Effects of angiotensin II on isolated toad (Bufo arenarum) aortic rings

1. The vascular response to Asn1-Val5 angiotensin II (A II) in aortic rings from Bufo arenarum toad was studied. 2. Tachyphylaxis in response to A II could be abolished after incubation with norepinephrine (NE). 3. Phentolamine treatment partially inhibited the pressor effects to A II. 4. Sar1-Ile8 A II and Sar1-Ala8 A II significantly attenuated the vascular effects of A II and did not affect the NE response. 5. We conclude that the pressor response to A II has a direct contractile effect and a catecholamine dependent component in aortic rings of Bufo arenarum toad.     

A cardiac specific analog of angiotensin I potentiated by converting enzyme inhibition

[1-Sarcosine, 7-Alanine] angiotensin I [( 1-Sar, 7-Ala] AI) and closely related analogs were tested for inotropic activity in the isolated cat heart, and for pressor activity in the intact conscious sheep both before and during converting enzyme inhibition (CEI). [1-Sar, 7-Ala] AI exhibited potent inotropic activity but was only weakly pressor. [1-Sar] AI, [1-Sar, 5-Val] AI, [1-Sar, 7-alpha MeAla] AI [1-Sar, 5-Val, 7-NMeAla] AI and [1-Sar, 5-Val, 7-Sar] were all potent agonists in both preparations. The action of [1-Sar, 7-Ala] AI was potentiated by CEI in both the isolated heart and the intact sheep. The activity of the remaining analogs was either partially or completely blocked by CEI. The activity of all analogs was inhibited by AII receptor blockade. These data indicate that the nature of the substitution in position 7 determines the affinity of the analog for converting enzyme. The [7-Ala] substitution appears to decrease the effect of the analog upon vascular receptors.     

Role of baroreflex in the pressor response of rats with hypertension developed by renal artery stenosis

We examined the interrelationships between the pressor response to the administration of norepinephrine and arginine vasopressin and baroreflex function in rats with hypertension of two days' duration induced by heminephrectomy and a clip placed on the right renal artery (2-day clipped rats). Mean arterial pressure was higher in the 2-day clipped rats than in heminephrectomized rats without clips (sham-operated rats). The pressor response in the 2-day clipped rats to both agents increased as compared to the sham-operated rats. This hyperresponsiveness was attenuated by administering an angiotensin II antagonist, [1-Sar, 8-Ile] angiotensin II. Baroreflex sensitivity was studied by measuring changes in arterial pressure and pulse interval in response to the injection of phenylephrine. Baroreflex sensitivity was not decreased but markedly increased in the 2-day clipped rats and unaffected by infusing the angiotensin II antagonist. These results provide evidence that 1) in the 2-day clipped rats there are exaggerated pressor responses to vasoconstrictors; 2) the hyperresponsiveness is not causally related to the change of baroreflex sensitivity; and 3) angiotensin II plays a significant role in the increased pressor responses; however, the baroreflex mechanism is not involved in attenuation of the hyperresponsiveness by the angiotensin II antagonist.     

A new class of angiotensin inhibitors: N-methylphenylalanine analogs

8-N-methylphenylalanine-; 1-sarcosine, 8-N-methylphenylalanine-; 1-sarcosine, 7-hydroxyproline, 8-N-methylphenylalanine-; 7-alanine, 8-N-methylphenylalanine-; and 1-sarcosine, 7-alanine, 8-N-methylphenylalanine-angiotensins II have been synthesized by the solid-phase method. Their agonist activity in several assays is greatly diminished. Three analogs have inhibitory properties, and 1-sarcosine, 8-N-methylphenylalanine-angiotensin II is more potent than the previously described 1-sarcosine, 8-isoleucine-angiotensin as an inhibitor of the pressor action of angiotensin in the rat.     

Antagonists of angiotensin II containing N-methyl-L-alanine and DL-nipecotic acid in position 7.

[1-sarcosine, 7-N-methyl-L-alanine, 8-isoleucine]-Angiotensin II and [1-sarcosine, 7-DL-nipecotic acid, 8-isoleucine]-angiotensin II were synthesized by the solid-phase method and purified by cation-exchange chromatography and high-pressure liquid chromatography. In the isolated rat uterus these analogs and less than 0.1% of the myotropic activity of angiotensin II and inhibited angiotensin II with pA2 values of 8.2 and 7.8, respectively. In the rat pressor assay (vagotomized ganglion blocked rat) these analogs had 0.9 and 2.8%, respectively, of the pressor activity of angiotensin II. The results show that the proline residue in position 7 of [Sar1,Ile8]-angiotensin II may be replaced by other secondary amino acids without disrupting interactions at angiotensin II receptors.     

