Comparison of the biological effects of two angiotensin II analogues in hypertensive patients with sodium depletion

The effects on blood pressure (BP), plasma aldosterone concentration (PAC) and plasma renin activity (PRA) of two angiotensin II analogues (AII A), i.e., 1-sarcosine, 8-isoleucine angiotensin II (Sar¹, I1e⁸-AII) and 1-sarcosine, 8-alanine angiotensin II (Sar¹, Ala⁸-AII), were investigated in patients with hypertension with various etiologies on sodium depletion. The changes of BP, PAC and PRA on infusion of Sar¹, Ile⁸-AII and Sar¹, Ala⁸-AII were very similar. With both compounds, there were significant inverse correlations between the pre-infusion PRA and the changes in BP and PAC, and a significant positive correlation between the pre-infusion PRA and change in PRA. The slopes of the regression lines for the correlations of changes in BP, PAC and PRA, plotted as functions of the pre-infusion PRA for Sar¹, Ile⁸-AII and Sar¹, Ala⁸-AII were not statistically different. In clinical investigations, the two compounds seemed equally useful for detecting renin-dependency in hypertension.     

Hypotensive activity of TCV-116, a newly developed angiotensin II receptor antagonist, in spontaneously hypertensive rats.

TCV-116, a recently developed angiotensin II (Ang II) receptor antagonist, was administered orally (1 mg/kg per day) to 10-week-old spontaneously hypertensive rats (SHR) for 2 weeks. Blood pressure and plasma components of the renin-angiotensin-aldosterone system were determined in these rats. TCV-116 produced a marked reduction in blood pressure without altering heart rate. Whereas plasma renin concentration (PRC), angiotensin I (Ang I) and angiotensin II (Ang II) all were significantly increased, plasma aldosterone was decreased by approximately 70% compared with control animals. These results not only indicate therapeutic efficacy of this agent in the chronic treatment of human hypertension, but support also the concept that the renin-angiotensin system plays an important role in the control of blood pressure in this animal model of human essential hypertension.     

Angiotensin II in essential hypertension.

Plasma concentrations of angiotensin II (PAC) were measured in a group of 146 hypertensive patients (diastolic pressure greater than 105 mm Hg) who had no apparent underlying cause for their condition and 113 randomly selected normotensive controls (diastolic pressure less than 90 mm Hg). There was no evidence of bimodality in the frequency distribution curves for plasma angiotensin II concentrations among the hypertensive patients. It was concluded that hypertension associated with low angiotensin II concentration and by implication "low-renin" hypertension is not a condition separate from essential hypertension.     

Role of angiotensin II in oral contraceptive hypertension in anesthetized rats

Rats fed a diet containing an oral contraceptive estrogen-progestogen mixture (Enovid) for 26 weeks developed a significant (P<0.01) elevation in mean arterial pressure (145 ± 6 mm Hg) when compared to a control group of rats fed the same diet without oral contraceptive steroids (mean arterial pressure averaged 117 ± 6 mm Hg). Infusion of [Sar¹, Ala⁸] angiotensin II, a competitive antagonist of angiotensin II, reduced the mean arterial pressure (P<0.01) in the oral contraceptive-treated rats to 117 ± 9 mm Hg, while infusion of this angiotensin II analogue into the control rats did not lower the arterial pressure. These studies provided evidence that angiotensin II may be involved in the hypertension produced by Enovid treatment in rats.     

Drinking to intracerebroventricularly infused angiotensin II, III, and IV in the SHR

Spontaneously hypertensive rats (SHR) revealed exaggerated water consumption to the intracerebroventricular (ICV) infusion of angiotensin II (AII), and angiotensin III (AIII), as compared with Wistar-Kyoto (WKY) and Sprague-Dawley (SD) normotensive rat strains, in agreement with an earlier report (30) that employed ICV bolus injections of AII and AIII. However, the ICV infusion of AII(3-8) (AIV) did not yield reliable drinking. A second hypothesis that the infusion of AII and AIII would yield equivalent drinking within members of each strain, as previously observed with bolus ICV injections in SD rats, was not confirmed. In contrast, ICV infusion of AII yielded greater water intake than AIII in members of each strain tested. These results suggest that the slow infusion of these ligands allowed endogenous aminopeptidases to adequately keep pace with the degradation of these peptides in contrast with bolus injections that could temporarily saturate the available aminopeptidases thus extending the half-life of the ligand.     

