Clinical Science (1979) 57,15-29
Neurogenic activity-angiotensin I1 interaction during the development and maintenance of renal hypertension in the rat G . B E L L I N I * , R. F l O R E N T I N l t , M. F E R N A N D E S , G. O N E S T I , H . H E S S A N , A. B. G O U L D , M . B I A N C H I , K . E. K I M A N D C. S W A R T Z Division of NephrologV and Hypertension, Hahnemann Medical College and Hospital, Philadelphia, Pennsylvania, U S A .
(Received 3 August 1978; accepted 9 March 1979) resulted in a greater decrease in blood pressure. This suggests the presence of a renal factor other than renin in the chronic phase of this hypertension.
Summary
1. Pentolinium tartrate (a ganglionic blocker) was injected in conscious rats during the early and late phases of two-kidney renal hypertension produced by aortic ligation. 2. In the early phase (5 days after aortic ligation), ganglionic blockade resulted in a decrease in blood pressure equal to that obtained in normotensive rats. Later, at days 12 and 40, for equally severe hypertension, ganglion blockade resulted in a greater decrease in blood pressure. 3. A 30 min infusion of [Sar', Ala8]angiotensin I1 during the pentolinium-induced nadir in blood pressure resulted in a further decrease in blood pressure at day 5 . Later, at days 12 and 40, this effect was smaller. 4. A 300 min infusion of [Sar', Ala*]angiotensin I1 normalized the blood pressure in hypertensive rats at day 40. This delayed response may be secondary to a central effect of the antagonist, reducing neurogenic tone or peripheral antagonism of locally generated angiotensin I1 in the blood vessel walls. 5. At day 40, removal of the small left kidney
Key words: angiotensin I1 antagonist, hypertension, neurogenic activity, nephrectomy, renin. Introduction
The participation of neurogenic mechanisms in hypertension has been demonstrated in the experimental animal (Dargie, Franklin .& Reid, 1977) and in man (Louis, Doyle & Anavekar, 1973). In chemically sympathectomized rats, adrenergic neural activity is not essential for the development of renal hypertension; the maintenance of the chronic state, however, depends on adrenergic innervation (Fernandes, Onesti, Fiorentini, Bellini, Gould, Kim & Swartz, 1977). The participation of the renin-angiotensin system in experimental renal hypertension is also well accepted (Thurston & Swales, 1974; MacDonald, Boyd & Peart, 1975). In a study utilizing the angiotensin I1 antagonist [Sar', Ala81angiotensin I1 we demonstrated that the renin-angiotensin system is activated during the early phase of twokidney renal hypertension; later, another mechanism of renal origin plays a pressor role (Fernandes, Fiorentini, Onesti, Bellini, Gould, Hessan, Kim & Swartz, 1978). It is the purpose of this study to evaluate the
* Present address: Istituto di Patologia Medica, Universita di Trieste, Trieste, Italy. f Present address: Cleveland Clinic Research Foundation, Cleveland, U S A . Correspondence: Dr Michael Fernandes, Division of Nephrology and Hypertension, Hahnemann Medical College and Hospital, 230 North Broad Street, Philadelphia, Pennsylvania 19102, U.S.A. 25
26
G . Bellini et al.
relative contribution of neurogenic and angiotensin I1 mechanisms during the development and maintenance of two-kidney renal hypertension in the rat. Methods
Male Sprague-Dawley rats weighing 3 10-375 g were maintained on standard Purina chow and tap water. Hypertension was induced by ligation of the aorta between renal arteries just below the origin of the superior mesenteric artery (Fernandes, Onesti, Weder, Dykyj, Gould, Kim & Swartz, 1976). Arterial pressure was measured in conscious rats through an indwelling Teflon/Tygon catheter in the left common carotid artery. Drugs were administered through a jugular catheter. Experiment 1 Groups of rats were studied 5, 12 and 40 days after aortic ligation. After a stable blood pressure recording was obtained, a maximal dose of pentolinium (20 mg/kg; 37.1 prnol/kg: Ansolysen, Wyeth) was injected through a jugular catheter. The decrease in blood pressure was prompt and lasted about 90 min. During the pentoliniuminduced nadir in blood pressure, [Sari, Alaslangiotensin I1 was infused through the jugular catheter at a rate of 5.54 nmol/min. After 30 min the infusion was stopped and a bolus of angiotensin I1 (48 pmol; Hypertensin, Ciba) was injected in order to confirm the completeness of angiotensin I1 blockade. This dose of angiotensin resulted in at least a 30 mmHg rise in blood pressure in control rats.