Effects of losartan on angiotensin receptors in the hypertrophic rat heart

The effects of losartan on angiotensin receptors in hypertrophic rat hearts were studied. The study was prompted by inconsistent findings of either an increase or decrease in the mRNA of the AT1 receptor in the hearts of cardiac hypertrophic rats treated with losartan, and a paucity of information on the effects of losartan on functional angiotensin receptors in the heart. Losartan, administered i.p. to aortic coarcted rats, dose-dependently attenuated the cardiac hypertrophy. Significant effect was observed with a dose of 2.72 micromol/kg/day for four days. Hypertrophy was accompanied by an increase in [125I]-Sar1-Ile8-angiotensin II binding sites (due mainly to an increase in AT2 binding) and AT2 receptor protein in cardiac ventricles of aortic coarcted rats. Treatment with effective anti-hypertrophic doses of losartan dose-dependently downregulated the [125I]-Sar1-Ile8-angiotensin II binding sites, constitutive AT1 receptor protein, and the over expressed AT2 receptor protein. It was suggested that the anti-cardiac hypertrophic action of losartan resulted from its ability to suppress the expression of both the basal and enhanced cardiac angiotensin receptors. This raises the question as to whether such drastic action could form the therapeutic basis for the use of losartan in cardiac pathologies.     

Heptapeptide analogues induce greater blockade of renal than femoral vascular responses to angiotensin

We characterized blockade induced by 2 octapeptide and 2 heptapeptide analogues of angiotensin in the vascular beds of the kidney and hindlimb. Bolus injections of angiotensin II and its 1-des Asp analogue (angiotensin III) at the dose which reduced blood flow by about 50 percent and graded infusions of the analogue-antagonists were made directly into each artery and flow responses were measured with an electromagnetic flowmeter in the anesthetized dog. With the dose of antagonist which produced 50 percent inhibition of the control angiotensin response (ID 50) as the index, inhibition was slightly greater in the kidney than in the hindlimb for both the potent octapeptide antagonist {1-Sar, 8-Ala angiotensin II: kidney ID 50 = 15.3±1.7 (SD) ng/kg/min; hindlimb ID 50 = 23.3±1.8 (SD) ng/kg/min} and the weak octapeptide antagonist {1-D-Asn, 8-Ala angiotensin II: kidney ID 50 = 178.7±2.0 (SD) ng/kg/min; hindlimb ID 50 = 266.7±1.9 (SD) ng/kg/min}. In contrast, both the potent and weak heptapeptide analogues were much more effective as antagonists in the renal than the femoral vascular bed {1-des Asp, 8-Ile AII: kidney ID 50 = 14.9±1.8 (SD) ng/kg/min; hindlimb ID 50 = 36.2±1.9 (SD) ng/kg/min}; {1-des Asp, 8-Ala angiotensin II: kidney ID 50 = 408.9±1.8 (SD) ng/kg/min; hindlimb ID 50 = 1270±2.8 (SD) ng/kg/min}. The difference in the influence of the analogues in the two vascular beds may reflect either a difference in their angiotensin receptors or in the rate at which heptapeptide analogues are degraded in their transit through the renal and femoral vasculature.     

Blockade of the systemic and renal vascular actions of angiotensisn II with the 1-sar, 8-ala analogue in the rat.

To assess the characteristics of blockade induced by 1-Sar, 8-Ala angiotensin II (P113) in the rat, dose-response relationships were established for angiotensin II and blood pressure, cardiac output and renal blood flow (measured with microspheres) and calculated total peripheral resistance. P113 infused at 1.0 μg/kg/min reduced renal and systemic vascular responses to angiotensin II, but did not modify the pressor response because of compensatory increase in cardiac output. Ganglionic blockade (pentolinium tartrate 2.5 mg) uncovered a significant influence of P113 at 1.0 μg/kg/ min on pressor responses to angiotensin II. P113 at 10 μg/kg/min totally prevented the pressor and renal vascular response to 1.0 μg/kg/min of angiotensin II. P113 at 10 and 100 μg/kg/min did not influence renal blood flow, cardiac output or total peripheral resistance, and had only a transient, small influence on blood pressure. P113 did not modify the renal or systemic vascular response to norepinephrine. The failure of P113 to influence renal blood flow in the rat and the relative insensitivity of the renal vasculature to angiotensin II suggest that the vascular receptor for angiotensin II in the rat differs from that in other species including the dog, rabbit and man.     

