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Proliferation of smooth muscle cells within the intima plays a key role in vascular occlusive disorders such as atherosclerosis and restenosis following balloon angioplasty. Among the factors that may be important in the development of vascular lesions, several authors have reported that the local angiotensin system participates in modulating the proliferation of smooth muscle cells after arterial injury. This study was therefore designed to characterize the antagonistic properties and to investigate the antiproliferative effect of a newly developed non-peptide angiotensin II AT1 receptor antagonist, SR 47436. This compound is a potent and competitive antagonist of the binding of [125I]angiotensin II to its receptor on cultured human aortic smooth muscle cells, exhibiting an IC50 value of 1.7 +/- 0.6 nM. SR 47436 was 10-fold more potent than DuP 753 (Losartan) (IC50 = 20.8 +/- 3.7 nM). In these same cells, SR 47436 potently inhibited the angiotensin II-induced [Ca2+]i increase (IC50 = 0.53 +/- 0.13 vs. 7.4 +/- 1.3 nM for DuP 753). Angiotensin II is a potent mitogen for human aortic smooth muscle cells in culture, exhibiting a maximum proliferative response at 1 microM. SR 47436 and Losartan prevented angiotensin II-induced proliferation of these cells in a dose-dependent manner (IC50 = 0.32 +/- 0.09 and 0.71 +/- 0.08 microM, respectively). SR 47436 displayed a marked in vitro inhibition of serum-induced smooth muscle cell proliferation (IC50 = 5.5 +/- 0.8 microM). A selective AT2 receptor antagonist, PD 123177 did not affect angiotensin II-induced responses in these cells.(ABSTRACT TRUNCATED AT 250 WORDS)

[3H]SR 47436, a selective and potent novel non-peptide antagonist of angiotensin receptors, was used to characterize the cardiac angiotensin AT1 receptors. In neonatal rat heart cells, Scatchard analysis showed a single class of high affinity binding sites (Kd = 0.24 nM, Bmax = 28 fmol/mg protein). The binding was saturable, reversible and prevented by angiotensin II and the AT1 subtype receptor antagonist DuP 753 whilst unaffected by the AT2 receptor antagonist PD123177. In the same cells, angiotensin II induced a twofold increase in the intracellular free Ca2+ concentration ([Ca2+]i), with a half-maximal effect (EC50) at 14 nM. This increase was prevented by SR 47436 (IC50 = 1.03 nM) and by the AT1 receptor antagonist DuP 753, but at higher concentrations (IC50 = 15.6 nM) and was unaffected by PD123177. These data directly demonstrate the presence of cardiac AT1 receptors in rat neonatal cardiomyocytes and confirm the involvement of AT1 receptors in cardiac Ca2+ homeostasis.

The possible interaction between the antianginal and antiarrhythmic drug amiodarone and the slow calcium channel was investigated by competition binding experiments in guinea-pig cerebral cortex and rat heart membranes using [3H]nitrendipine as radioligand. Amiodarone displaced specifically bound [3H]nitrendipine from cerebral cortex and cardiac membranes in an apparently competitive manner. In saturation binding experiments, apparent affinity for [3H]nitrendipine progressively decreased with increasing concentrations of amiodarone, whereas maximal binding capacity (Bmax remained unchanged. Both diltiazem and verapamil reversed the inhibitory effect of amiodarone on [3H]nitrendipine binding to cerebral cortex membranes. Together these results suggest that amiodarone exerts a pseudocompetitive inhibition on [3H]nitrendipine binding by acting at a site in allosteric interaction with the 1,4 dihydropyridine binding site associated with the calcium channel. The data are compatible with the existence of a common binding site for diltiazem, verapamil, and amiodarone. These observations are discussed in connection with the pharmacological properties of the drug.