LOW DIHYDROPYRIDINE RECEPTOR DENSITY IN VASA DEFERENTIA OF CASTRATED RATS

Radioligand binding studies in crude membrane preparations of vasa deferentia of normal rats, with the 1,4-dihydropyridine (+)-[H-3]-PN200-110 (isradipine) showed typical saturation isotherms. The binding exhibited a K(D) of 259 +/- 60 pM and B(max) of 144 +/- 20 fmol mg-1 protein. The low K(D) and the stereoselective displacement of (+)-[H-3]-PN200-110 binding by (+)- and (-)-PN200-110 and by nifedipine suggests that these tissues contain dihydropyridine receptors probably coupled to voltage-sensitive, L-type calcium channels. In membrane preparations from vasa deferentia from rats castrated 30 days previously the maximum specific binding was 25 +/- 10 fmol mg-1 protein, representing only 11% of total binding; thus, the calculation of reliable K(D) values was not feasible. These findings suggest that a testicular hormone, possibly testosterone, plays an important role in the regulation of dihydropyridine-sensitive, voltage-dependent calcium channels in the rat vas deferens.