tion was measured by a modification of the bioassay procedure (Gould & Goodman, 1970) so that angiotensin could be determined by means of radioimmunoassay (Fernandes et al., 1978). Another group of hypertensive rats (40 days after aortic ligation) were subjected to extraperitoneal removal of the small left kidney through a flank incision under ether anaesthesia. Blood pressure and re$n were determined 24 h later. Statistical analysis was performed by using the t-test. Results
Experiment 1 (Table 1, Fig. 1) The decrease in blood pressure after injection of pentolinium. in hypertensive rats at day 5 was similar to that obtained in normotensive controls (49 +_ 10 vs 40 f 16 mmHg). At days 12 and 40, the decrease in blood pressure was greater than in normotensive rats (75 f 16 and 80 7 1 mmHg; P < 0.005). Only at day 5 did the infusion of [Sari, Alanlangiotensin I1 result in a greater decrease in blood pressure than that obtained in normotensive rats (33 f 1 1 vs 12 13 mmHg; P < 0.05). Experiment 2 (Table 2) At day 40, a 300 rnin infusion of [Sari, Alas]angiotensin I1 lowered the blood pressure more than that after 30 rnin (P < 0.001). At this time, a 30 min infusion stimulated plasma renin concentration more than that after 300 min (P < 0-025). At day 40, the decrease in blood pressure 24 h after removal of the small kidney was more than that after 30 or 300 rnin infusion of the antagonist.
Experiment 2 Groups of hypertensive rats (40 days after aortic ligation) were infused with [Sar', Ala81angiotensin I1 at a rate of 5.54 nmol/min for 30 rnin and 300 min. The blood pressure began to decrease at 30 rnin and remained stable. Later, at 45 rnin, a further gradual decline occurred, reaching a nadir at 270300 min. Extending the duration of infusion to 660 rnin (Riegger, Lever, Millar, Morton & Slack, 1977) did not result in a further decrease in blood pressure. A sample (0.7 ml) of blood was drawn from the carotid artery before and after the infusion for the determination of plasma renin concentration. After withdrawal of the second sample of blood, the completeness of angiotensin I1 blockade was assessed. Plasma renin concentra-
Discussion
In hypertensive rats at day 5 , ganglionic blockade resulted in a fall in blood pressure equal to that obtained in normotensive controls. If the decrease in blood pressure in response to ganglionic blockade is considered an index of a neural component in hypertension (Tarazi & Dustan, 1973), a neurogenic mechanism is not responsible for the early rise in blood pressure. Nakamura & Nakamura (1977) have demonstrated that an enhanced sympathetic activity in young spontaneously hypertensive rats is not the trigger mechanism for genetic hypertension. In this study, 12 and 40 days after the production of hypertension, for equivalent blood pressures ganglionic blockade induced a fall in
27
Pressor interactions in hypertension
TABLE1. Effect of infiusion of pentolinium on the mean arterial pressure in normotensive controls and hypertensive rats at 5 , 12 and 40 days and effect of subsequent iqfusion of [Sar', Ala8jangiotensinII (P113)during thepentoliniuminduced nadir in blood pressure Mean values f SDare shown. N.S., not significant. ~~
~~
Mean arterial pressure (mmHg)
Control
Baseline
After pentolinium
Difference
After P113
Difference
130 k 9
9 0 + 12
40rt 16
78 f 8
12 k 13
175 f 7
126 f 12
49 f 10
93
+7
33 rt 11
(n = 7) Day 5 (n = 6)
Difference from response in controls Day 12 (n = 6)
174 f 10
9 9 + 11
75 & 16
75 f 12
24 f 7
90 f 10
3f5
P < 0.005
Difference from response in controls Day 40 (n = 6)
P < 0.05
N.S.
