Role of the renin-angiotensin system in the control of vasopressin and ACTH secretion during the development of renal hypertension in dogs

Patterns of Renal Function in Hypertension Due to Unilateral Renal Artery Occlusion

We performed renal function studies in dogs with chronic renovascular hypertension produced by complete occlusion of a renal artery. In addition, we evaluated in anesthetized dogs the acute effects of a novel angiotensin converting enzyme inhibitor, CGS 16,617, on renal function and plasma neurohormones (epinephrine, norepinephrine and vasopressin) 4 weeks after initiation of 2 kidney, 1 clip hypertension. CGS 16,617 effectively decreased blood pressure in renal hypertensive animals. This response was associated with suppression of angiotensin II indicating effective converting enzyme inhibition. In the non-clipped kidney, acute administration of CGS 16,617 increased effective renal plasma flow but not glomerular filtration rate and urinary sodium excretion. In the clipped kidney, CGS 16,617 caused no change in any parameter of renal function. Plasma norepinephrine, epinephrine and vasopressin were unaffected by administration of CGS 16,617. These studies showed that chronic occlusion of a renal artery does not result in renal infarction because of a compensatory increase in the amount of blood provided through capsular collateral vessels. The collateral circulation which has developed in the clipped kidney explains the lack of a converting enzyme inhibitor effect.

Norepinephrine kinetics in dogs with experimentally induced renal vascular hypertension

OBJECTIVE

To determine norepinephrine (NE) kinetics in dogs with experimentally induced renal vascular hypertension.

ANIMALS

4 mixed-breed dogs.

PROCEDURE

The study comprised a control and hypertensive period. The hypertensive period followed induction of renal vascular hypertension achieved by surgical placement of clips on both renal arteries to reduce diameter by approximately 80%. Arterial blood pressure, renal clearance, and NE kinetics were measured during each period while dogs were receiving a low-sodium diet. Measurements of NE kinetics and renal clearance during the hypertensive period were made 5 days after induction of hypertension.

RESULTS

Five days after induction of hypertension, arterial blood pressure increased by 15 to 20 mm Hg. Mean (+/- SEM) plasma NE concentration and NE spillover rate increased significantly from 151.5+/-14.1 pg/ml and 8.03+/-0.62 ng/kg/min, respectively, during the control period to 631.4+/-30.5 pg/ml and 54.0+/-5.2 ng/kg/min, respectively, during the hypertensive period. Norepinephrine clearance rate also increased (54.0+/-2.4 vs. 86.0+/-9.3 ml/kg/min). Positive associations between mean arterial pressure (MAP) and NE concentration and spillover rate were detected. However, MAP and NE clearance rate were not associated.

CONCLUSIONS AND CLINICAL RELEVANCE

Increased blood pressure during the hypertensive period was likely attributable to increased NE spillover rate, which resulted in a significant increase in plasma NE concentration. Analysis of these results suggests that central sympathetic outflow was increased and may be responsible for the pathogenesis of high blood pressure during the acute phase of renal vascular hypertension in dogs.

Effects of a single dose and short-term captopril treatment on some pressor and depressor humoral factors in healthy subjects

14 healthy subjects (8 males and 6 females), aged 25-40 years, were studied before and after oral administration of a single dose of 50 mg captopril as well as after 3 d treatment with 100 mg captopril daily per os. We found that in addition to the well-known effects on the renin-angiotensin-aldosterone system, captopril, after 3 d treatment, significantly increases plasma and urinary kallikrein activity, plasma vasopressin and urinary prostaglandin (PG) E2. Atrial natriuretic peptide did not change significantly after either the single dose or the short-term treatment. We conclude that the blood pressure lowering effect of captopril could be mediated by increasing activity of the kallikrein-kinin system and of PGE2 without any participation of atrial natriuretic peptide.

Alterations in sodium metabolism as an etiological model for hypertension

An adequate matching for race, sex, stage of the menstrual cycle, family history of hypertension, and the amount of sodium and other electrolytes in the diet should be a prerequisite for valid conclusions when interpreting the erythrocyte concentration and fluxes of sodium in essential hypertensive patients in comparison with normal subjects. Alterations in intracellular sodium concentration and transmembrane sodium transport systems as causes of essential hypertension are postulated. This review article describes how this abnormal sodium and calcium metabolism translates into increased systemic vascular resistance through altered vasoactive responses and/or vasculature structural changes.

