The action of angiotensin on the isolated perfused cat heart

Tissue-specific modulation of angiotensin-converting enzyme (ACE) in hyperthyroidism

We have previously demonstrated the interaction between the RAS and thyroid hormones (TH). The present study was designed to determine the role of TH in the local regulation of ACE activity and expression in different tissues. Adult male Wistar rats were randomized into three groups: T4-25 and T4-100 (0.025 and 0.100mg/kg of body weight/day of l-thyroxine for 14 days, respectively) and control. Hemodynamic parameters as well as cardiac and renal hypertrophy were evaluated. ACE activity and mRNA levels were determined by Fluorimetric and Northern blot assays, respectively. Both doses increased SBP and HR, as well as inducing cardiac and renal hypertrophy. Pulmonary and serum ACE levels were comparable among the groups. Both doses promoted increased renal ACE activity and expression but surprisingly ACE was diminished in the heart in both hyperthyroid groups. This change was mediated by a tissue-specific transcription mechanism.

Regulation by angiotensin II of phosphate transport in cardiac myocytes

The purpose of this study was to test the hypothesis that angiotensin II (ang II) affects the transport of inorganic phosphate in adult ventricular myocytes. Ventricular myocytes were isolated from Dahl rats and allowed to take up 32P inorganic phosphate (32P-Pi). The intracellular 32P concentration increased rapidly and reached a peak at 5 min. Ang II, 10(-10) to 10(-5) M, produced a significant (P less than 0.05) and concentration-dependent reduction in 32P-Pi uptake that plateaued at 0.1 to 1.0 microM. Ang II at 1 microM produced a 30% reduction in Vmax and a slightly greater reduction in the Km of 32P-Pi uptake, compared to myocytes that were not exposed to ang II. The ang II receptor antagonist saralasin (Sar1-Val5-Ala8-angiotensin II), significantly (P less than 0.05) antagonized the action of ang II on 32P-Pi uptake. TPA (12-O-tetradecanoylphorbol-13-acetate) also produced a significant (P less than 0.05) reduction of 32P-Pi uptake, suggesting that protein kinase C is involved in the transduction of ang II effects on intracellular Pi. 32P Efflux from myocytes, pulsed with 32P-Pi and chased with Pi-free medium, was accentuated markedly by ang II; this effect was blunted by saralasin. These data suggest that ang II is capable of regulating total intracellular Pi in the heart via two actions: (i) by inhibiting uptake of Pi into the myocyte, and (ii) by increasing the efflux of phosphates out of the cell.

Does endocardial endothelium mediate positive inotropic response to angiotensin I and angiotensin II?

The positive inotropic response to angiotensin I and II in cardiac tissue of most mammalian species, as well as the exact site in the heart for conversion of local and systemic angiotensin I into angiotensin II, remains to be elucidated. In isolated cat papillary muscles, angiotensin I and angiotensin II (0.1 nM to 1 microM, 35 degrees C, 1.25 mM Ca2+) increased, in a dose-dependent manner, peak twitch tension with typical slight prolongation of twitch duration. This typical response did not necessitate the presence of an intact endocardial endothelium (EE), as a similar response was observed in muscles where the EE had been damaged by a 1-second exposure to 0.5% Triton X-100. After addition of captopril, an angiotensin converting enzyme inhibitor, the positive inotropic response to angiotensin I was completely abolished, both in the presence and the absence of an intact EE. Hence, the heart possesses angiotensin converting enzyme, which mediates the positive inotropic response to angiotensin I. An intact EE was not a prerequisite for this response; thus, myocytes as well as nonmyocytes may be possible locations (in addition to the EE) for angiotensin converting enzyme. In the presence of an intact EE, and after addition of captopril, the positive inotropic response to angiotensin II was significantly diminished (desensitization). By contrast, in the absence of an intact EE, but also after addition of captopril, the positive response to angiotensin II was potentiated (sensitization). Both desensitization and sensitization (in the presence or absence of an intact EE, respectively) of the response to angiotensin II induced by the addition of captopril were inhibited by indomethacin, a cyclooxygenase inhibitor, suggesting a role for prostaglandins.