175 f 12
93 f 13
N.S.
81f7
P < 0.005
Difference from response in controls
N.S.
TABLE2. Mean arterial pressure and plasma renin before and qfter 30 and 300 rnin illfusion of [Sar', Alas]angiotensin I1 (PI 13) and 24 h qfler removal of the small left kidney in hypertensive rats 40 days qfter aortic ligation G.U., Goldblatt units. 30 min (n = 8 ) Before P113
Mean arterial pressure (mmHg) lo-' x Plasma renin (G.U.)
169 k 17 1.75 f0.78
300 min ( n = 8)
After P113
Before P113
After P113
176 f 14
126 f 15
169 f 22
98 f 18
1.73 f 1.32
5.57 k4.18
1.76 f 1.02
1.47 f 1.62
152 f 21 14.1 f 11.21
Nephrectomy (n = 8)
blood pressure greater than at day 5 . This sensitivity to pentolinium suggests a progressive neurogenic influence on the blood pressure in chronic two-kidney renal hypertension. This finding is in keeping with the lower blood pressures in the chronic phase of this model when adrenergic innervation was destroyed by chemical sympathectomy (Fernandes et al., 1977). A 30 min infusion of [Sar', Ala*]angiotensin during the pentolinium-induced nadir in blood pressure resulted in a greater decrease in blood pressure at days 5 and 12 compared with that in normotensive controls. At day 40, the decrease in blood pressure was minimal. If the decrease in blood pressure after a 30 min infusion of the antagonist is considered an index of the direct
Before
After
vasoconstrictor effect of angiotensin 11, this mechanism is activated during the blood pressure elevation at days 5 and 12 (Fernandes et al., 1978). Later, at day 40, angiotensin does not appear to exert a direct pressor effect. A 300 min infusion of [Sar', Ala8]angiotensin I1 in hypertensive rats 40 days after aortic ligation resulted in reduction of the blood pressure. Riegger et al. (1977) previously reported correction of renal hypertension in the rat by prolonged infusion of angiotensin inhibitors. In their study, infusion of Bar', Ala*langiotensin I1 at a rate of 11 pmol min-I kg-I for 660 min resulted in normalization of two-kidney renal hypertension in the rat (28-60 days after clipping; mean 42 days). This slower blood pressure-lowering mechanism may be due to
28
G. Bellini et al.
180
160
-
h
p 140E E
-
v
e 2 en
-.m
120-
: f L
-
100-
5
80 60 L
1
I
I
I
0
5
12
40
Duration (days)
FIG. 1. 0 , Mean arterial pressure before and after aortic ligation. 0 , Effect of infusion of pentolmium on mean arterial pressure. 4 Effect of subsequent infusion of [Sar', Ala'langiotensin I1 on mean
arterial pressure. Vertical bars indicate +SD.
antagonism of angiotensin within the central nervous system (Sweet, Columbo & Gaul, 1976), resulting in a decrease in neurogenic tone. Stimulation of renin after angiotensin I1 blockade has been attributed to interruption of a local negative feedback mechanism regulating renin release (Shade, Davis, Johnson, Gotshall & Spielman, 1973) and to a decrease in arterial blood pressure (Vander, 1967). The lesser stimulation of renin at 300 rnin than after 30 min, despite a greater fall in blood pressure, is consistent with a decrease in neurogenic tone to the kidney (Davis, 1971). Alternatively, this slower mechanism may result from antagonism of locally generated angiotensin I1 within the walls of blood vessels (Gould, Skeggs & Kahn, 1964). In a study on the blood pressure response of nephrectomized renal hypertensive rats to [Sar', Ala81angiotensin 11, Thurston & Swales (1977) have provided evidence for the presence and participation of vascular renin activity in the maintenance of two-kidney renal hypertension in the rat.