Respiratory control during exercise: hormones, osmolality, strong ions, and PaCO2

For optimal performance of exercising muscle, the charge state of proteins must be maintained; the pH environment of protein histidine imidazole groups must be coordinated with their pK. During exercise, increasing temperature and osmolality as well as changes in strong ions affect the pK of imidazole groups. Production of strong organic anions also decreases the concentration difference between strong cations and anions (strong ion difference, or [SID]), causing a metabolic acidosis in peripheral tissues. Central chemoreceptors regulate PCO2 in relation to the [SID] of brain fluids to maintain a "constant" brain [H+]. In addition, increased osmolality, angiotensin II, and vasopressin during exercise may stimulate circumventricular organs of the brain and interact with chemical control of ventilation. Changes in [SID] of brain fluids during exercise are negligible compared to systemic decreases in [SID]; thus, regulation of PCO2 to maintain brain [H+] homeostasis cannot simultaneously compensate for greater changes in [SID] in peripheral tissues.

Antihypertensive activity of the non-peptide angiotensin II receptor antagonist, SK&F 108566, in rats and dogs

The antihypertensive activity of the nonpeptide angiotensin II receptor antagonist, SK&F 108566 (E)-alpha-[[2-butyl-1-[(4-carboxyphenyl)methyl]-1H-imidazol-5- yl]methylene]-2-thiophene propanoic acid), was examined in rats and dogs. SK&F 108566 produced dose-dependent decreases in blood pressure in renin-dependent hypertensive rats. At 10 mg/kg intraduodenally, mean arterial blood pressure fell from between 150-160 mm Hg to approximately 124 mm Hg. A sustained infusion of SK&F 108566 at 25 micrograms/min intraduodenally normalized blood pressure during 3 days of infusion and for 18 h following cessation of the infusion. Evaluation of the systemic hemodynamic effects of SK&F 108566 in chronically instrumented renin-dependent hypertensive rats demonstrated that the antihypertensive effects of SK&F 108566 were accompanied by a significant increase in cardiac output with little change in stroke volume. In dogs made acutely hypertensive by an intravenous infusion of angiotensin I, SK&F 108566 resulted in dose-dependent decreases in blood pressure. The antihypertensive activity of SK&F 108566 at 10 mg/kg p.o. was maintained for between 13-15 h, a similar duration of action as observed with enalapril (1 mg/kg, p.o.). Administration of DuP 753 (losartan) intravenously caused a small and short-lived fall in blood pressure in the angiotensin I-infused hypertensive dog. However, the active metabolite of losartan, EXP 3174, resulted in a response of longer duration. In dogs made hypertensive by placement of an ameroid constrictor on the left renal artery, SK&F 108566 (10 mg/kg, p.o.) or enalapril (1 mg/kg, p.o.) resulted in antihypertensive responses of at least 12 h duration. The data indicate that SK&F 108566 is a long-acting antihypertensive agent in the rat and dog.

Effect of baroreceptor denervation on stimulation of drinking by angiotensin II in conscious dogs

Angiotensin II causes marked stimulation of drinking when it is injected centrally but is a relatively weak dipsogen when administered intravenously. However, it has been proposed that the dipsogenic action of systemically administered angiotensin II may be counteracted by the pressor action of the peptide. To test this hypothesis, the dipsogenic action of angiotensin II was investigated in dogs, in which low and high baroreceptor influences had been eliminated by denervation of the carotid sinus, aortic arch, and heart. In five sham-operated dogs, infusion of angiotensin II at 10 and 20 ng.kg-1.min-1 increased plasma angiotensin II concentration to 109.2 +/- 6.9 and 219.2 +/- 38.5 pg/ml and mean arterial pressure by 20 and 29 mmHg, respectively, but did not induce drinking. In four baroreceptor-denervated dogs, the angiotensin II infusions produced similar increases in plasma angiotensin II concentration and mean arterial pressure but, in contrast to the results in the sham-operated dogs, produced a dose-related stimulation of drinking. Water intake with the low and high doses of angiotensin II was 111 +/- 44 and 255 +/- 36 ml, respectively. The drinking responses to an increase in plasma osmolality produced by infusion of hypertonic sodium chloride were not different in the sham-operated and baroreceptor-denervated dogs. These results demonstrate that baroreceptor denervation increases the dipsogenic potency of intravenous angiotensin II and provides further support for the hypothesis that the dipsogenic action of intravenous angiotensin II is counteracted by the rise in blood pressure.