Potentiation of pressor response to angiotensin II at the preoptic area in spontaneously hypertensive rat

Cardiovascular responses to angiotensin II(AII) at the preoptic area (POA) were compared between normotensive Wistar Kyoto rat (WKY) and spontaneously hypertensive rat(SHR) by measuring blood pressure and heart rate under unrestrained, conscious state via a catheter implanted chronically into the abdominal aorta and by injection of drugs into POA through a chronic guide cannula. AII injected into POA at doses of 0.3 ng and 1 ng produced a dose-dependent pressor response, accompanied with a slight decrease of heart rate, in both WKY and SHR. However, in SHR, the pressor response to AII was more than 2 times greater than that in WKY and was quick in onset and lasted about 30 min. When AII in combination with [Sar1, Ile8]-angiotensin II (0.5 microgram), an AII receptor antagonist, were simultaneously administered to POA, the pressor response to AII was strongly inhibited in both WKY and SHR. The results suggest that the pressor response to AII due to its receptor stimulation at POA is markedly potentiated in SHR.     

Action of a human plasma fraction on tetradecapeptide,angiotensin I and angiotensin II

A protein fraction designated PF70 was isolated from human plasma and partially purified on Sephadex G-100. PF70 proteins, molecular weight 37, 000 to 41, 500, formed angiotensin I (AI) and angiotensin II (AII) from ¹⁴C-tetradecapeptide renin substrate (TDP) at 37 C. Hydrolysis was maximal at pH 6.9 but there was no change in the relative quantity of AI and AII formed at different pH values. Data indicate that AI was formed first and at a faster rate than AII, but typical converting enzyme activity was not detected. Radiolabeled AII was converted to Des-Asp¹-angiotensin II (angiotensin III); [³H]AI was degraded to a single tritiated product, possibly the nonapeptide. These aspartyl hydrolase reactions were apparently inhibited by TDP and were not involved in AI or AII generation from TDP. It is concluded that these enzymic activities represent two or more enzymes that are associated with the renin-angiotensin system.     

Effects of losartan, a nonpeptide angiotensin II receptor antagonist, on cardiac hypertrophy and the tissue angiotensin II content in spontaneously hypertensive rats.

Losartan, a recently developed nonpeptide angiotensin II (Ang II) receptor antagonist, was administered orally to 10-week-old spontaneously hypertensive rats (SHR) for 2 weeks. Cardiac weight and tissue Ang II, as well as plasma renin activity (PRA) and Ang II, were determined. Treatment with Losartan (10 mg/kg per day) lowered blood pressure markedly. Losartan reduced significantly the left ventricular weight by 11% compared with control rats. The left ventricular Ang II content was lowered by Losartan (18.6 +/- 0.9 pg/tissue; 21.9 +/- 0.9 pg/tissue, control, p less than 0.05), whereas PRA and plasma Ang II concentration were increased by the treatment. With the control and Losartan-treated animals, there was a significant positive correlation between the left ventricular weight and the tissue Ang II content (r = 0.563, p less than 0.05). These results provide evidence that cardiac tissue Ang II, rather than circulating Ang II, plays an important role in the pathophysiology of left ventricular hypertrophy of this animal model of human hypertension.     

