AJP - Gastrointestinal and Liver Physiology, Vol 257, Issue 1 86-G93, Copyright © 1989 by American Physiological Society

ARTICLES

Two calcium channels in basolateral membranes of rabbit ileal epithelial cells

F. R. Homaidan, M. Donowitz, G. A. Weiland and G. W. Sharp
Department of Pharmacology, New York State College of Veterinary Medicine, Cornell University, Ithaca 14853.

The actions of three different types of calcium channel blockers on short-circuit current (Isc) in rabbit ileum were studied. These included the phenylalkylamines, verapamil and (l)-desmethoxyverapamil (D888); the dihydropyridines, nifedipine and nitrendipine; and the benzothiazepine, diltiazem. All of the drugs decreased Isc, a change associated with increased Na and Cl absorption. Verapamil and D888 had the largest effects. The dihydropyridine, BAY K 8644, a calcium channel activator, increased Isc and decreased Na and Cl absorption, effects not inhibited by tetrodotoxin. The phenylalkylamines had an additional effect on Isc in the presence of a maximally inhibitory concentration of the dihydropyridines, suggesting the possibility of two distinct calcium channels, one of which is the L-type voltage-activated, dihydropyridine- and phenylalkylamine-sensitive channel, and the other is a channel only sensitive to phenylalkylamines but not to dihydropyridines. [3H]nitrendipine and [3H]D888 binding to an enriched preparation of basolateral membranes from ileal epithelial cells was characterized. Each ligand bound specifically and saturably to an apparently single population of high-affinity sites with [3H]D888 having three times as many binding sites as [3H]nitrendipine. [3H]nitrendipine binding was partially inhibited by verapamil and D888 and was increased by diltiazem; whereas [3H]D888 binding was inhibited completely by verapamil but only partially by nitrendipine and diltiazem. These transport and binding studies suggest the presence of two types of Ca2+ channels in ileal epithelial cells, one of which interacts with the dihydropyridines, the phenylalkylamines, and the benzothiazepines at three different sites and the other channel that only binds the phenylalkylamines.

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