Direct inhibitory effect of digitalis on progesterone release from rat granulosa cells

1. Digoxin (10^-7-10^-5 M) or digitoxin (10^-7-10^-5 M) decreased the basal and human chorionic gonadotropin (hCG)-stimulated release of progesterone from rat granulosa cells. 2. Digoxin (10^-5 M) or digitoxin (10^-5 M) attenuated the stimulatory effects of forskolin and 8-bromo-cyclic 3′:5′-adenosine monophosphate (8-Br-cAMP) on progesterone release from rat granulosa cells. 3. Digoxin (10^-5 M) or digitoxin (10^-5 M) inhibited cytochrome P450 side chain cleavage enzyme (cytochrome P450_scc) activity (conversion of 25-hydroxyl cholesterol to pregnenolone) in rat granulosa cells but did not influence the activity of 3β-hydroxysteroid dehydrogenase (3β-HSD). 4. Neither progesterone production nor P450scc activity in rat granulosa cells was altered by the administration of ouabain. 5. Digoxin (10^-5 M) or digitoxin (10^-5 M), but not ouabain, decreased the expression of P450scc and steroidogenic acute regulatory (StAR) protein in rat granulosa cells. 6. The present results suggest that digoxin and digitoxin decrease the progesterone release by granulosa cells via a Na⁺,K⁺-ATPase-independent mechanism involving the inhibition of post-cyclic AMP pathway, cytochrome P450scc and StAR protein functions.