KCl-induced insulin secretion from RINm5F cells is mediated through Ca2+ influx along L-type Ca2+ channels

In order to characterize the voltage-dependent Ca2+ channels of insulin secretory RINm5F cells, we have studied the binding of the dihydropyridine (DHP) type Ca2+ antagonist PN 200-110 and its effect on insulin release. In the membrane preparation from RINm5F cells [3H]-(+)-PN 200-110 bound to a high affinity binding site in a stereoselective manner (KD: 7.0 nM, Bmax: 858 fmol/mg protein). The benzothiazepine type Ca2+ antagonist D-cis-diltiazem increased the binding of [3H]-(+)-PN 200-110 in a temperature-dependent manner. The phenylalkylamine-type Ca2+ antagonist verapamil decreased PN binding with an IC50 of 100 microM. (+)-PN 200-110 inhibited KCl-(25 mM)-induced insulin release (IC50 = 10 nM). Effects on binding and hormone release occurred over comparable concentration ranges: 1 microM PN 200-110 produced 100% displacement and totally abolished depolarization-mediated insulin release. The N-type Ca(2+)-antagonist omega-conotoxin showed no effect on KCl-induced insulin release. The data suggest that in RINm5F cells only L-type Ca2+ channels are involved in the mechanism of depolarization-mediated insulin release.