Effects of converting enzyme inhibition and alpha receptor blockade on the angiotensin pressor response in the American alligator.

This study examines the effects of two converting enzyme inhibitors (captopril and enalaprilat) and two alpha-adrenergic receptor antagonists (phentolamine and phenoxybenzamine) on the pressor response produced by exogenous angiotensin I ([Asp1, Val5, Ser9] ANG I, fowl) and [Val5] angiotensin II (ANG II) in the American alligator (Alligator mississippiensis). Bolus administration of ANG I at 0.1, 0.5, and 1.0 micrograms/kg; ANG II at 0.05, 0.1, and 0.5 micrograms/kg; or norepinephrine (NE) at 2 micrograms/kg elicited dose-dependent increases in arterial blood pressure. Captopril (0.5 mg/kg/hr) and enalaprilat (300 micrograms/kg/hr) significantly reduced the response to ANG I, but not ANG II or NE. Both phenoxybenzamine (0.25 mg/kg/min) and phentolamine (1 mg/kg/hr) effectively blocked the NE pressor response (84 and 88%, respectively) and attenuated (42-80%) the pressor effects of ANG I and ANG II. These results support previous work suggesting the alligator may possess a renin-angiotensin system with characteristics similar to those found in mammals and other vertebrates. In addition, the pressor response to exogenously administered ANG I and ANG II was attenuated by alpha adrenergic receptor blockade and thus may be due, in part, to secondary catecholamine release.     

Identification and synthesis of [1-asparagine, 5-valine, 9-glycine] angiotensin I produced from plasma of American eel Anguilla rostrata

The major peptide produced by incubation of American eel (Anguilla rostrata) plasma with the eel kidney extract was identified as [1-asparagine, 5-valine, 9-glycine] angiotensin I (I). Two minor peptides were also identified as [1-aspartic acid, 5-valine, 9-glycine] angiotensin I (II) and [5-valine, 9-glycine] angiotensin I-(3-10)-octapeptide (III). These structures were further confirmed by comparison of these peptides on high-pressure liquid chromatography (HPLC) and high-performance thin-layer chromatography (HPTLC) with the synthetic peptides and the tryptic and chymotryptic digests of the synthetic and natural angiotensins. In the rat pressor bioassays, the synthetic decapeptides I and II possessed 45.4 and 52.2%, respectively, of the pressor activity of [1-aspartic acid, 5-isoleucine] angiotensin II. The pressor activity of I and II was blocked with the converting enzyme inhibitor captopril. Study of the conversion of asparaginyl decapeptide into aspartyl decapeptide in eel plasma indicated that the presence of thimerosal (sodium ethylmercurithiosalicylate) in the incubation mixture inhibited the conversion of I into II. These results suggest that (a) I is the natural form of angiotensin inherent in the American eel while II may be formed during incubation with plasma; (b) eel plasma contains an enzyme which is capable of converting asparaginyl angiotensins into aspartyl angiotensins; and (c) pressor activity of I and II is due to their conversion into the corresponding octapeptides. In a previous work when thimerosal was not included in the incubation mixture, the major peptide produced by incubation of Japanese eel (Anguilla japonica) plasma with its kidney extract was identified as II. (Y. Hasegawa, T. Nakajima, and H. Sokabe, 1983, Biomed. Res. 4, 417-420).     

No Substrate Specificity of Converting Enzyme for N-Terminal Substituted Angiotensin I in Man

In 5 normal men sarcosinel-angiotensin II (Sarl-ANG II) (Exp. 1) and sarcosinel-angiotensin I (Sarl-ANG I) (Exp. 2) infused iv at a rate of 5 pmol/kg-min from 0900 h to 0930 h caused the same degree of rise in blood pressure (BP). But 100 mg of captopril given orally at 0800 h (Exp. 3) completely abolished the BP rise by Sarl-ANG I. In Exps. 1 and 2 plasma renin activity (PRA) decreased and plasma aldosterone (PA) increased after the infusions. In Exp. 3 PRA increased markedly and PA decreased 60 min after captopril, and at 30 min of Sarl-ANG I infusion PRA decreased to the pre-captopril level despite no BP change but PA was kept at the pre-infusion level. Hence, substrate specificity of converting enzyme previously demonstrated for N-terminal deleted ANG I was not shown for N-terminal substituted ANG I in man because the conversion of Sar¹-ANG I to Sar¹-ANG II was 100%. Sar¹-ANG I may possibly inhibit renin release in normal men.     

