Activation of ovine fetal adrenal function by pulsatile or continuous administration of adrenocorticotropin-(1-24). II. Effects on adrenal cell responses in vitro.

We have examined 1) the effects of mode of ACTH administration to fetal sheep in vivo on the pattern of corticosteroid output by dispersed adrenal cells in vitro, and 2) the time course of fetal adrenal activation during pulsatile ACTH administration to the fetus. Fetal sheep received the same amount of ACTH either as a continuous infusion (C-ACTH; 0.5 microgram/h) or as 15-min pulses every 2 h for 72 h (P-ACTH). Other fetuses received P-ACTH until labor occurred (mean, 100 h) or saline for 72 or 100 h. Adrenal cells from fetuses that received C-ACTH for 72 h produced more corticosterone from endogenous precursors after the addition of ACTH in vitro and after the addition of 0.3 microM progesterone (P4) or pregnenolone (P5) than cells from fetuses treated with P-ACTH for 72 h. There was no difference in cortisol (F) response between the two groups, although in both groups F output was greater than in controls. Adrenal cells from control fetuses produced more P4 in vitro during incubation with ACTH plus guanosine-5'-(beta, gamma-imido)triphosphate [Gpp(NH)p] or N6,O2'-dibutyryladenosine 3',5'-cyclic monophosphate [(Bu)2cAMP] than after ACTH alone. There was no significant F response to the agonists, and F output was quantitatively less than that of P4. After 72-h P-ACTH, the mean P4 output after ACTH addition was not significantly different from that after ACTH plus Gpp(NH)p or (Bu)2cAMP. By 100 h of P-ACTH, the output of F exceeded that of P4 and was not different in response to ACTH, ACTH plus Gpp(NH)p, or (Bu)2cAMP. After 72 h of P-ACTH, incorporation of exogenous P4, but not P5, 17 alpha-hydroxyprogesterone, or 17 alpha-hydroxypregnenolone, into F was greater than that in cells from control fetuses. By 100 h of P-ACTH, incorporation of all substrates into F was greater than that after P-ACTH for 72 h. We conclude that 1) both C-ACTH and P-ACTH for 72 h increase fetal adrenal responses, but the pattern of corticosteroid output in vitro is determined by the mode of ACTH administration in vivo; 2) adrenal responsiveness is increased further between 72 and 100 h of P-ACTH, and activation probably involves changes in ACTH receptor-GTP coupling as well as enzyme activities on the pathway to F biosynthesis.