Angiotensins and the failing heart. Enhanced positive inotropic response to angiotensin I in cardiomyopathic hamster heart in the presence of captopril

We examined the hypothesis that the positive inotropic effect of angiotensin I (Ang I) may be retained in the presence of angiotensin converting enzyme inhibitors so that it may have a direct beneficial effect on the heart. Accordingly, isolated perfused hearts (Langendorff preparation) of 300-day-old cardiomyopathic hamsters (a model of spontaneous cardiomyopathy) and age-matched normal hamsters (controls) were infused with Ang I in the presence of captopril; propranolol was added to the perfusing medium to block catecholamine-mediated effects of angiotensins on the heart. Left ventricular developed pressure and the rate of increase in left ventricular developed pressure increased significantly (p less than 0.001) in both the cardiomyopathic and the normal hamster heart despite concomitant reduction in myocardial flow rate favoring a direct inotropic effect of Ang I in both normal and myopathic hearts; these changes were significantly higher by almost threefold in the cardiomyopathic than in the normal hamsters (p less than 0.01) and were blocked by the angiotensin II (Ang II) antagonist [Sar1,Thr8]Ang II. Comparing dose-left ventricular contractility response curves for Ang I and Ang II, ED50 for responses was identical in both normal and myopathic hearts, whereas peak responses to Ang II were double those to Ang I in normal hearts but were almost identical in the myopathic hearts. Binding of [125I]Ang II in six cardiomyopathic and four normal hamster hearts was of high affinity, but there was no evidence for Ang I-saturable high-affinity binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of atriopeptin and angiotensin on the rat right ventricle

1. Atriopeptin had no effect, whereas angiotensin increased the force of the electrically stimulated ventricle muscle. 2. Atriopeptin and angiotensin had no effect on the combined response to muscle stimulation and isoprenaline. 3. Following incubation of the ventricle with [3H]noradrenaline, atriopeptin and angiotensin had no effect on the spontaneous or nerve-evoked outflow of radioactivity.

Role of the endothelium on the facilitatory effects of angiotensin I and angiotensin II on noradrenergic transmission in the caudal artery of the rat

Perfusion of carbogen gas through the lumen of the rat caudal artery abolished the dilator response to acetylcholine (1 mumol 1(-1) in artery segments which had been precontracted with noradrenaline (50 nmol 1(-1]. Hence, it was assumed that gas perfusion was effective in removing the vascular endothelium. Angiotensin I (30 nmol 1(-1] and angiotensin II (10 nmol 1(-1] enhanced the responses of artery segments to field stimulation of their sympathetic nerves (0.5 Hz, 10 s). In arteries with an intact endothelium the ability of each peptide to enhance responses to stimulation was the same whether applied through the lumen or to the adventitial surface. Removal of the endothelium, by gas perfusion, did not significantly alter the facilitatory effects of extraluminally or intraluminally applied angiotensin I or angiotensin II. The converting enzyme inhibitor enalaprilat was equally effective in inhibiting the facilitatory effect of angiotensin I in the presence and absence of an intact endothelium. It is concluded that in the rat caudal artery, conversion of angiotensin I to angiotensin II does not depend on an intact endothelium and that the facilitatory effect of angiotensin II on noradrenergic neuroeffector transmission is not modified by, or dependent on, an intact endothelium.

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.

Effects of locally generated angiotensin II on noradrenergic transmission in guinea-pig isolated atria

The extent to which cardiac neuroeffector function may be modulated by angiotensin II (AII) generated locally from angiotensin I (AI) was investigated in guinea-pig spontaneously beating isolated atria radiolabelled with [3H]noradrenaline. AI and AII were equipotent in increasing the rate and force of atrial contractions with a threshold concentration between 0.1 and 1.0 nM. In contrast, AI was approximately twenty times less potent than AII in enhancing the stimulation-induced (S-I) efflux of radioactivity. The actions of AI and AII on rate and force of atrial contractions, as well as those on S-I efflux of radioactivity, were substantially blocked by the AII receptor antagonist saralasin. On the other hand, the converting enzyme inhibitors captopril and MK-422 selectively blocked the actions of AI without affecting those of AII. These results suggest that significant local conversion of AI to AII occurs in guinea-pig isolated atria. This locally generated AII can act on myocardial AII receptors to increase rate and force of atrial contractions, and on AII receptors located on sympathetic nerve terminals to facilitate sympathetic neurotransmission. The lower potency of AI compared to AII in modulating transmitter release, in contrast to the equal potency of the peptides in mediating chronotropic and inotropic responses, may be due to conversion of AI to AII at sites remote from the neuroeffector junction.

Facilitation of noradrenergic transmission by locally generated angiotensin II in guinea-pig isolated atria and in the perfused caudal artery of the rat

In guinea-pig isolated atria, angiotensin I and angiotensin II produced concentration dependent increases in the rate of spontaneous beating and in the release of noradrenaline produced by field stimulation of sympathetic nerves. In rat isolated caudal artery preparations, both angiotensin I and angiotensin II had direct vasoconstrictor actions and also produced concentration dependent increases in constrictor responses to periarterial sympathetic stimulation. All the above effects of angiotensin I and angiotensin II were blocked by the receptor antagonist saralasin, but only those of angiotensin I were blocked by the converting enzyme inhibitor enalaprilat (MK-422). The findings confirm that angiotensin II generated locally from precursor angiotensin I within cardiac and vascular tissues may modulate noradrenergic transmitter release.