The influence of plasma on basal and ACTH-stimulated in vitro adrenocortical steroidogenesis

Early descriptions of in vitro ACTH bioassays all emphasised the need to us e extracted plasma samples due to interference by an unidentified component . The aim of these studies was to elucidate the effects of whole plasma on ACTH steroidogenic activity in vitro and to identify the responsible factor . A sensitive in vitro dispersed bovine adrenocortical cell bioassay was es tablished. The addition of 10% ACTH-depleted human pooled plasma to the inc ubation media resulted in basal steroidogenesis equivalent to that achieved with 10(-9) M ACTH(1-24) and potentiated the steroidogenic activity of 10( -9) M ACTH(1-24) by 7.8-fold. This potentiation was dependent on the concen tration of both ACTH and plasma in the media, but did not result from the m itogenic effect of plasma. A pituitary source was excluded and the potentia ting activity was not extractable by Vycor glass. Column chromatography dem onstrated two peaks of activity corresponding to molecular weights of 650 a nd 220 x 10(3) Da. These peaks did not correspond to the plasma binding of I-125-ACTH which resulted from non-specific binding to albumin. Lipoprotein -deficient serum had no effect on either basal or ACTH-stimulated steroidog enesis, but both were restored by the addition of purified lipoproteins. Ho wever, novel findings demonstrated a differential effect of low (LDL) and h igh (HDL) density lipoproteins on basal and ACTH-stimulated steroid product ion; thus, LDL exerted a greater effect an the former, whilst HDL potentiat ed the steroidogenic activity of added ACTH more than LDL. The addition of the lipoproteins to lipoprotein-deficient serum restored its basal and ACTH potentiating effects, the cholesterol concentrations of the chromatographi c fractions exactly paralleling their ACTH potentiating effect. These findi ngs suggest that not only are lipoproteins the plasma factor(s) which poten tiates ACTH steroidogenic activity in in vitro bioassays, but also that the y exert differential effects on basal and ACTH-stimulated steroid productio n.