Although ganglionic blockade or a 300 rnin infusion of an angiotensin inhibitor does result in a decrease in blood pressure in two-kidney renal hypertension at day 40, the lower blood pressure recorded 24 h after removal of the small left kidney suggests a renal influence other than renin is present at this phase (MacDonald et al., 1975; Fernandes et al., 1978). A comparison between a 300 min and 24 h response does represent a valid objection. It has to be considered, however, that the reduction in blood pressure after infusion of the antagonist does reach a plateau at 300 min in this model at day 4 0 and that persistent vascular renin activity does contribute to blood pressure elevation in recently nephrectomized renal hypertensive rats (Thurston & Swales, 1977). It is not clear how neurogenic mechanisms maintain the blood pressure in hypertension. It is also not clear whether studies employing sympathetic denervation can be used to exclude the participation of the nervous system in the intact animal.
Pressor interactions in hypertension In hypertensive rabbits, Conway (1955) noted a relationship between increased sensitivity to noradrenaline and increased response to ganglionic blockade. The progressive neurogenic influence on the blood pressure was attributed to increased vascular sensitivity to a normal sympathetic discharge. In the spontaneously hypertensive rat, Iriuchijima (1973) observed an increased sympathetic discharge rate. After right or bilateral splanchnicectomy, the difference in blood pressure between the hypertensive and normotensive strains was not significant. Hypertension, a normal renin and normalization of the blood pressure after left nephrectomy are the criteria for phase I1 renal hypertension (Brown, Cuesta, Davies, Lever, Morton, Padfield, Robertson, Trust, Bianchi & Schalekamp, 1976). The differential response of the blood pressure to brief versus prolonged infusion of the angiotensin antagonist and increasing sensitivity to pentolinium are additional features. At day 40, hypertension is maintained by the action of angiotensin on relatively inaccessible receptors or is the result of a hormonal influence (other than renin) originating from the left kidney, altering either neural discharge or vascular sensitivity. Acknowledgments Bar', Ala81angiotensin I1 was supplied by Dr Alan Castellion of the Norwich Pharmacal Company. The authors appreciate the assistance of Mr Luis Maldonado and Mr Jose Maldonado. Ms Franny Dienes prepared this manuscript. This study was supported by the March of Dimes and the National Science Foundation Grant: PCM 75-21933.
References BROWN, JJ., CUESTA,V., DAVES,D.L., LEVER, A.F., MORTON, J.J., PADPIELD, P.L., ROBERTSON, J.I.S., TRUST,R., BIANCHI, G. & SCHALEKAMP, M.A.D. (1976) Mechanism of renal hypertension. Lancet, i, 1219-1221. CONWAY,J. (1955) The bchaviour of the blood pressure in normal and hypertensive rabbits in response to L-noradrenaline and to ganglion block by hexa- or pentamethonium. Journal of Physiology (London), 127.69-80. DARGIE,HJ., FRANKLIN, S.S. & REID, J.L. (1977) Plasma noradrenaline concentrations in experimental renovascular
29