Blood pressure homeostasis in the snake, Ptyas korros

Three putative pressor systems, the alpha-adrenergic system (AS), the renin angiotensin system (RAS), and the arginine vasotocin system (AVT-S), were studied for their roles in blood pressure regulation and their possible interactions in the rat snake. Ptyas korros. Norepinephrine (NE), angiotensin I (ANG I), and arginine vasotocin (AVT) increased the mean arterial pressure (MAP) of the snake while administration of phentolamine, an alpha-adrenergic antagonist, and captopril, an ANG-converting enzyme inhibitor, but not KBIV24, an AVT antagonist, decreased the MAP. Treatment with any combination of two of these antagonists/inhibitor invariably decreased the MAP. Treatment with the agonist of the remaining third system invariably returned the MAP to normal or above. Phentolamine and KBIV24 attenuated the vasopressor effect of ANG I. Phentolamine and captopril enhanced the vasopressor effect of AVT. The pressor effect of NE was not altered by KBIV24 and captopril. It was concluded that there were at least two pressor systems (AS and RAS) regulating the basal MAP in the snake. There was also interaction among the three systems which could affect the MAP.     

Effect of Captopril on Angiotensin II Release from Vascular Tissues in Rats

Isolated rat hindlegs were perfused with Krebs-Ringer solution and released angiotensin I (ANG I) and ANG II were determined. The release of ANG I and ANG II in nephrectomized rats did not differ from those in control group. Pretreatment with captopril (50 mg/kg/day) for 3 days or addition of captopril (2 × 10^?6 M) to the perfusate induced increase in ANG I release and decrease in ANG II release. These findings suggest that ANG II is locally generated and released from the vascular tissues. Captopril may inhibit the conversion of vascular ANG I into ANG II.     

Captopril attenuates pressor responses to norepinephrine and vasopressin through depletion of endogenous angiotensin II

The influence of captopril on pressor responses to exogenously administered vasopressor substances was investigated in normal subjects. Norepinephrine (0.05, 0.1 and 0.2 μg/kg · min^?1; n = 5), angiotensin II (5, 10 and 20 ng/kg · min^?1; n = 5) and vasopressin (2 mU/kg · min^?1; n = 5) were infused each for 10 minutes; each infusion was repeated twice. Captopril (50 mg orally) significantly attenuated the pressor response to norepinephrine (0.1 [p < 0.05], 0.2 [p < 0.01]μg/kg · min^?1; n = 7) and to vasopressin (p < 0.01, n = 5), but not to angiotensin II; these responses were reproducible. Attenuation of the pressor responses to norepinephrine did not occur when a subpressor dose of angiotensin II (ng/kg · min^?1) was infused in addition to captopril (n = 5). Infusion of a subpressor dose of bradykinin (0.1 ng/kg · min^?1) had no influence on the pressor responses to norepinephrine (n = 5). In the five subjects treated with indomethacin (225 mg/54 hours) captopril still attenuated the pressor responses to norepinephrine. These results suggest that the attenuation by captopril of the pressor responses to norepinephrine and vasopressin might have been due to reduction of endogenous angiotensin II.     

Chemical structures of angiotensins formed by incubating plasma with the kidney and the corpuscles of Stannius in the chum salmon, Oncorhynchus keta

The chemical structures of salmon angiotensins produced by incubating tissue extract of the kidney or the corpuscles of Stannius (CS) with homologous plasma are proposed, Two angiotensins, [Asp¹, Val⁵, Asn⁹] and [Asn¹, Val⁵, Asn⁹] angiotensin I, were proposed from both kidney and CS incubations by amino acid analysis and the fluorescent peptide-mapping techniques. CS angiotensis were not organ specific, because these two angiotensins were produced by both kidney and CS incubations in a ratio of 1:2 under the same conditions. Whether [Asp¹, Val⁵, Asn⁹] angiotensin I is a naturally occurring form remains to be clarified; however, [Asn¹, Val⁵, Asn⁹] angiotensin I may be the major form of angiotensin formed from plasma by salmon kidney and CS.     

