al). Branches off this first-order arteriole were denoted as second-order arterioles (A2). Branches off A2s were A3s and off A3s were A&. RESULTS A total of 888 observations were made in 17 SHRs and 21 NCRs. The dose-response curves for the two groups of animals and the four vessel categories are shown in Fig. 1. The control injection was the injection of the vehicle (Normosol-R, Abbott Laboratories) and is labeled by “Norm.” The abscissa corresponds to the concentration of active base in grams per milliliter and is plotted logarithmically. The ordinate presents a normalized averaged diameter. A value of 1.0 indicates that the diameter remained unchanged after the isoproterenol injection. A value of 1.6 indicates that the vessel dilated to 160% of its control diameter (e.g., from a control diameter of 10 pm to an average postinjection diameter of 16 pm). The values represent the mean of all observations in this category + SEM. The average control diameter for Al vessels was 106 pm; A2, 59 pm; A3,25 pm; and A4, 12 pm. The p values were computed from Student’s two-tailed t test. The total observations are shown in parentheses above the probability values. It is seen from Fig. 1 that there are statistically significant differences at the higher concentrations of isoproterenol in the Al and A2 vessel categories. In these vessels there seemed to be a slight reduction in vessel size with the higher concentrations of isoproterenol. The third- and fourth-order arterioles of the SHR exhibited a lower threshold than did similar vessels in the NCR. However, at the highest concentrations of isoproterenol the NCR fourth-order arterioles demonstrated a greater dilatation than did A4s from SHR. Since these doses gave a maximum response, it may be concluded that the maximum dilatation of the SHR was less than that of the NCR in the smallest arterioles. The mean blood pressure for the SHR was 122.0 -t 2.3 mmHg; while NCR mean blood pressure averaged 107.9 & 2.6 mmHg. DISCUSSION It appears from Fig. 1 that there are two apparent differences between the arteriolar microvasculature of the SHR and that of the NCR. These are (1) a lower threshold for the SHR, and (2) a greater dilation by the NCR to isoproterenol. The lowered threshold for the SHR appears at the 10m8(A3) and 10e7 (A3 and A4) concentrations of isoproterenol. The threshold for the NCR was observed to be at 10e6 g/ml. Assuming that p agonists work through c-AMP, this reduced threshold for the SHR is in agreement with the data of Triner et al. (1972) who showed that in lo-12 wk SHR the basal adenylate cyclase activity was elevated. In addition, they observed an increase in c-AMP in the SHR. However, this finding was not confirmed by Amer (1973), who found significantly lower levels of c-AMP in SHR and stress
104
HUTCHINS.GREENEANDRAINS 130)
08
PC 6
--
IO
--3-
08 1 0 dF,RST
-c .t0s-Lz 'F --I 'c‘ .-I
(30)
':o' _ I -_
ORDER ARTERldLES
(Al)
ORDER ARTERIOLES
(A31
‘THIRD
(35) ( 01
1
I .6.
‘FOURTH I
Norm
ORDER ARTERIOLES I
I
10m8
10m7
(A41 1
1
I
low6
lO-5
lO-4
ISOPROTERENOL FIG. 1. Response of the four arteriolar categories to different concentrations of isoproterenol base. Values represent the arithmetic mean k 1 SEM. The numbers just above the standard error bars represent p values computed from a two-tailed t test. The numbers in parentheses just above the p values are the number of observations. The dashed lines represent the SHR and the solid lines, NCR.