Angiotensins in plasma of hypertensive rats and human

The plasma levels of des-aspartate-angiotensin I (DAA-I) in three models of hypertensive rats and hypertensive subjects were determined and compared with their normotensive controls. The rationale for the study was based on our earlier findings showing that DAA-I is a physiological angiotensin peptide that is involved in the pathophysiology of the cardiovascular system. The determination was carried out by the technique of capillary electrophoresis. Plasma level of angiotensin I, angiotensin II, and angiotensin III was also determined as a measurement of the status of the renin-angiotensin system in the different models of hypertension. DAA-I was found to be significantly lower in the spontaneously hypertensive rats (SHR) (46.6 +/- 2.5 pmol/l compared to 66.1 +/- 3.4 pmol/l for the normotensive control Wistar Kyoto rats), renal hypertensive rats (54.2 +/- 5.1 pmol/l compared to 72 +/- 2.5 pmol/l for the normotensive control Sprague-Dawley rats), and essential human hypertensive subjects (15.2 +/- 0.9 pmol/l compared to 19.5 +/- 2.5 pmol/l for the normotensive adult), whilst plasma concentration of angiotensin I and angiotensin II is reflective of the state of the renin-angiotensin system in the particular model of hypertension. When the SHR and human hypertensive subjects were treated with an angiotensin converting enzyme (ACE) inhibitor, the plasma level of DAA-I increased significantly. These findings suggest that the low plasma level of DAA-I could be a characteristic defect of the renin-angiotensin system in the two genetic models of hypertension (SHR and human essential hypertensive subjects). The increase of the nonapeptide following ACE inhibitor treatment could be an important hitherto unrecorded contributory factor to the effectiveness of ACE inhibitors in combating heart pathology.     

Heart rate reflex responses during gestation in normotensive and spontaneously hypertensive rats following angiotensin II and vasopressin

Gestation in the human and in rats is accompanied by a decrease in blood pressure and a reduction of the pressor response to vasoconstrictor agents. In humans, the decreased vascular reactivity to angiotensin II (AII) may occur simultaneously with a state of increased baroreceptor sensitivity. We have consequently evaluated the heart rate response to elevation of blood pressure following administration of either AII or arginine8-vasopressin (AVP) in conscious unrestrained, nonpregnant, or term-pregnant normotensive rats (Sprague-Dawley, SDR; Wistar-Kyoto, WKR) and in spontaneously hypertensive rats (SHR). The decrease in heart rate in response to increase in blood pressure by AII in nonpregnant animals was similar in SDR and SHR, but much greater in WKR. The heart rate response to increase in blood pressure by AVP was similar in all three strains of cycling rats. Gestation (20th day) did not change the heart rate response to increase in blood pressure by AII in normotensive animals, but increased slightly the reflex responses in SHR, as shown by a significant increase of the slope of the relationship of the decrement in heart rate versus the increment of blood pressure. The heart rate response to increase in blood pressure by AVP was greater during gestation in normotensive SDR and WKR, but not in SHR. These results show that the heart rate responses to an increase in blood pressure by vasoconstrictor peptides is dependent on the strain of animals used and suggest that the baroreceptor reflexes play a minor role in the blunted effect of vasconstrictor agens at the end of gestation in normotensive and spontaneously hypertensive rats.     

Reversible hypertension caused by calcium overloading in a patient with postoperative hypoparathyroidism

A 37-year-old woman with postoperative hypoparathyroidism had hypertension, and elevated plasma renin activity (PRA) and subsequent hyperaldosteronism during a two-month hypercalcemic period caused by vitamin D and excessive calcium supplements. The hypertension with elevated PRA, however, was resistant to the angiotensin II (AII) analog [Sar1, Ile8] ALL. PRA further increased and plasma aldosterone decreased in response to the [Sar1, Ile8] ALL. When the patient became normocalcemic, normotensive and normoreninemic, calcium gluconate (5 mg calcium/kg/h) was infused for one hour. The calcium infusion reproduced hypercalcemic hypertension mediated by an increase in total peripheral resistance. These observations suggest that the hypertension observed while taking vitamin D and excessive calcium supplements may be caused by a direct effect of calcium on peripheral blood vessels and the renin-angiotensin system may play a negligible role.     

Hemodynamic and metabolic effects of angiotensin II on the liver

To ascertain the mechanism of interaction between angiotensins (AI and AII) and the liver, an angiotensin-converting enzyme inhibitor (captopril) and a receptor antagonist (losartan) were used. Monovascular or bivascular liver perfusion was used to assess both hemodynamic (portal and arterial hypertensive responses) and metabolic (glucose production and oxygen consumption) effects. Microphysiometry was used for isolated liver cell assays to assess AII or losartan membrane receptor-mediated interaction. Captopril abolishes portal hypertensive response (PHR) to AI but not the AII effect. AII infused via the portal pathway promotes calcium-dependent PHR but not a hypertensive response in the arterial pathway (AHR); when infused into the arterial pathway AII promotes calcium-dependent PHR and AHR. Losartan infused into the portal vein abolishes PHR to AII but not the metabolic response; when infused via both pathways it abolishes the hypertensive responses and inhibits the metabolic effects. Isolated liver cells specifically respond to AII. Sinusoidal cells, but not hepatocytes, respond to 10 nM losartan. We conclude that AI has to be converted to AII to produce PHR. Quiescent stellate cells interacts in vitro with AII and losartan. Hemodynamic responses to AII are losartan-dependent but metabolic responses are partially losartan-independent. AII hemodynamic actions are mainly presinusoidal.     

