Voltage-dependent Ca2+ channels are involved in regulation of steroid synthesis by bovine but not rat fasciculata cells

Abstract

Bovine but not rat fasciculata cells show a concentration-dependent stimulation of the production of corticosteroids by addition of external Ca2+ to the incubation medium. Both cell types respond to ACTH in a concentration-dependent manner. Increasing concentrations of K+ (0-20 mM) cause increased production of corticosteroids and accelerated influx of Ca2+ in bovine fasciculata cells but no change in either of these two parameters in rat fasciculata cells. Two inhibitors of Ca2+ channels (nifedipine and PY108-068) inhibit both the production of steroids by unstimulated bovine cells and the stimulation produced by three agents (ACTH, (Bu)2 cAMP, and K+). Half-maximal inhibition of these responses was produced in each case by submicromolar or low micromolar concentrations of the inhibitors. These inhibitors are without effect in rat fasciculata cells. One Ca2(+)-channel agonist (BAY K8644) stimulated synthesis of steroids by bovine cells and potentiated the response to ACTH. The agonist acts in the low micromolar range but was without effect on rat cells or on the responses of steroid synthesis by these cells to either ACTH or dibutyryl cAMP. Moreover, bovine fasciculata cells show specific binding sites for [+]PN 200-110, a specific ligand for 1,4-dihydropyridine receptors associated with voltage-dependent Ca2+ channels [dissociation constant (Kd), 14.3 nM; maximum number of binding sites (Bmax), 0.52 pmol/10(6) cells]. Rat cells show no specific binding of PN 200-110. We conclude that bovine fasciculata cells possess voltage-dependent Ca2+ channels which are involved in the regulation of steroid synthesis in these cells and that rat fasciculata cells are without such channels.