SR33557, an indolizinsulfone blocker of Ca2+ channels: identification of receptor sites and analysis of its mode of action.

SR33557 belongs to a new class of molecules (indolizinsulfones) that act on the same receptor complex that has been characterized for other classical calcium channel effectors. The main binding properties of SR33557 to rabbit skeletal muscle are as follows. (i) Unlabeled SR33557 completely inhibits the specific binding of all classes of calcium channel antagonists such as dihydropyridines [(+)-[3H]PN200-110], phenylalkylamines ([3H] verapamil), benzothiazepines (d-(cis)-[3H]diltiazem), and diphenybutylpiperidines ([3H]fluspirilene). In all these cases inhibition of binding is of a noncompetitive nature. (ii) [3H]SR33557 binds with high affinity to T tubule membranes (KD = 0.08 nM) and the maximum binding capacity (Bmax = 78 pmol/mg of protein) is the same as that found for other classes of Ca2+ channel antagonists. Photoaffinity labeling confirms that [3H]SR33557 associates with the same protein of Mr 165,000 that binds the classical calcium channel inhibitors. 45Ca2+ uptake experiments performed with the rat aortic cell line A7r5, the insulin-secreting cell line RINm5F, and the pheochromocytoma cell line PC12 demonstrate that SR33557 fully inhibits the 1,4-dihydropyridine-sensitive 45Ca2+ uptake elicited by depolarization. A very good correlation was found between inhibition of 45Ca2+ uptake and of [3H]dopamine release in PC12 cells and between inhibition of 45Ca2+ uptake and of L-type Ca2+ current in A7r5 cells under whole-cell patch-clamp conditions.