hypertension in the rat. Clinical Science and Molecular Medicine, 52,477-483. DAVIS,J.O. (1971)What signals the kidney to release renin? Circulation Research, 27,301-306. FERNANDES, M., FIORENTINI,R., ONESTI,G., BELLINI, G., C. (1978) GOULD,A.B., HESSAN,H., KIM,K.E. & SWARTZ, Effect of administration of Sar-Alan-angiotensin I1 during the development and maintenance of renal hypertension in the rat. Clinical Science and Molecular Medicine, 54, 633637. FERNANDES, M., ONESTI,G., FIORENTINI, R., BELLMI,G., GOULD,A.B., KIM, K.E. & SWARIZ,C. (1977) Role of adrenergic innervations in experimental renal hypertension. LLfe Sciences, 20,623-626. FERNANDES, M., ONESTI,G., WEDER,A., DYKYJ, R., GOULD, A.B., KIM, K.E. & SWARIZ,C. (1976)Experimental model of severe renal hypertension. Journal of Laboratory and Clinical Medicine, 87,561-567. GOULD,A.B. & GOODMAN, S.A. (1970)The effect of hypoxia on the renin-angiotensin system. Laboratory Investigation, 22,443-447. GOULD, A.B., SKEGGS,L.T. & KAHN, J.R. (1964) The presence of renin activity in blood vessel walls. Journal oJExperimenfal Medicine, 119,348-399. IRIUCHUIMA, J. (1973) Sympathetic discharge rate in spontaneously hypertensive rats. Japanese Heart Journal, 14, 350-356. LOUIS,W.J., DOYLE,A.E. & ANAVEKAR, S. (1973) Plasma norepinephrine levels in essential hypertension. New England Journal of Medicine, 288,599-601. MACDONALD, G.J., BOYD, G.W. & PEART,W.S. (1975) Effect of the angiotensin blocker I-Sar-8-Ala-angiotensin I1 on renal artery clip hypertension in the rat. Circulation Research, 31,640-646. NAKAMURA, K. & NAKAMURA, K. (1977) Enhanced sympathetic activity in young spontaneously hypertensive rats is not the trigger mechanism for genetic hypertension. Naunyn-Schmieakbergs Archives of Pharmacology, 299, 143-148. RIEGGER, A.J.G., LEVER,A.F., MILLAR,J.A., MORTON, J.J. & SLACK,B. (1977)Correction of renal hypertension in the rat by prolonged infusion of angiotensin inhibitors. Lancet, ii, 1317-1319. SHADE, R.E., DAVIS, J.O., JOHNSON,J.A., GOTSHALL, R.W. & SPIELMAN,W.S. (1973) Mechanism of action of angiotensin I1 and antidiuretic hormone on renin secretion. American Journal of Physiology, 224,926-929. SWEET,C.S., COLUMBO, J.M. & GAUL,S.L. (1976) Central antihypertensive effects of inhibitors of the renin-angiotensin Physiology, 231, systems in rats. American Journal 17961799. TARAZI, RC.& DUNSTAN, H.P. (1973)Newogenic participation in essential and renovascular hypertension assessed by acute ganglionic blockade: correlation with haemodynamic indices and intravascular volume. Clinical Science, 44, 197-2 12. THURSTON, H. & S w h e s , J.D. (1974)Comparison of andotensin I1 antagonist and antiserum infusion with nephrectomy in the rat with two-kidney Goldblatt hypertension. Circulation Research, 35,325-329. . THURSTON, H.& SWALES, J.D. (1977)Blood pressure response of nephrectomized hypertensive rats to converting enzyme inhibition: evidence for persistent vascular re& activity. Clinical Science and Molecular Medicine, 52,299-304. VANoeK AJ. (1967) Control of renin release. Physiological Reviews, 47,359-382.
Neurogenic activity-angiotensin I1 interaction during the development and maintenance of renal hypertension in the rat G . B E L L I N I * , R. F l O R E N T I N l t , M. F E R N A N D E S , G. O N E S T I , H . H E S S A N , A. B. G O U L D , M . B I A N C H I , K . E. K I M A N D C. S W A R T Z Division of NephrologV and Hypertension, Hahnemann Medical College and Hospital, Philadelphia, Pennsylvania, U S A .
(Received 3 August 1978; accepted 9 March 1979) resulted in a greater decrease in blood pressure. This suggests the presence of a renal factor other than renin in the chronic phase of this hypertension.
