Abstract
Previous studies have indicated that digoxin (DG) inhibits testosterone production by rat testicular interstitial cells through both in vivo and in vitro experiments. DG and digitoxin (DT), but not ouabain, inhibit the progesterone, pregnenolone, and corticosterone secretion by rat granulosa cells, luteal cells, and zona fasciculata‐reticularis (ZFR) cells, respectively. However, the effect of DG and DT on the enzyme kinetics of cytochrome P450 side chain cleavage enzyme (P450scc), the protein expression of P450scc and steroidogenic acute regulatory protein (StAR), and mRNA expression of StAR are unclear. ZFR cells were prepared from adrenocortical tissues of ovariectomized rats, and then challenged with adrenocorticotropin (ACTH), 8‐Br‐cAMP, forskolin, A23187, cyclopiazonic acid (CPA), nicotinic acid adenine dinucleotide phosphate (NAADP), trilostane, 25‐OH‐Cholesterol, progesterone, or deoxycorticosterone in the presence of DG, DT, or ouabain for 1 h. Enzyme kinetics of P450scc, protein expression of acute regulatory protein (StAR) and P450scc, and mRNA expression of StAR were investigated. DG and DT but not ouabain suppressed basal and other evoked‐corticosterone release significantly. DG and DT also inhibited pregnenolone production. The V~max~ of the DG and DT group was the same as the control group, but the K~m~ was higher in DG‐ and DT‐treated group than in control group. DT and ouabain significant suppressed mRNA expression of StAR. DG and DT had no effect on the P450scc and StAR protein expression at basal state, but diminished ACTH‐induced StAR protein expression to basal level. These results indicated that DG and DT have an inhibitory effect on corticosterone production via a Na^+^, K^+^‐ATPase‐independent mechanism by diminishing actions on cAMP‐, Ca^2+^‐pathway, competitive inhibition of P450scc enzyme and reduction of StAR mRNA expression. J. Cell. Biochem. © 2005 Wiley‐Liss, Inc.