Chronic sodium depletion suppresses the area postrema pressor pathway

Sodium depletion in dogs is known to affect both the renin-angiotensin as well as the sympathetic nervous system. The effect of this dietary regime upon the area postrema pressor pathway, as evaluated by the cardiovascular responses to centrally acting angiotensin II, has not been determined previously. With this in mind, male mongrel dogs were maintained on either a normal or a sodium restricted diet supplemented with furosemide and dose-response curves for intravertebral and intravenous angiotensin II (range: 1-20 ng/kg/min) were obtained. Sodium depletion results in not only a blunted intravenous pressor response to angiotensin II but also the abolition of the centrally mediated pressor responses mediated by the area postrema. Because accumulating evidence indicates that in sodium depleted dogs sympathetic nerve activity is reduced while central noradrenergic inhibitory activity is increased the reduced effects of angiotensin II upon the central sympathetically mediated pressor response may in part be related to decreases in sympathetic nerve activity.     

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.     

Effect of dietary sodium intake on the pressor reactivity to angiotensin II in rats with experimental cirrhosis of the liver

The present experiments were designed to evaluate vascular reactivity to angiotensin II in rats with experimental cirrhosis of the liver (induced with CCl4 and phenobarbital) before ascites appearance. The systemic pressor response to angiotensin II in conscious animals and the contractile effect of angiotensin II in isolated femoral arteries were studied. In addition, the effect of high sodium intake on these parameters was also analyzed. Both renin and aldosterone plasma concentrations were similar in control and cirrhotic rats on the normal or on the high sodium diet. Basal mean arterial pressure was higher in control rats than in cirrhotic rats on the normal sodium (116 +/- 4 vs. 101 +/- 4 mmHg (1 mmHg = 133.3 Pa), p less than 0.05) or on the high sodium diet (118 +/- 7 vs. 98 +/- 6 mmHg). No differences in plasma renin activity or plasma aldosterone were found between control and cirrhotic rats. Upon injection of angiotensin II, control rats show a dose-dependent increase in mean arterial pressure which is higher in high sodium than in normal sodium rats. Cirrhotic rats showed a lower hypertensive response to angiotensin II than their corresponding control rats. In addition, no difference between pressor responses to angiotensin II was observed when normal sodium and high sodium cirrhotic rats were compared. On application of angiotensin II, femoral arteries of control and cirrhotic rats exhibited a dose-dependent contraction. However, maximal contraction was higher in high sodium control rats (145 +/- 12 mg) than in normal sodium control rats (99 +/- 6 mg, p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Central hypertensive actions of angiotensin I, II and III in conscious rats

The effects of intracerebroventricular administrations of three natural angiotensins, angiotensin I (ANG I 3.8 X 10-11-9.4 X10-10 mol/kg body weight), II (9.6 X 10-12-2.4 X 10-10 mol/kg body weight) and III (2.7 X 10-10 2.5 X 10-9 mol/kg body weight) on systemic blood pressure were investigated in conscious rats. Angiotensin II (ANG II), ANG I and angiotensin III (ANG III), increased blood pressure in a dose-related manner. The order of potency of angiotensins was ANG II greater than ANG I greater than ANG III. The intraventricular administration of a converting enzyme inhibitor (SQ 14225, 6.9 X10-8 mol/kg) abolished the central effect of ANG I, while an angiotensin II analogue ([Sar1-Ala8]ANG II, 1.1 X 10-8 mol/kg) administered intraventricularly inhibited the central pressor effects of these three angiotensins. These results suggest that ANG II is a main mediator of the renin-angiotensin system in the central nervous system.     

The effect of angiotensin II and indomethacin on immunoreactive prostaglandin “A” levels in man

The effect of angiotensin II on peripheral levels of immunoreactive prostaglandin A₂ (IR-PGA) was determined in 17 normal male volunteers. IR-PGA rose from 338 ± 65 (SE) pg/ml to 635 ± 142 in response to pressor infusions of angiotensin II (p <0.05 on paired analysis). This increase was not observed when indomethacin, 75 mg p.o., was given to 8 patients two hours prior to a repeat infusion. Five patients of the original group were placed on a low sodium diet (10–20 mEq). The response to angiotensin was now exaggerated (278 ±52 pg/ml to 916 ± 284). These five patients were kept on a low sodium intake and given indomethacin 50 mg p.o. q 6 hourly for 4 days. There was no significant rise with angiotensin infusion (106 ± 31 pg/ml to 120 ± 70). Pressor infusions of angiotensin II raise peripheral levels of IR-PGA, and this response is exaggerated by a low sodium diet and blocked by either acute or chronic indomethacin administration. This data supports the concept that vasodilatory prostaglandins may be released by endogenous angiotensin and thus provide a dynamic antagonism to the renin angiotensin system in man.     