The effect of prolonged infusion and withdrawal of angiotensin II in the spontaneously hypertensive rat

In spontaneously hypertensive rats (SHR) and their normotensive Wistar-Kyoto controls (WKY), prolonged intravenous administration of angiotensin II (AII, 0.2 microgram X kg-1 X min-1 for 3h) resulted in similar increases in arterial blood pressure. Heart rate decreased in WKY and increased in SHR. At the end of the infusion, blood pressure dropped substantially in SHR, but not in WKY: at 5 h after AII withdrawal, blood pressure in SHR had fallen from a control value of 172 +/- 3.3 to 146 +/- 3.9 mmHg (p less than 0.01), whereas pressure in WKY had fallen from 116 +/- 3.0 to 107 +/- 4.2 mmHg (statistically non significant). Thus, pressure at 5 h after AII withdrawal was still substantially higher (p less than 0.01) in the SHR than in the WKY. The results demonstrate that the fall in blood pressure following withdrawal of a prolonged infusion of AII in SHR is much less than that reported to occur following withdrawal of a prolonged infusion of vasopressin (AVP) in SHR.     

Aldosterone response to angiotensin II during hypoxemia

Exercise in humans causes increases in plasma renin activity (PRA) and plasma aldosterone concentrations (PAC) except when performed at high altitude or while the subjects breathe hypoxic gas. Under those conditions, PRA increases with exercise but PAC does not. We speculated that the PAC suppression during hypoxemic exercise was due to hypoxemia-induced release of a circulating inhibitor of angiotensin II-mediated aldosterone secretion. To test this hypothesis, we measured the PAC response to graded infusions of angiotensin II during hypoxemia and normoxemia. Eight normal volunteers were given increasing doses of angiotensin II (first 2 ng X kg-1 X min-1 and then 4, 8, and finally 12 ng X kg-1 X min-1, each for 20-min periods) on 2 separate days, once while breathing room air and the other day while breathing hypoxic gas adjusted to maintain the subjects' hemoglobin saturation at 90%. The PAC response to different doses of angiotensin II did not significantly differ during hypoxemia from normoxemia. We conclude that our model of hypoxemia does not cause release of an inhibitor of angiotensin II-mediated aldosterone release.     

Discrimination of two angiotensin II receptor subtypes with a selective agonist analogue of angiotensin II, p-aminophenylalanine6 angiotensin II

Angiotensin II receptor binding sites in rat liver and PC12 cells differ in their affinities for a nonpeptidic antagonist, DuP 753, and p-aminophenylalanine6 angiotensin II. In liver, which primarily contains the sulfhydryl reducing agent-inhibited type of angiotensin II receptor, which we refer to as the AII alpha subtype, DuP 753 displays an IC50 of 55 nM, while p-aminophenylalanine6 angiotensin II displays an IC50 of 8-9 microM. In PC12 cells, which primarily contain the angiotensin II receptor type whose binding affinity is enhanced by sulfhydryl reducing agents (AII beta), DuP 753 displays an IC50 in excess of 100 microM, while p-aminophenylalanine6 angiotensin II displays an IC50 of 12 nM. p-Aminophenylalanine6 angiotensin II binding affinity in liver is decreased in the presence of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) suggesting that this analogue is an agonist.     