Baroreflex control of renal sympathetic nerve activity is potentiated at early phase of two-kidney, one-clip Goldblatt hypertension in conscious rabbits

Conscious normotensive and two-kidney, one-clip Goldblatt hypertensive rabbits were studied to determine the sensitivity of the arterial baroreflex control of renal sympathetic nerve activity (RSNA) and heart rate. The relations of the mean arterial pressure-RSNA and mean arterial pressure-heart rate were examined over a wide range of blood pressures produced by infusions of phenylephrine and nitroglycerin. The maximum slope obtained by logistic function analysis was considered to represent the baroreflex sensitivity. In the early hypertensive group (n = 8; mean arterial pressure +/- SEM, 88 +/- 2 mm Hg) on day 5 after renal clip application, the maximum slope of the mean arterial pressure-RSNA relation was -11.3 +/- 1.2, which was significantly greater than that of the sham normotensive group (-6.9 +/- 0.3, p less than 0.05). The maximum slope (-4.3 +/- 0.2) of the mean arterial pressure-RSNA relation in the late hypertensive group (n = 8; mean arterial pressure, 96 +/- 3 mm Hg) on day 21 after renal clipping was significantly smaller than that of another sham group (-7.2 +/- 0.2, p less than 0.05). In contrast to these changes in the baroreflex control of RSNA, the control of heart rate was attenuated according to the magnitude of mean arterial pressure. To elucidate the mechanisms underlying the potentiated baroreflex, the effects of endogenous neuropeptides were investigated. First, plasma concentrations of angiotensin II and arginine vasopressin that are known to affect the baroreflex were determined. Plasma concentrations of vasopressin (3.1 +/- 0.6 pg/ml) as well as of angiotensin II (34 +/- 7 pg/ml) were increased in the early hypertensive group, and the plasma vasopressin returned to a similar level to the sham group in the late hypertensive group (1.3 +/- 0.4 pg/ml). Second, to study endogenous effects of these neuropeptides on the baroreflex, the maximum slopes of the baroreflex curves during infusions of antagonists for the peptides were determined in the early hypertensive group. The maximum slope of mean arterial pressure-RSNA during intravertebral arterial [Sar1, Ala8]-angiotensin II (-16.4 +/- 1.5) was significantly greater (p less than 0.05), whereas the maximum slope during intravertebral arterial infusion of d(CH2)5Tyr(Me)arginine vasopressin (-4.7 +/- 0.5) was significantly smaller (p less than 0.05) than that during vehicle infusion (-11.3 +/- 1.2).(ABSTRACT TRUNCATED AT 400 WORDS)

Peripheral chemoreceptor control of fetal renin responses to hypoxia and hypercapnia

The renin response to hypoxia in late gestation fetal sheep has been well characterized. However, the renin response to asphyxia--the combination of hypoxia and hypercapnia--has not been extensively studied. The purpose of this study was to determine 1) the interaction of hypoxia and hypercapnia in the control of renin secretion in late gestation fetal sheep and 2) the role of peripheral arterial chemoreceptors therein. Chronically catheterized fetal sheep (intact or sinoaortic denervated) were exposed to hypoxia and/or hypercapnia for 30 minutes. Hypercapnia alone had no effect on plasma renin activity or aldosterone but did result in a significant increase in angiotensin II. Hypercapnia combined with hypoxia resulted in a significant increase in renin activity, angiotensin II, and aldosterone. Sinoaortic denervation attenuated the renin and angiotensin II responses to hypercapnia plus hypoxia. The increase in renin and angiotensin II in response to hypercapnia with or without concomitant hypoxia strongly correlated with the magnitude of the decrease in arterial pH in intact fetuses only. Hypoxia alone and in concert with hypercapnia increased mean arterial pressure and decreased heart rate in intact but not sinoaortic denervated fetuses. We conclude that 1) hypercapnia more potently increases plasma renin activity than does hypoxia in late gestation fetal sheep, 2) arterial pH may be the relevant signal perceived by the peripheral arterial chemoreceptors for the control of the renin-angiotensin system during asphyxia, and 3) the cardiovascular response to hypoxia is mediated, in part, by peripheral arterial chemoreceptors.