Enhancement of the pressor response to norepinephrine by angiotensin in the conscious rabbit

The pressor interactions between angiotensin II and norepinephrine were investigated in conscious New Zealand white rabbits receiving a low sodium diet. Angiotensin II was administered continuously by intraperitoneal osmotic pumps in a subpressor dose so as to avoid the potentially confounding effects of experimentally-induced hypertension. Norepinephrine challenges were given as a series of graded intravenous boluses. During the 3 days of study the baseline blood pressure in the angiotensin-treated rabbits (n=10) did not differ from that in controls (n=10) whose intraperitoneal pumps contained only diluent. After 24 hours the systolic and diastolic blood pressure responses to norepinephrine in the angiotensin-treated group were, on average, 45% and 30% higher than in the controls; after 72 hours, they were 46% and 34% higher. Although the pressor amplitudes were increased by angiotensin II, they were not prolonged. Thus, facilitation by the subpressor angiotensin II of the blood pressure responses to norepinephrine did not seem dependent upon alterations in endogenous sympathetic mechanisms or the uptake of norepinephrine; nor could it be explained by sodium retention. It is possible that angiotensin II exhibits its effect by enhancing contractile responsiveness to norepinephrine at the postreceptor level.     

Offset of actions of angiotensin II, angiotensin III, and their analogue antagonists in renal and femoral vascular beds: further evidence for different vascular angiotensin receptors.

Half-time of the offset of antagonist action was used to assess the possibility that factors which determine the duration of action of angiotensin antagonists were responsible for regional differences in their effectiveness: thus, for example, enhanced degradation of angiotensin III analogues in the limb circulation would reduce their effectiveness there despite an angiotensin receptor identical to that in the kidney. In the anesthetized dog blood flow in the renal and femoral vascular beds was measured with an electromagnetic flowmeter; the octapeptide analogue saralasin (1-Sar, 8-Ala AII) and a heptapeptide analogue (des-Asp, 8-Ile AII) were infused intravenously (1 μg/kg/min) for 10 minutes and, after stopping the infusion, the effectiveness of their blockade of angiotensin II was assessed over time. The half-time of offset of the antagonist action was determined from semilogarithmic plots of percent inhibition during recovery. Offset of heptapeptide-induced inhibition in the hindlimb would have been more rapid if increased rate of degradation was the explanation for its reduced effectiveness and such was not the case: Indeed offset was more rapid in the renal (5.8 ± 1.1 min) than the femoral vascular bed (11.7 ± 2.1 min) (p > 0.05). Saralasin showed identical offsets in the two beds (renal 17.2 ± 1.5 min; femoral 15.1 ± 2.9 min) (p > 0.5). Consistent with these observations, the offset of the agonist action of angiotensin III was shorter in the kidney (0.69 ± 0.06 min) than in the limb (1.46 ± 0.13 min; p < 0.001). This study has confirmed the relatively greater efficacy of heptapeptide analogues in the renal than in the femoral vascular bed and has ruled out degradation as accounting for that difference: The difference is most likely to lie in a different angiotensin receptor in the two regions.     

Effects of angiotensin I and II and their interactions with some prostanoids in perfused human umbilical arteries

In perfused human umbilical arteries both angiotensin I and II induced vasoconstriction with a monophasic response. Angiotensin I and II induced vasoconstrictions at doses greater than or equal to 10(-8) M and 10(-9) M respectively. Captopril inhibited the angiotensin I response while the angiotensin II receptor blocker Sar1-Ala8 AII inhibited the effect of both angiotensins. PGI2 attenuated the angiotensin II response in a dose dependent pattern. PGE2 and PGF2 alpha in concentrations below the critical levels for creating pressure responses per se, also attenuated the angiotensin II response. The cyclooxygenase inhibitor indomethacin potentiated the angiotensin II response indicating that endogenous production of prostanoids is of importance in the modulation of angiotensin effects.     