Influence of physical training on blood pressure, plasma renin, angiotensin and catecholamines in patients with ischaemic heart disease

Eighteen patients with ischaemic heart disease were trained for 3 months, three times a week. The effectiveness of the training programme was demonstrated by increases of 27% in peak oxygen uptake and 29% in exercise duration, and by a decrease in resting and submaximal heart rates. Blood pressure, however, was not significantly affected during the training period. At rest and at submaximal exercise plasma renin activity (PRA) was lower after training. Plasma angiotensin I concentration (PA I) and angiotensin II concentration (PA II) were not significantly affected. Plasma aldosterone concentration (PAC), only measured at rest, was not significantly changed after the training period, while plasma norepinephrine (PNE) and epinephrine (PE) concentrations were significantly decreased, but only at high levels of exercise. A reduced sympathetic tone after training, suggested by the lower heart rates and the tendency to a decrease in PNE, is a likely explanation for the decrease in PRA. However, despite this decrease, PA I, PA II, and PAC were not significantly changed after training; the reason for this disrepancy is unknown.     

Role of the pituitary in the effects of tonin on adrenal secretion of the rat

Chronically catheterized conscious rats were infused intravenously with tonin at 2.4 and 12 micrograms x kg-1 x min-1 for 2 h. Plasma aldosterone concentration (PAC) at the end of the experiment was 11.2 +/- 2.4 ng% in controls, 8.5 +/- 2.8 ng% in rats infused with tonin at the lower rate, and 26.2 +/- 3.6 ng% (p less than 0.01 vs. controls) in rats infused at the higher rate. Plasma corticosterone (PC) was significantly higher (p less than 0.05) in the group infused at the high rate while plasma renin activity (PRA) was significantly reduced in this group of rats. Plasma angiotensin II (AII) concentration was similar in all three groups. PAC was elevated after tonin infusion in the presence of AII blockade. PAC in conscious sodium-depleted rats infused with tonin was not significantly changed, but PRA was significantly reduced (p less than 0.01). In chronically hypophysectomized rats, PAC remained unchanged by tonin infusion. The failure of tonin to stimulate aldosterone in hypophysectomized animals indicates a role of a pituitary hormone (probably ACTH) in the effect of tonin on adrenal secretion.     

Bromobenzofurans : a new class of potent, non-peptide antagonists of angiotensin II

This paper describes the synthesis and pharmacology of a novel series of benzofurans which are antagonists of angiotensin II. One of these, the bromobenzofuran 11b, is a potent (apparent pK_B=9.8) and specific antagonist of angiotensin II which, after oral administration (10mg/Kg), causes marked and long-lasting ( > 24h) falls in blood pressure in renal hypertensive rats.     

Arteriovenous ratios of angiotensin II during acute infusion experiments: a model-based nalysis

The hormone angiotensin II (AII) is a vascocontrictor known to participate in the natural regulation of blood pressure via the renin-angiotensin system. A third-order model was developed which describes the dynamics of venous and arterial plasma AII concentrations (PAC) and mean arterial blood pressure (BP) during acute constant rate AII infusion experiments. The model is calibrated using approximate blood circulation rates and steady-state PAC and BP data for published experiments in sheep. Analysis of the dynamic model demonstrates that local changes in PAC during the first several minutes of acute infusion are characterized by the comparatively rapid distribution of exogenous AII making its forward passage across the blood circulation, combined with the more gradual elevation of exogenous AII recycled through the circulation. This analysis explains the observed divergence in physiological levels of venous and arterial PAC at steady state in terms of the monotonic net clearance of elevated levels of circulating AII along the circulatory path between the point of infusion and the two sites at which the PAC measurements are taken. The model suggests that the differing arteriovenous AII concentration ratios and differing PAC and BP relationships reported for different dose-response experiments may be explained in part by differences in the specific infusion and measurement sites employed in those experiments.     

Reduced number of adrenal angiotensin II receptors in the spontaneously hypertensive rat

[¹²⁵I]angiotensin II binding to adrenal subcellular particles was compared between spontaneously hypertensive and Wistar-Kyoto normotensive control rats. The number of angiotensin II receptors was reduced in adrenals of spontaneously hypertensive rats (P < 0.05) without a change in the affinity of angiotensin II for the binding sites at animal ages of 5, 10 and 15 weeks. This decline of receptor content antedated the abrupt rise in blood pressure noted between 5 and 10 weeks. The data suggest the presence of an alteration of the receptor number in the renin-angiotensin- aldosterone system in the spontaneously hypertensive rat.