Testosterone sensitive dihydropyridine binding in the Harderian gland of the male hamster

It is well known that in different tissues, dihydropyridines bind at nanomolar concentrations to a receptor and block voltage-operated Ca2+ channels. In studies reported here, Harderian gland tissue homogenates from intact male hamsters exhibited significant dihydropyridine binding (Bmax = 1700 fmoles/mg protein) of high affinity (Kd = 1.1 nM). Tissue homogenates from female animals exhibited a similar Kd value (1.35 nM) but receptor density per mg protein was significantly reduced (Bmax = 270 fmoles). Dihydropyridine binding of Harderian gland tissue homogenates from castrated males was reduced greater than 80% (Bmax = 225 fmoles/mg protein). Treatment of castrated males with subcutaneous testosterone pellets resulted in significant restoration of dihydropyridine binding activity (approximately 80%, Bmax = 1630 fmoles/mg protein) with a comparable binding constant (Kd = 1.50 nM) as observed for noncastrated, control animals. Addition of testosterone (ex vivo) to homogenates from castrated hamsters did not restore dihydropyridine binding to control levels. These data indicate: (a) the Harderian gland from male hamsters exhibits significant dihydropyridine binding; (b) ligand binding is abolished following castration; and (c) significant restoration of dihydropyridine binding occurs following in vivo testosterone treatment. The dependence of dihydropyridine binding restoration upon in vivo steroid hormone administration suggests probable involvement of the steroid at the transcriptional level although non-genomic mechanisms such as the binding of testosterone to a receptor resident in the plasma membrane and subsequent activation of Ca2+ channels can not be ruled out.