Erythrocyte concentrations and transmembrane fluxes of sodium and potassium in essential hypertension: role of intrinsic and environmental factors

The intraerythrocyte sodium concentration is increased in the erythrocytes of Zaïrean Bantu with untreated hypertension, while the red blood cell potassium is not different from that of normotensive subjects. Compared with whites, normotensive healthy blacks have a higher intracellular concentration of sodium due to a depressed activity of the sodium-potassium pump. Normotensive healthy males with a positive familial background of hypertension display higher erythrocyte sodium and lower cotransport activity. None of the two measurements offer a clear-cut genetic marker of essential hypertension. In healthy women, the erythrocyte sodium concentration is lowered during the luteal as compared with the follicular phase of the menstrual cycle. This variability explains the difference observed between men and women. A low-sodium diet stimulates the activity of the sodium-potassium ATPase pump, which leads to a decrease in the erythrocyte sodium concentration. Both alterations reverse only slowly during sodium repletion. It is therefore suggested that an adequate matching for race, sex, stage of the menstrual cycle (in women), family history of hypertension, and the amount of sodium in the diet should be a prerequisite for valid conclusions when interpreting the erythrocyte concentration and fluxes of sodium.

Importance of endogenous angiotensin II in the cardiovascular responses to sympathetic stimulation in conscious rabbits

Pharmacological evidence indicates that angiotensin (Ang II) converting enzyme inhibitors attenuate cardiovascular responses to sympathetic stimulation. To investigate the physiological significance of this attenuation, the pressor and heart rate responses to bilateral carotid occlusion (BCO) were studied before and after administration of captopril and again during Ang II replacement in conscious, aortic nerve-sectioned rabbits with chronically implanted carotid occluders. In the control period, BCO produced increases (p less than 0.05) in mean arterial pressure (MAP) and heart rate (HR) of 37.3 +/- 3.0 mm Hg and 21.7 +/- 5.4 beats/min from baseline values of 79.1 +/- 2.5 mm Hg and 255.4 +/- 16.7 beats/min. Captopril (5 mg/kg i.v.) markedly reduced (p less than 0.05) both the pressor (10.2 +/- 2.6 mm Hg) and HR (5.0 +/- 4.0 beats/min) responses to BCO, in parallel with a decrease in plasma Ang II of 75%. Infusion of a subpressor dose of Ang II (5-25 ng/kg/min i.v.) increased plasma Ang II to precaptopril levels and fully restored (p less than 0.05) the pressor (33.0 +/- 5.7 mm Hg) and HR (19.8 +/- 7.7 beats/min) responses to BCO. In two additional series of experiments, the mechanism of the effects of captopril and Ang II were investigated. In the first series, cardiac baroreflex curves (pulse interval versus MAP) were generated by increasing or decreasing blood pressure with phenylephrine or nitroprusside (5-20 micrograms/kg/min i.v.). The slope of the linear region of the curve (2.9 msec/mm Hg) was not changed significantly by captopril treatment (3.1 msec/mm Hg) or Ang II replacement (3.2 msec/mm Hg), indicating that cardiac baroreflex sensitivity was not altered by blockade of the renin-angiotensin system. In the second series, the effect of captopril on the pressor response to exogenous norepinephrine (0.1-2.5 micrograms/kg/min i.v.) was tested. The response was reduced by less than 40%, indicating only a modest postsynaptic component to the action of captopril. These results provide physiological evidence for an important action of endogenous Ang II in facilitating the cardiovascular responses to sympathetic stimulation in conscious rabbits. This facilitation is not due to an action upon the baroreflex per se but results, at least in part, from a presynaptic action of Ang II.