Summary
1. Pentolinium tartrate (a ganglionic blocker) was injected in conscious rats during the early and late phases of two-kidney renal hypertension produced by aortic ligation. 2. In the early phase (5 days after aortic ligation), ganglionic blockade resulted in a decrease in blood pressure equal to that obtained in normotensive rats. Later, at days 12 and 40, for equally severe hypertension, ganglion blockade resulted in a greater decrease in blood pressure. 3. A 30 min infusion of [Sar', Ala8]angiotensin I1 during the pentolinium-induced nadir in blood pressure resulted in a further decrease in blood pressure at day 5 . Later, at days 12 and 40, this effect was smaller. 4. A 300 min infusion of [Sar', Ala*]angiotensin I1 normalized the blood pressure in hypertensive rats at day 40. This delayed response may be secondary to a central effect of the antagonist, reducing neurogenic tone or peripheral antagonism of locally generated angiotensin I1 in the blood vessel walls. 5. At day 40, removal of the small left kidney
Key words: angiotensin I1 antagonist, hypertension, neurogenic activity, nephrectomy, renin. Introduction
The participation of neurogenic mechanisms in hypertension has been demonstrated in the experimental animal (Dargie, Franklin .& Reid, 1977) and in man (Louis, Doyle & Anavekar, 1973). In chemically sympathectomized rats, adrenergic neural activity is not essential for the development of renal hypertension; the maintenance of the chronic state, however, depends on adrenergic innervation (Fernandes, Onesti, Fiorentini, Bellini, Gould, Kim & Swartz, 1977). The participation of the renin-angiotensin system in experimental renal hypertension is also well accepted (Thurston & Swales, 1974; MacDonald, Boyd & Peart, 1975). In a study utilizing the angiotensin I1 antagonist [Sar', Ala81angiotensin I1 we demonstrated that the renin-angiotensin system is activated during the early phase of twokidney renal hypertension; later, another mechanism of renal origin plays a pressor role (Fernandes, Fiorentini, Onesti, Bellini, Gould, Hessan, Kim & Swartz, 1978). It is the purpose of this study to evaluate the
* Present address: Istituto di Patologia Medica, Universita di Trieste, Trieste, Italy. f Present address: Cleveland Clinic Research Foundation, Cleveland, U S A . Correspondence: Dr Michael Fernandes, Division of Nephrology and Hypertension, Hahnemann Medical College and Hospital, 230 North Broad Street, Philadelphia, Pennsylvania 19102, U.S.A. 25
26
G . Bellini et al.
relative contribution of neurogenic and angiotensin I1 mechanisms during the development and maintenance of two-kidney renal hypertension in the rat. Methods
Male Sprague-Dawley rats weighing 3 10-375 g were maintained on standard Purina chow and tap water. Hypertension was induced by ligation of the aorta between renal arteries just below the origin of the superior mesenteric artery (Fernandes, Onesti, Weder, Dykyj, Gould, Kim & Swartz, 1976). Arterial pressure was measured in conscious rats through an indwelling Teflon/Tygon catheter in the left common carotid artery. Drugs were administered through a jugular catheter. Experiment 1 Groups of rats were studied 5, 12 and 40 days after aortic ligation. After a stable blood pressure recording was obtained, a maximal dose of pentolinium (20 mg/kg; 37.1 prnol/kg: Ansolysen, Wyeth) was injected through a jugular catheter. The decrease in blood pressure was prompt and lasted about 90 min. During the pentoliniuminduced nadir in blood pressure, [Sari, Alaslangiotensin I1 was infused through the jugular catheter at a rate of 5.54 nmol/min. After 30 min the infusion was stopped and a bolus of angiotensin I1 (48 pmol; Hypertensin, Ciba) was injected in order to confirm the completeness of angiotensin I1 blockade. This dose of angiotensin resulted in at least a 30 mmHg rise in blood pressure in control rats.
tion was measured by a modification of the bioassay procedure (Gould & Goodman, 1970) so that angiotensin could be determined by means of radioimmunoassay (Fernandes et al., 1978). Another group of hypertensive rats (40 days after aortic ligation) were subjected to extraperitoneal removal of the small left kidney through a flank incision under ether anaesthesia. Blood pressure and re$n were determined 24 h later. Statistical analysis was performed by using the t-test. Results
Experiment 1 (Table 1, Fig. 1) The decrease in blood pressure after injection of pentolinium. in hypertensive rats at day 5 was similar to that obtained in normotensive controls (49 +_ 10 vs 40 f 16 mmHg). At days 12 and 40, the decrease in blood pressure was greater than in normotensive rats (75 f 16 and 80 7 1 mmHg; P < 0.005). Only at day 5 did the infusion of [Sari, Alanlangiotensin I1 result in a greater decrease in blood pressure than that obtained in normotensive rats (33 f 1 1 vs 12 13 mmHg; P < 0.05). Experiment 2 (Table 2) At day 40, a 300 rnin infusion of [Sari, Alas]angiotensin I1 lowered the blood pressure more than that after 30 rnin (P < 0.001). At this time, a 30 min infusion stimulated plasma renin concentration more than that after 300 min (P < 0-025). At day 40, the decrease in blood pressure 24 h after removal of the small kidney was more than that after 30 or 300 rnin infusion of the antagonist.