The pressor response to exogenous angiotensin I and its blockade by angiotensin II analogues in the American alligator.

We examined the pressor response to exogenous, nonnative angiotensin I (ANG I; bullfrog, turtle, and fowl) in the conscious American alligator, Alligator mississippiensis. In addition, the inhibitory effects of three ANG II analogues ([Sar1, Ala8], [Sar1, Thr8], and [Sar1, Ile8]ANG II) on the pressor responses to angiotensin I (fowl ANG I, [Asp1, Val5, Ser9]) were also examined. Intravenous administration of bullfrog, turtle, and fowl ANG I at 0.1, 0.5, and 1.0 micrograms/kg produced dose-dependent increases in arterial blood pressure. [Val5]ANG II at 0.05, 0.1, and 0.5 micrograms/kg, or NE at 2 micrograms/kg also produced dose-dependent increases in blood pressure. [Sar1, Ile8]ANG II and [Sar1, Ala8]ANG II (10 micrograms/kg/min) both attenuated the pressor response to fowl ANG I whereas [Sar1, Thr8]ANG II (10 micrograms/kg/min) produced no significant blockade. These data demonstrate: (1) All three exogenous ANG I molecules exert potent vasopressor responses in the alligator, (2) [Sar1, Ile8]ANG II is the most effective ANG antagonist, and (3) the alligator appears to possess a renin-angiotensin system similar to that found in other vertebrates.     

Update: Role of the angiotensin type-2 (AT2) receptor in blood pressure regulation

In the past, virtually all of the physiologic actions of angiotensin II (ANG II) were thought to be mediated by the type-1 ANG II receptor. However, there is now a compelling body of evidence suggesting that the type-2 (AT₂) receptor is an important regulator of renal function and blood pressure (BP). The AT₂ receptor stimulates a bradykinin (BK)-nitric oxide (NO)-cyclic GMP vasodilator cascade in blood vessels and in the kidney. Recent studies have shown that absence of the AT₂ receptor lends to pressor and natriuretic hypersensitivity to ANG II. Furthermore, there is now excellent evidence that the AT2 receptor mediates pressure natriuresis. The AT₂ receptor also stimulates the conversion of prostaglandin E2 (PGE₂) to PGF₂. In addition, it is now apparent that the therapeutic reduction in BP with AT₁ receptor blockade (eg, losartan, valsartan, candesartan) is mediated by ANG II stimulation of the AT₂ receptor, leading to increased levels of BK, NO, and cGMP. Current evidence predicts that AT₂ receptor agonists would be beneficial in the treatment of hypertension.     

Effect of low doses of angiotensin II perfusion on the hypotensive action of captopril in anaesthetized normotensive and spontaneously hypertensive rats

The effect of intravenous (i.v.) captopril on mean arterial blood pressure (MABP) of anaesthetized normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats perfused i.v. with two doses of angiotensin II (ANG II; 2.9 and 5.8 pmol/kg per min) was studied to determine the role of the suppression of plasma ANG II in the hypotensive action of captopril. The reduction of MABP by captopril was attenuated in WKY and abolished in SHR by the highest dose of ANG II; it was unchanged in WKY and attenuated in SHR by the lowest dose of ANG II. The suppression of plasma ANG II thus explains a minor part of the acute reduction of MABP by captopril in WKY and a major part of this action in SHR. Plasma ANG II contributes to the maintenance of high blood pressure in SHR.     

Pressor responses to alligator Angiotensin I and some analogs in the spectacled caiman (Caiman crocodilus)

