Inhibitory Effects of Digoxin on Testosterone Production in Rat Leydig Cells

Abstract

Digoxin has beneficial effects on heart failure due to a direct positive inotropic action as an inhibitor of Na^+- K^+-ATPase and endogenous digoxin-like immunoreactive factors (DLIFs) found in mammalian blood also correlates to the same issue. We have demonstrated that digoxin inhibits rat testosterone secretion but the mechanisms remain unclear. In the present study, the direct effects of digoxin on the expression of steroidogenic acute regulatory protein (StAR) and cytochrome P450 side chain cleavage enzyme (P450scc) in rat Leydig cells were evaluated. Digoxin inhibited not only testosterone production after incubation of Leydig cells with or without human chorionic gonadotropin (hCG), forskolin, 8-Br-cAMP, but also pregnenolone production following stimulation of 25-hydroxy-cholesterol, suggesting that digoxin reduced adenylyl cyclase and P450scc activity. Digoxin significantly reduced the expression of StAR protein, but not that of P450scc. The stimulation of testosterone production induced by A23187 was decreased by digoxin indicating that digoxin might influence Ca^2+ utilization. The analog of digoxin, ouabain, did not affect testosterone production in rat Leydig cells, which indicated the drug specificity. These results suggested that, in rat Leydig cells, digoxin directly inhibited testosterone production partly by reducing P450scc activity, StAR protein expression, and Ca^2+ utilization. This report not only demonstrates the mechanism digoxin acts, but also implies a probable interaction between gonadal and adrenal axis linked by DLIFs (i.e. digoxin).