Experiment 2 Groups of hypertensive rats (40 days after aortic ligation) were infused with [Sar', Ala81angiotensin I1 at a rate of 5.54 nmol/min for 30 rnin and 300 min. The blood pressure began to decrease at 30 rnin and remained stable. Later, at 45 rnin, a further gradual decline occurred, reaching a nadir at 270300 min. Extending the duration of infusion to 660 rnin (Riegger, Lever, Millar, Morton & Slack, 1977) did not result in a further decrease in blood pressure. A sample (0.7 ml) of blood was drawn from the carotid artery before and after the infusion for the determination of plasma renin concentration. After withdrawal of the second sample of blood, the completeness of angiotensin I1 blockade was assessed. Plasma renin concentra-
Discussion
In hypertensive rats at day 5 , ganglionic blockade resulted in a fall in blood pressure equal to that obtained in normotensive controls. If the decrease in blood pressure in response to ganglionic blockade is considered an index of a neural component in hypertension (Tarazi & Dustan, 1973), a neurogenic mechanism is not responsible for the early rise in blood pressure. Nakamura & Nakamura (1977) have demonstrated that an enhanced sympathetic activity in young spontaneously hypertensive rats is not the trigger mechanism for genetic hypertension. In this study, 12 and 40 days after the production of hypertension, for equivalent blood pressures ganglionic blockade induced a fall in
27
Pressor interactions in hypertension
TABLE1. Effect of infiusion of pentolinium on the mean arterial pressure in normotensive controls and hypertensive rats at 5 , 12 and 40 days and effect of subsequent iqfusion of [Sar', Ala8jangiotensinII (P113)during thepentoliniuminduced nadir in blood pressure Mean values f SDare shown. N.S., not significant. ~~
~~
Mean arterial pressure (mmHg)
Control
Baseline
After pentolinium
Difference
After P113
Difference
130 k 9
9 0 + 12
40rt 16
78 f 8
12 k 13
175 f 7
126 f 12
49 f 10
93
+7
33 rt 11
(n = 7) Day 5 (n = 6)
Difference from response in controls Day 12 (n = 6)
174 f 10
9 9 + 11
75 & 16
75 f 12
24 f 7
90 f 10
3f5
P < 0.005
Difference from response in controls Day 40 (n = 6)
P < 0.05
N.S.
175 f 12
93 f 13
N.S.
81f7
P < 0.005
Difference from response in controls
N.S.
TABLE2. Mean arterial pressure and plasma renin before and qfter 30 and 300 rnin illfusion of [Sar', Alas]angiotensin I1 (PI 13) and 24 h qfler removal of the small left kidney in hypertensive rats 40 days qfter aortic ligation G.U., Goldblatt units. 30 min (n = 8 ) Before P113
Mean arterial pressure (mmHg) lo-' x Plasma renin (G.U.)
169 k 17 1.75 f0.78
300 min ( n = 8)
After P113
Before P113
After P113
176 f 14
126 f 15
169 f 22
98 f 18
1.73 f 1.32
5.57 k4.18
1.76 f 1.02
1.47 f 1.62
152 f 21 14.1 f 11.21
Nephrectomy (n = 8)
blood pressure greater than at day 5 . This sensitivity to pentolinium suggests a progressive neurogenic influence on the blood pressure in chronic two-kidney renal hypertension. This finding is in keeping with the lower blood pressures in the chronic phase of this model when adrenergic innervation was destroyed by chemical sympathectomy (Fernandes et al., 1977). A 30 min infusion of [Sar', Ala*]angiotensin during the pentolinium-induced nadir in blood pressure resulted in a greater decrease in blood pressure at days 5 and 12 compared with that in normotensive controls. At day 40, the decrease in blood pressure was minimal. If the decrease in blood pressure after a 30 min infusion of the antagonist is considered an index of the direct
Before
After
vasoconstrictor effect of angiotensin 11, this mechanism is activated during the blood pressure elevation at days 5 and 12 (Fernandes et al., 1978). Later, at day 40, angiotensin does not appear to exert a direct pressor effect. A 300 min infusion of [Sar', Ala8]angiotensin I1 in hypertensive rats 40 days after aortic ligation resulted in reduction of the blood pressure. Riegger et al. (1977) previously reported correction of renal hypertension in the rat by prolonged infusion of angiotensin inhibitors. In their study, infusion of Bar', Ala*langiotensin I1 at a rate of 11 pmol min-I kg-I for 660 min resulted in normalization of two-kidney renal hypertension in the rat (28-60 days after clipping; mean 42 days). This slower blood pressure-lowering mechanism may be due to