Angiotensin II-like material extracted from the rat brain is distinct from authentic angiotensin II

Specific radioimmunoassay and radioreceptor assay for angiotensin II (A II) were used for the possible identification of this peptide in the rat brain. An A II-like material (A II-LM) was detected with both assays applied to acidic extracts of various brain structures. The regional distribution of A II-LM was uneven, but absolute levels (in A II equivalents) could not be accurately determined, as they were highly dependent on the assay used. Partial purification of A II-LM by Sep-Pak C 18 chromatography and affinity chromatography using anti-A II antibodies bound to Ultrogel gave a compound coeluting with authentic A II in reverse-phase HPLC. However, gel filtration through Sephadex G-25 and TSK Spherogel 3000 SW as well as anion exchange HPLC demonstrated that A II-LM did not correspond to authentic A II. Partial characterization of A II-LM indicated that this compound was probably a peptide with an apparent molecular weight of 5,000-7,000 (instead of 1,046 for A II) and more polar but less positively charged than A II. Whether A II-LM is, in fact, the endogenous ligand of A II binding sites in brain remains an interesting hypothesis for further investigations.

Effect of intravertebral angiotensin II on cardiac output and its distribution in conscious dogs

Intravertebral infusion of angiotensin II (Ang II) increases mean arterial pressure (MAP), but the contribution of cardiac output (CO) and total peripheral resistance (TPR) to this increase is unclear. In the present study, the effects of Ang II infusion on CO and regional blood flow was determined by the microsphere technique in eight conscious, chronically catheterized dogs. Ang II was infused into both vertebral arteries at 0.33 and 1.0 ng/kg/min, and intravenously at 0.66, 2.0 and 5.0 ng/kg/min. Intravertebral infusion of Ang II at 0.33 ng/kg/min increased MAP by increasing CO without changing TPR or peripheral plasma Ang II concentration. MAP also was increased with intravertebral infusion of Ang II at 1.0 ng/kg/min, but this resulted from small increases in both CO and TPR. In contrast, intravenous infusion of Ang II at 2.0 and 5.0 ng/kg/min increased MAP by increasing TPR in association with a decrease in CO. The increase in CO with intravertebral infusion of Ang II at 0.33 ng/kg/min was distributed primarily to the muscles, kidneys, heart, and brain. Intravenous infusion of Ang II at 5.0 ng/kg/min and, to a lesser extent, 2.0 ng/kg/min decreased blood flow to the skin, splanchnic region, and kidneys. These data indicate that the increase in MAP produced by a low intravertebral dose of Ang II results from an increase in CO, which is distributed primarily to the muscle, kidney, heart, and brain. In contrast, the increase in MAP produced by a higher intravertebral dose of Ang II results from increases in CO and TPR. This latter action is apparently due to a peripheral action of Ang II to increase resistance in the skin, splanchnic, and renal circulations.

Effect of vasopressin blockade on blood pressure during water deprivation in intact and baroreceptor-denervated conscious dogs

Previous studies have provided evidence that vasopressin plays an important role in blood pressure regulation during water deprivation. However, these investigations have been complicated by reflex compensatory increases in cardiac output and renin secretion. The aim of the present study was to investigate the effect of blockade of the vasoconstrictor action of vasopressin in conscious water-deprived dogs in which the low- and/or high-pressure baroreceptors were denervated to minimize reflex responses. Vasopressin blockade in sham-operated dogs (n = 7) did not change arterial pressure. Heart rate rose from 78 +/- 9 to 119 +/- 13 beats/min (P less than 0.01), and plasma renin activity increased from 10.9 +/- 2.1 to 21.6 +/- 4.6 ng.ml-1.3 h-1 (P less than 0.01). In carotid sinus-denervated dogs (n = 6), vasopressin blockade again failed to decrease arterial pressure. Heart rate increased from 105 +/- 10 to 132 +/- 10 beats/min (P less than 0.01), and plasma renin activity rose from 6.8 +/- 1.7 to 15.5 +/- 2.4 ng.ml-1.3 h-1 (P less than 0.01). The antagonist also failed to change blood pressure in cardiac-denervated dogs (n = 5). Heart rate increased from 111 +/- 9 to 119 +/- 1 beats/min (P less than 0.01), but plasma renin activity did not increase significantly. In marked contrast, vasopressin blockade in sinoaortic/cardiac-denervated dogs (n = 7) promptly decreased arterial pressure from 115 +/- 8 to 94 +/- 7 mmHg (P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Role of vasopressin in stimulation of ACTH secretion by angiotensin II in conscious dogs