hypertensive rats. He also noted that adenylate cyclase activity was normal but less responsive to stimulation in the hypertensive group. Spector et al. (1969) also report that aortic strips from SHR demonstrate a greater relaxation to IO-’ g/ml of isoproterenol, a finding which was confirmed in the smaller arterioles of this study. The smaller dilatory capacity in response to isoproterenol may be due to the SHR arterioles being partially dilated. We have previously reported (Hutchins and Darnell, 1974) that the small arterioles of the SHR have larger diameters than their NCR counterparts but are fewer in number. The effect of a prior existing, partial dilation
BETA RECEPTORSIN HYPERTENSION
IO5
would be that the maximum dilation would be reduced when computed on a percentof-control basis. Evidence for a functionally reduced maximum dilation was reported by Triner et ul. (1972) who found that the concentration of isoproterenol needed to increase the c-AMP level a set amount was five to ten times higher in the SHR than the NCR. As stated above, Amer (I 973) also found that adenylate cyclase activity was less responsive to stimulation. The differences between the SHR and NCR Al and A2 vessels at 10m6(Al), lo-’ (Al, and 10e8 (A2) g/ml of isoproterenol is most probably related to the observed change in threshold in the A3 and A4 arterioles. However, if one assumes a longitudinal pressure drop similar to that reported by Fronek and Zweifach (1973) these changes may be somewhat inconsequential in terms of the vascular bed as a whole. In addition to the documented alterations in beta activity of the heart, these results suggest that there is a modification of the beta receptor in the skeletal muscle vascular bed of the spontaneously hypertensive rat. ACKNOWLEDGMENTS This study was supported in part by Public Service Grant HL 13936 and a Grant from the North Carolina Heart Association. The assistance of Mrs. Lib Stewart in the preparation of this manuscript is gratefully acknowledged. REFERENCES AMER, M. (1973). Cyclic adenosine monophosphate and hypertension in rats. Science 179, 807-809. ARMSTRONG,J. (1972). Vascular reactivity to noradrenaline of 5-hydroxytryptamine in hypertensive rats. Brit. J. Pharmol. 45, 183P-184P. BOHR,D., SITRIN, M., AND HANSEN,T. (1971). Vascular smooth muscle in experimental hypertension. Clin. Sci. 40, 1P. CLARK, D. (1971). Effects of immunosympathectomy on development of high blood pressure in genetically hypertensive rats. Circ. Res. 28, 330-336. CONWAY,J. (1970). Hemodynamics aspects of hypertension. Circ. Res. Suppl. 126,27,143-148. ENGLEMAN,K., PORTNOY,C., AND SJOERDSMA, A. (1970). Plasma catecholamine concentrations in patients with hypertension. Circ. Res. Suppl. I. 26,27,1141-1146. FOLKOW,B., HALLBACK, M., LUNDGREN,Y., AND WEISS,L. (1971). The effects of “Immunosympathectomy” on blood pressure and vascular “reactivity” in normal and spontaneously hypertensive rats. Acta Physiol. Stand. 82, 27A. FOLKOW, B., HALLBACK, M., LUNDGREN,Y., AND WEISS,L. (1971). The effect of intense treatment with hypotensive drugs on structural design of the resistance vessels in spontaneously hypertensive rats. Acta Physiol. Stand. 83,280-282. FOLKOW, B., LUNDGREN,Y., AND WEISS,L. (1972). Effect of prolonged propranolol treatment on blood pressure and structural design of the resistance vesselsin young spontaneously hypertensive rats (SH R). Acta Physiol. Stand. 84,8A-9A. FROHLICH,E. (1971). Beta adrenergic blockage in the circulatory regulation of hyperkinetic states. Amer. J. Cardiol. 27, 195-199. FRONEK,K., ANDZWEIFACH,B. (1973). Comparison of the effect of papaverine on terminal vasculature of the cat mesentery and skeletal muscle. Microvas. Res. 6, 121-122. FUJIWARA,M., KUCHII, M., AND SHIBATA,S. (1972). Differences of cardiac reactivity between spontaneously and normotensive rats. Eur. J. Pharmol. 19, l-l 1. HAEULSER,G., AND HAEFELY,W. (1970). Pre- and postjunctional supersensitivity of the mesenteric artery preparation from normotensive and hypertensive rats. Naunyn Schmied. Arch. Pharmak. 266, 18-23.
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