Discovery of the chemical structure of alligator (Alligator mississipiensis) [Asp(1), Val(5), Ala(9)]-Angiotensin I (ANG I) has permitted the investigation of cardiovascular responses to this peptide and its analogs in spectacled caimans (Caiman crocodilus), close relatives of alligators. ANG I and [Asp(1), Val(5)]- Angiotensin II (ANG II) i.v. gave dose-dependent increases in mean arterial pressure but there was no pressor response to [Val(4)]-ANG III (ANG III). Pressor responses to a series of doses of ANG II were compared with a range of doses of norepinephrine (NE) and epinephrine (E) which were found to be only about 1/100 as potent as ANG II on a molar basis. The replacement of d-leu(10)in the alligator ANG I molecule with l-leu(10) almost stopped its conversion to ANG II and attenuated the pressor response. [Asp(1), Val(5), Ala(9)]-ANG I (1-9), and ANG (1-7) both failed to increase arterial blood pressure, even at the relatively high non-physiological test dose of 194pmolkgbw(-1) i.v. Captopril blocked angiotensin converting enzyme (ACE) and prevented the pressor response to ANG I whereas the mammalian AT(1) inhibitor Losartan attenuated, but did not completely block the pressor response to ANG II. These are the first experiments which test the cardiovascular responses to alligator ANG I and its analogues in any crocodilian species.     

Differential pressor and renal vascular reactivity to angiotensin II in spontaneously hypertensive and Wistar-Kyoto rats

The suggestion has been made that the Okamoto strain of spontaneously hypertensive rats (SHR) shares some features with a subgroup of patients with essential hypertension, called nonmodulators. One feature of nonmodulators is a renal blood flow response to angiotensin II (ANG II) that is blunted on a high salt diet; the blunted renal vascular response is corrected by converting enzyme inhibition. Renal blood flow (electromagnetic flowmeter) and pressor responses to graded ANG II doses (5-300 ng) were assessed in 24 SHR and 24 Wistar-Kyoto rats (WKY) ingesting 1.6% Na. In comparison to WKY, blood pressure was higher in SHR (155 +/- 4 vs 106 +/- 2 mm Hg; p less than 0.001), renal blood flow was lower (6.9 +/- 0.5 vs 8.2 +/- 0.4 ml/min/g; p less than 0.05), and the pressor response to ANG II was enhanced, (p less than 0.0005) but the renal vascular response was blunted (p less than 0.005). Captopril (1-30 mg/kg) reduced blood pressure more in SHR than in WKY but increased renal blood flow similarly in both strains. The blunted renal vascular response to ANG II in SHR was reversed by captopril, but inhibition of converting enzyme in the kidney did not parallel systemic inhibition. Maximum blockade of converting enzyme in the kidney appears to require a larger captopril dose than is required for systemic inhibition. These results suggest that the renal blood supply in SHR also shares some of the characteristics of nonmodulators and that the action of captopril on the renal blood flow probably reflects reversal of inappropriate intrarenal ANG II formation.     

Renal handling of angiotensins I and II in patients with renovascular hypertension

Plasma renin (PRC) and angiotensin I (pANG I) and II (pANG II) concentrations were determined at renal vein catheterization in 38 hypertensive patients suspected of renal or renovascular aetiology. Veno-arterial differences in pANG I across the affected kidney in patients with lateralization of the renin secretion indicated release of angiotensin I (ANG I) in considerable amounts. Veno-arterial differences in pANG II of around zero indicated that generation and elimination of angiotensin II (ANG II) were equal in that kidney. Across the contralateral kidney the veno-arterial differences in PRC and pANG II were both close to zero, while negative differences in pANG II indicated the removal of ANG II. In patients without lateralization of the renin secretion the veno-arterial differences in pANG I were close to zero or positive, those of pANG II being close to zero or negative. In 14 of the 38 patients, data were obtained either during maintenance treatment with captopril or after a single dose of this converting enzyme inhibitor. Systemic PRC and pANG I values were extremely high and pANG II values low. The pANG I gradient across the affected kidney was further increased compared with pre-captopril levels, whereas the contralateral kidney still eliminated ANG I.     

Interaction of angiotensin-converting enzyme inhibitors with the function of the sympathetic nervous system