Effect of angiotensin II on RNA synthesis by isolated nuclei

Peptide hormones are known to bind to cell surface receptors as the first step in the generation of their effects on target tissues. However, it remains uncertain whether internalized hormone might also play a role in the production of longterm or trophic effects of peptide hormones. Because the peptide hormone angiotensin II appears to be internalized by target cells, we studied the effect of this peptide on isolated hepatic nuclei. At both 5 X 10(-7)M and 5 X 10(-9)M, angiotensin II significantly increased RNA synthesis. This effect was not mimicked by Sar1-Ala8-angiotensin II (saralasin) or the unrelated nonapeptide teprotide.     

Effects of an angiotensin II antagonist; [sarcosine 1, isoleucine 8] angiotensin II, on blood pressure, plasma renin activity and plasma aldosterone concentration in hypertensive and normotensive subjects taking oral contraceptives.

To examine the involvement of renin-angiotensin-aldosterone system in the etiology of oral contraceptive induced hypertension, normal women (Group I), normotensive (Group II) and hypertensive (Group III) women taking Ovulen (R) were infused with a competitive angiotensin II (AII) antagonist, [1-sarcosine, 8-isoleucine] angiotensin II. The angiotensin II antagonist was infused at a rate of 600 ng/kg/min for 30 min 1.5 hrs after intravenous injection of 40 mg of furosemide. Blood pressure was monitored and pre-infusion and post-infusion plasma renin activity (PRA) and plasma aldosterone concentration (PAC) were determined. Pre-infusion PRA and PAC showed no significant differences among these three groups. In response to the AII antagonist infusion blood pressure rose in Groups I and II, but blood pressure responses in Group III were variable. Four out of the total 6 subjects had pressor responses and only one subject had a significant blood pressure reduction. In both Groups I and II, PRA decreased and PAC rose after infusion of the antagonist. In Group III, PRA decreased to a lesser degree and PAC showed no consistent change. These data suggest that the renin-angiotensin-aldosterone system in hypertensive women or oral contraceptives is different from that of the normotensive users. However, the pathophysiology of oral contraceptive induced hypertension is not homogenous and angiotensinogenic hypertension is uncommon.     

The actions of vasoactive compounds in the foetus and the effect of perfusion through the placenta on their biological activity

In acute experiments on the in utero foetal lamb, angiotensin II was a more potent pressor agent that either noradrenaline or adrenaline, and the response to angiotensin II was not consistently modified by the combined administration of alpha and beta-adrenergic blocking agents. A significant reduction in the pressor response of the foetus to angiotensin II and noradrenaline occurred with infusion of these compounds to the foetus by the umbilical artery when compared with the response obtained with infusions of the same doses of these drugs by the umbilical vein. Moreover, the concentration of angiotensin II (pg. ml-1) present in the foetal circulation was less following umbilical arterial infusions compared with umbilical vein infusions of the same doses. A similar reduction in the pressor activity of adrenaline and the cardio-stimulant effect of isoprenaline occurred when these drugs were infused by the umbilical artery. It is concluded that the foetus, like the adult animal, is more sensitive to angiotensin II than to catecholamines and that the biological activities of noradrenaline, angiotensin II, adrenaline and isoprenaline are reduced by perfusion through the foetal placenta.     

Stimulation of renal gluconeogenesis by angiotensin II.

Renal gluconeogenesis was studied in suspended tubule fragments isolated by collagenase treatment of rat kidney cortices. Angiotensin II increased glucose formation from pyruvate, lactate, and to a lesser extent from oxoglutarate and glutamine, but not from other substrates such as malate, succinate, dihydroxyacetone or fructose. Stimulation was significant with peptide concentration exceeding 1 . 10(-8) M and was also shown with an 8-Sar derivative. Other peptides such as 4-Ala-8-Ile-angiotensin II, hexapeptide and bradykinin had no effect. The stimulatory action of angiotensin II was additive to that of L-lysine, and 3',5'-adenosine cyclic monophosphate, suggesting a different mechanism of action. In the presence of maximally stimulatory concentrations of oleate, phenylephrine and 3',5'-guanosine cyclic monophosphate, however, the stimulatory effect of angiotensin II was absent. Cyclic GMP levels, however, did not increase in tubules after angiotensin II and phenylephrine addition, making a messenger function of this nucleotide unlikely. Omission of Ca2+ from the medium markedly reduced basal gluconeogenesis but did not result in a complete loss of angiotensin II effect. Reduction of medium potassium to 2 mM, however, increased basal gluconeogenesis and blunted the peptide effect. 1 mM ouabain was also able to inhibit the stimulatory effect of angiotensin II. Therefore changes in intracellular potassium levels are discussed as a possible mechanism of angiontensin action, whereas calcium seems not to be specifically linked to this metabolic action of angiotensin on the proximal tubule.