Calciseptine binding to a 1,4-dihydropyridine recognition site of the L-type calcium channel of rat synaptosomal membranes.

Calciseptine (CaS) is a natural peptidic L-type Ca2+ channel blocker consisting of 60 amino acids with four disulfide bonds. The effects of synthetic CaS on the binding of various ligands to Ca2+ channels of rat brain synaptosomal membranes were studied. The membranes possessed specific binding sites for L-type Ca2+ channel ligands [3H]nitrendipine, [3H]diltiazem and [3H]verapamil, derivatives of 1,4-dihydropyridine, benzothiazepine and papaverine, respectively, and also for N-type Ca2+ channel ligand omega-[125I]-conotoxin GVIA (omega-[125I]CTX). Lineweaver-Bulk plot analysis disclosed that CaS competitively inhibited the binding of [3H]nitrendipine, with maximal binding capacity of 0.19 pmol/mg protein and dissociation constant (Kd) of 290 nM, being about 10(3) times the Kd value of [3H]nitrendipine. Similar to nitrendipine, CaS noncompetitively enhanced the binding of [3H]diltiazem, but did not affect the binding of [3']verapamil. CaS at up to 10.0 microM did not affect the binding of omega-[125I]CTX. These observations indicate that CaS shares the properties of 1,4-dihydropyridine derivatives, and allosterically modulates the binding of other L-type Ca2+ channel ligands.