The effects of the converting enzyme inhibitors captopril and SQ 20881 and the angiotensin II antagonist saralasin were studied on neurogenic vasoconstriction in the rat using both in vivo and in vitro techniques. In the pithed rat the presser response to nerve stimulation at 1 to 30 Hz was reduced by captopril (0.1 and 1 mg/kg), saralasin (4 gmg/kg/min) and SQ 20881 (10 mg/kg). The pressor responses to noradrenaline (10 to 500 ng total dose) were also antagonized by captopril (1 mg/kg only), SQ 20881 and saralasin. In animals bilaterally nephrectomized 18 to 24 hours previously, captopril and saralasin were without effect on responses to nerve stimulation, but captopril still had a small residual effect on responses to noradrenaline.In isolated mesenteric vessels perfused with Krebs solution exogenous angiotensin I and II potentiated vasoconstrictor responses to nerve stimulation in doses that themselves did not have a direct vasoconstrictor effect. This potentiating effect of angiotensin I was antagonized by captopril (6.7 × 10^?8 to 2 × 10^?6mol) and by saralasin (10^?8 mol). The potentiating action of angiotensin II was blocked only by saralasin. These concentrations of the antagonists themselves had no effect on vasoconstrictor responses to nerve stimulation in the absence of angiotensin although higher concentrations of captopril (10^?4 to 3 × 10^?4mol/liter) did antagonize vasoconstrictor responses to both nerve stimulation and noradrenaline.These results indicate that inhibitors of the renin-angiotensin system may impair neurogenic vasoconstriction by interfering with both a preand postjunctional action of angiotensin. In addition, very high concentrations of captopril antagonize neurogenic vasoconstriction by a nonangiotensin-dependent mechanism. This important interaction of angiotensin with the sympathetic nervous system may help to explain the effectiveness of converting enzyme inhibitors as antihypertensive agents.     

The renin-angiotensin system in nonmammalian vertebrates

The most primitive components of the RAS appeared early in the phylogenetic history of vertebrate animals. It is probable that renin granules were present in the kidneys of ancestral chordates before divergence in the evolution of actinopterygian fish and tetrapods occurred. Granulated juxtaglomerular cells similar to the renin-containing cells of the mammalian nephron are found in most extant vertebrate species although not in agnathan and elasmobranch fish. A macula densa occurs in amphibians, birds and mammals; and an extraglomerular mesangium, only in birds and mammals. Renin-like activity and angiotensin-like pressor material have been demonstrated in all classes of vertebrates. The amino acid sequences of native ANG I have been determined for representative species of teleost fish, amphibian, reptile and bird. These peptides differ from mammalian angiotensins at positions 1, 5 and 9. The RAS appears to be involved in osmoregulation, ionoregulation and the control of blood circulation. Prolonged hypovolemic hypotension or sodium depletion increases renin levels. Angiotensins elicit drinking and stimulate transepithelial ion transport. However, direct steroidogenic and antidiuretic hormone-releasing activities, which would promote mineral and fluid conservation, have not been demonstrated unambiguously in nonmammalian vertebrates. ANG II raises blood pressure by direct vasoconstrictor action on arteriolar muscles in some animals, but perhaps more generally by acting on the nervous system and adrenal paraneurons. In birds the hormone also has a hypotensive effect. ANG II stimulates the SNS in agnathans, elasmobranchs, teleosts, amphibians, reptiles, birds and mammals. Thus, modulation of sympathetic activity may be one of the most primitive and conservative functions of the RAS. For this reason, nonmammalian vertebrates are valuable models for studying the neurogenic actions of angiotensin II relevant to hypertensive disease.     

Recent Updates on the Proximal Tubule Renin-Angiotensin System in Angiotensin II-Dependent Hypertension

It is well recognized that the renin-angiotensin system (RAS) exists not only as circulating, paracrine (cell to cell), but also intracrine (intracellular) system. In the kidney, however, it is difficult to dissect the respective contributions of circulating RAS versus intrarenal RAS to the physiological regulation of proximal tubular Na⁺ reabsorption and hypertension. Here, we review recent studies to provide an update in this research field with a focus on the proximal tubular RAS in angiotensin II (ANG II)-induced hypertension. Careful analysis of available evidence supports the hypothesis that both local synthesis or formation and AT₁ (AT_1a) receptor- and/or megalin-mediated uptake of angiotensinogen (AGT), ANG I and ANG II contribute to high levels of ANG II in the proximal tubules of the kidney. Under physiological conditions, nearly all major components of the RAS including AGT, prorenin, renin, ANG I, and ANG II would be filtered by the glomerulus and taken up by the proximal tubules. In ANG II-dependent hypertension, the expression of AGT, prorenin, and (pro)renin receptors, and angiotensin-converting enzyme (ACE) is upregulated rather than downregulated in the kidney. Furthermore, hypertension damages the glomerular filtration barrier, which augments the filtration of circulating AGT, prorenin, renin, ANG I, and ANG II and their uptake in the proximal tubules. Together, increased local ANG II formation and augmented uptake of circulating ANG II in the proximal tubules, via activation of AT₁ (AT_1a) receptors and Na⁺/H⁺ exchanger 3, may provide a powerful feedforward mechanism for promoting Na⁺ retention and the development of ANG II-induced hypertension.     