Three series of experiments were performed in conscious dogs to test the possibility that the stimulation of adrenocorticotropin (ACTH) release by angiotensin II (ANG II) is mediated by arginine vasopressin (AVP). In the first protocol, the effect of ANG II on ACTH release was studied in dogs in which endogenous AVP levels had been increased by water deprivation. Water deprivation for 24 h increased plasma AVP concentration from 3.0 +/- 0.5 to 7.7 +/- 0.5 pg/ml (P less than 0.01) and increased the AVP response to the highest dose of ANG II (20 ng X kg-1 X min-1). Despite these changes, water deprivation failed to increase the ACTH response to ANG II. Next, the contribution of endogenous AVP to the stimulation of ACTH release by ANG II was examined using the V1-receptor antagonist, d(CH2)5Tyr[Met]-AVP (10 micrograms/kg iv). The ACTH response to ANG II in the presence of the AVP antagonist (66.4 +/- 3.1 to 100.1 +/- 15.9 pg/ml) was not significantly less than that in its absence (53.0 +/- 4.8 to 72.2 +/- 11.1 pg/ml). Finally, ANG II and AVP were infused in combination to determine whether there is a synergism between these two peptides in the release of ACTH. In one protocol, AVP and ANG II were infused separately and in combination. The ACTH response to ANG II and AVP in combination (48.7 +/- 6.5 to 61.5 +/- 8.5 pg/ml) was not enhanced compared with the responses to ANG II (59.8 +/- 7.3 to 71.0 +/- 10.1 pg/ml) or AVP (48.8 +/- 5.7 to 55.6 +/- 6.5 pg/ml) alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Are hypertensive effects of aldosterone, angiotensin, vasopressin, and norepinephrine chronically additive?

The effects of chronic combined administration of angiotensin II, norepinephrine, aldosterone, and arginine vasopressin were compared with the response to each of these hormones administered alone. The studies were performed in dogs to determine the extent to which moderately inappropriate elevations of these hormones could enhance each other's ability to produce chronic hypertension and influence Na and water homeostasis. Blood pressure sensitivity to Na intake was also evaluated by infusing the hormones for 11 days at normal levels of Na intake followed by 11 days at high Na intake with ad libitum drinking. Combined hormone administration did not enhance each hormone's singular hypertensive actions. With aldosterone infusion alone and normal Na intake, mean arterial pressure rose nearly 15 mm Hg and an additional 3 mm Hg during high Na intake. Combined hormone infusion also resulted in a nearly 15 mm Hg rise during normal Na intake and an additional 3 mm Hg rise in mean arterial pressure during high Na intake. Marked Na retention and hypernatremia were observed with aldosterone infusion, while hyponatremia characterized arginine vasopressin infusion. The combined hormone infusion resulted in a tendency toward hypernatremia, although daily Na balance was not significantly changed. Daily water turnover was substantially increased and urine osmolality fell to hypoosmotic levels, despite elevated arginine vasopressin levels. Even with high Na intake, dogs receiving either angiotensin II, arginine vasopressin, or norepinephrine at the same concentrations showed 4 to 10 mm Hg increases in mean arterial pressure. Thus, humoral summation or synergism of these hormones probably does not play a major role in the development of chronic hypertension.

Correlation of plasma angiotensin II concentration and plasma renin activity during acute hypoxia in dogs

The effects of isocapnic hypoxia on plasma renin activity (PRA) and angiotensin II (AII) concentration were studied in anaesthetized, artificially ventilated dogs. Regression analysis of plasma AII concentration vs PRA, both measured by radioimmunoassay (RIA), was used as an index of converting enzyme activity in vivo. PaO2 decreased from 82 to 30 mmHg but regression analysis did not reveal any inhibition of AII production within the limits of detection of this method (less than 20% inhibition). We conclude that systemic converting enzyme activity, assessed by in vivo measurement and correlation of PRA and AII, is not inhibited by severe hypoxia.