Adrenal cholesterol esters as substrate source for steroidogenesis.

Adrenocortical cells obtained from tissues of unstimulated rats and which contained a high concentration of endogenous esterified cholesterol, were labeled in vitro with unesterified [4-14C]cholesterol, or incubated in the presence of [2-14C]acetate or lipoprotein [4-14C]cholesterol oleate (LP-CE). Incubations were conducted in the absence and presence of ACTH, and changes in the specific radioactivity (SA) of the secreted corticosterone were used to assess the primary sources of cholesterol substrate used for steroidogenesis. Incubations of cells containing [4-14C]cholesterol with ACTH resulted in a marked increase in the output of corticosterone mass, but not of labeled corticosterone. Thus, the SA of corticosterone when cells were incubated with ACTH was only 6.5% of that obtained from cells incubated in the absence of ACTH. During incubations with [2-14C]acetate, the ACTH-induced increase in the output of corticosterone mass was not associated with increased isotope output, and the SA of corticosterone was only 15% of that in control incubations. This dilution was not altered in cells isolated from adrenals of rats treated with 4-aminopyrazalopyrimidine (4-APP), in which increased cholesterogenesis was demonstrable. The uptake, and hydrolysis of LP-CE, and formation of labeled corticosterone was lipoprotein concentration dependent, and was not influenced by ACTH. However, in the presence of ACTH, the SA of the secreted corticosterone was only 4-8% of that in unstimulated cells. The consistent dilution of the SA of corticosterone in ACTH-treated cells in all studies suggest that the large stores of cytoplasmic cholesterol esters in these cells may normally serve as a primary source of the immediate precursor sterol used for steroidogenesis.