28
G. Bellini et al.
180
160
-
h
p 140E E
-
v
e 2 en
-.m
120-
: f L
-
100-
5
80 60 L
1
I
I
I
0
5
12
40
Duration (days)
FIG. 1. 0 , Mean arterial pressure before and after aortic ligation. 0 , Effect of infusion of pentolmium on mean arterial pressure. 4 Effect of subsequent infusion of [Sar', Ala'langiotensin I1 on mean
arterial pressure. Vertical bars indicate +SD.
antagonism of angiotensin within the central nervous system (Sweet, Columbo & Gaul, 1976), resulting in a decrease in neurogenic tone. Stimulation of renin after angiotensin I1 blockade has been attributed to interruption of a local negative feedback mechanism regulating renin release (Shade, Davis, Johnson, Gotshall & Spielman, 1973) and to a decrease in arterial blood pressure (Vander, 1967). The lesser stimulation of renin at 300 rnin than after 30 min, despite a greater fall in blood pressure, is consistent with a decrease in neurogenic tone to the kidney (Davis, 1971). Alternatively, this slower mechanism may result from antagonism of locally generated angiotensin I1 within the walls of blood vessels (Gould, Skeggs & Kahn, 1964). In a study on the blood pressure response of nephrectomized renal hypertensive rats to [Sar', Ala81angiotensin 11, Thurston & Swales (1977) have provided evidence for the presence and participation of vascular renin activity in the maintenance of two-kidney renal hypertension in the rat.
Although ganglionic blockade or a 300 rnin infusion of an angiotensin inhibitor does result in a decrease in blood pressure in two-kidney renal hypertension at day 40, the lower blood pressure recorded 24 h after removal of the small left kidney suggests a renal influence other than renin is present at this phase (MacDonald et al., 1975; Fernandes et al., 1978). A comparison between a 300 min and 24 h response does represent a valid objection. It has to be considered, however, that the reduction in blood pressure after infusion of the antagonist does reach a plateau at 300 min in this model at day 4 0 and that persistent vascular renin activity does contribute to blood pressure elevation in recently nephrectomized renal hypertensive rats (Thurston & Swales, 1977). It is not clear how neurogenic mechanisms maintain the blood pressure in hypertension. It is also not clear whether studies employing sympathetic denervation can be used to exclude the participation of the nervous system in the intact animal.
Pressor interactions in hypertension In hypertensive rabbits, Conway (1955) noted a relationship between increased sensitivity to noradrenaline and increased response to ganglionic blockade. The progressive neurogenic influence on the blood pressure was attributed to increased vascular sensitivity to a normal sympathetic discharge. In the spontaneously hypertensive rat, Iriuchijima (1973) observed an increased sympathetic discharge rate. After right or bilateral splanchnicectomy, the difference in blood pressure between the hypertensive and normotensive strains was not significant. Hypertension, a normal renin and normalization of the blood pressure after left nephrectomy are the criteria for phase I1 renal hypertension (Brown, Cuesta, Davies, Lever, Morton, Padfield, Robertson, Trust, Bianchi & Schalekamp, 1976). The differential response of the blood pressure to brief versus prolonged infusion of the angiotensin antagonist and increasing sensitivity to pentolinium are additional features. At day 40, hypertension is maintained by the action of angiotensin on relatively inaccessible receptors or is the result of a hormonal influence (other than renin) originating from the left kidney, altering either neural discharge or vascular sensitivity. Acknowledgments Bar', Ala81angiotensin I1 was supplied by Dr Alan Castellion of the Norwich Pharmacal Company. The authors appreciate the assistance of Mr Luis Maldonado and Mr Jose Maldonado. Ms Franny Dienes prepared this manuscript. This study was supported by the March of Dimes and the National Science Foundation Grant: PCM 75-21933.
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