AT1 Receptor Blockade Prevents the Increase in Blood Pressure and the Augmentation of Intrarenal ANG II Levels in Hypertensive Cyp1a1-Ren2 Transgenic Rats Fed With a High-Salt Diet

IntroductionThis study was performed to determine the effects of high-salt diet on the magnitude of the increases in systolic blood pressure (SBP) and kidney tissue angiotensin (ANG) II levels that occur after induction of ANG II-dependent malignant hypertension in Cyp1a1-Ren2 transgenic rats with inducible expression of the mouse Ren2 renin gene [strain name: TGR(Cyp1a1Ren2)].MethodsCyp1a1- Ren2 rats (n = 6) were fed a normal diet containing 0.3% indole-3- carbinol (I3C) for 10 days to induce ANG II-dependent malignant hypertension.ResultsRats induced with I3C exhibited increases in SBP and elevations of ANG II levels in kidney cortex and medulla. In a second group of rats (n = 6), high-salt intake alone did not alter basal SBP; however, subsequent dietary administration of 0.3% I3C during continued high-salt intake elicited a substantially greater increase in SBP than observed in rats fed a normal salt diet. ANG II levels in kidney cortex and medulla of rats induced with I3C and fed a high-salt diet were elevated similarly to those in rats induced with I3C alone. Chronic administration of the AT₁ receptor antagonist, losartan (100 mg/L in drinking water, n = 6), markedly attenuated the I3C-induced increase in SBP and prevented the augmentation of ANG II levels in kidney cortex and medulla in rats induced with I3C and maintained on a high-salt diet.ConclusionsActivation of AT₁ receptors contributes to the augmented blood pressure and elevated kidney tissue ANG II levels that occur in Cyp1a1-Ren2 transgenic rats with malignant hypertension maintained on a high-salt diet.     

An endocrinological update in toads: disparity between the cardiovascular effects of two angiotensin II analogs

This study was undertaken to test the hypothesis that, in toads, the pressor effects of angiotensin II (ANG II) are partly due to the secondary effects of catecholamines. In Bufo marinus, blood pressure responses to bullfrog ([Val(5)]) ANG II administration were measured in animals pretreated (experimental group) and not pretreated (control group) with phentolamine, an alpha-adrenergic antagonist. At 1, 2, and 4 min following ANG II administration (50 ngkg(-1)), mean arterial pressure (MAP) of the experimental group was significantly less than in the control group (27.9+/-1.0, 28.2+/-1.6, and 25.8+/-2.1 mmHg versus 45.1+/-1.0, 39.4+/-2.8, and 35.5+/-3.6, respectively; n=6). Heart rate (f(H)) was unaffected by phentolamine. These data support our hypothesis. Previous authors, using human ([Ile(5)]) ANG II, have concluded that alpha-adrenergic mechanisms are not involved in the pressor effects of ANG II in B. marinus. In a separate group of toads, we examined MAP and f(H) responses to similar doses of human and bullfrog ANG II (100 ngkg(-1)). Bullfrog ANG II caused MAP to reach higher peak values (56.5+/-2.6 versus 37.2+/-0.4 mmHg; n=7) in less time (approximately 1 min versus approximately 8 min) than human ANG II. Furthermore, bullfrog ANG II caused f(H) to significantly increase from 41.3+/-5.1 to 79.3+/-2.3 beats min(-1) (at 1 min) whereas human ANG II caused no significant changes in f(H) throughout the measured time course. Higher doses of human ANG II (100 microgkg(-1)) invoked slow but large increases in MAP with non-significant decreases in f(H). These additional data suggest that the choice of exogenously administered hormone is an important one to consider in comparative endocrinological studies.