On the Role of Calcium in Adrenocorticotropin-Induced Changes in Mitochondrial Pregnenolone Synthesis*

Abstract

The importance of Ca in the ACTH-induced increase in adrenal mitochondrial pregnenolone synthesis was evaluated. In mitochondria prepared in the absence of EDTA and albumin, Ca enhanced the binding of cholesterol to cytochrome P-450 and subsequent pregnenolone synthesis. Although these effects of Ca were slightly greater in control than in ACTH-treated mitochondria, a sizeable effect of ACTH remained even at high Ca levels (500 .mu.M). In mitochondria prepared from adrenals homogenized in fluid containing EDTa and albumin, ACTH-induced effects on pregnenolone synthesis were relatively poor unless Ca was added to the incubation mixture. High concentrations of added Ca (500 .mu.M or greater) obviated the need for the labile protein required for ACTH-induced effects in intact mitochondria, presumably by disrupting mitochondria and allowing an unrestrained interaction of cholesterol with cytochrome P-450. Cholesterol-rich mitochondria from ACTH plus cycloheximide-treated rats produced large amounts of pregnenolone when high (probably unphysiological) Ca concentrations were present. Ca may be required at the mitochondrial level for ACTH-induced effects on pregnenolone synthesis, and the reported ACTH-induced increase in intraadrenal Ca may amplify the effects of ACTH on steroidogenesis. It seems unlikely that Ca is the agent primarily responsible for mediating the ACTH-induced steroidogenic effect at the mitochondrial level.