On the biopotency and site of action of drugs affecting endocrine tissues with special reference to the anti-steroidogenic effect of anaesthetic agents.

Dispersed guinea-pig adrenal cells or mouse Leydig cells were stimulated with a saturating dose of adrenocorticotrophin (ACTH, 50 ng/1) or luteinizing hormone (LH, 5IU/1), respectively. The incubations were performed in the presence of increasing concentrations (10(-9) - 5 X 10(-4)mol/l) of the anaesthetic agents propofol, thiopentone and etomidate. At the end of this stimulation period, cortisol (from the adrenal preparation) or testosterone (from the Leydig cell culture) were assayed by radioimmunoassay. Propofol, thiopentone and etomidate all inhibited ACTH-stimulated cortisol secretion in a dose-related fashion. Similar inhibition of LH-stimulated testosterone output was found with propofol and thiopentone whereas etomidate was without effect at any concentration employed, an observation in accordance with its known site of action, 11 beta-hydroxylase, an enzyme which is not involved in the biosynthesis of testosterone. The concentration (mumol/l) of anaesthetics which gave 50% inhibition (ED50) of ACTH-stimulated cortisol secretion was 0.1 +/- 0.002 (n = 7), 160 +/- 18 (n = 3) and 170 +/- 18 (n = 3) (mean +/- s.e.m.) for etomidate, thiopentone and propofol, respectively. The corresponding values for the LH stimulated testosterone output from the Leydig cell preparations were 186 (thiopentone) and 180 (propofol) mumol/l. In a separate series of experiments adrenal cells were stimulated with (a) the cortisol precursor steroids (all at 10(-5)mol/l) pregnenolone, 17-hydroxypregnenolone, progesterone, 17-hydroxyprogesterone and 11-deoxycortisol, (b) dibutyryl cAMP (10(-3)mol/l) or (c) ACTH (100 ng/l) in the presence and absence of either etomidate (5 X 10(-5)mol/l), propofol (2.5 X 10(-4)mol/l) or thiopentone (5 X 10(-4)mol/l). All the stimulators increased cortisol production by > 7-fold over that seen in their absence. Propofol depressed ACTH and dibutyryl cAMP induced cortisol output by > 60% (P < 0.05) but was without effect when the steroid precursors were used, suggestive of an inhibition between the sequence involving ACTH binding -> pregnenolone production. In contrast, etomidate and thiopentone reduced cortisol secretion by > 40% (P < 0.05) regardless of the stimulator used, indicating that at least one site of action was at the level of the final enzymic step of cortisol synthesis, i.e. 11beta-hydroxylase.