Influence of season and low-level oestradiol immunoneutralization on episodic LH and testosterone secretion and testicular steroidogenic enzymes and steroidogenic acute regulatory protein in the adult ram

The regulation of LH-dependent and -independent increases in testosterone s ecretion by key proteins in the testes of adult rams was investigated. Seri al blood samples were collected from groups of four control and passively i mmunized (oestradiol antiserum for 3 weeks) rams and the animals were gonad ectomized in either the non-breeding season (April) or the breeding season (September). LH pulse frequency and basal (interpulse) concentrations were several times greater (P < 0.01) in the breeding season than in the non-bre eding season. Neither of these parameters nor LH pulse amplitude were affec ted by oestradiol immunization. Parameters of testosterone episodic secreti on and response to an injection (i.v.) of 15 mu g NIH-LH-S25 were also grea ter (P < 0.05) in the breeding season and, with the exception of pulse freq uency, in immunized rams versus controls. Substrate utilization established that testosterone biosynthesis was predominantly via the 5-ene pathway. In creases in blood testosterone concentration in the breeding season were ass ociated with a fivefold higher (P < 0.01) activity of cytochrome P450 17 al pha-hydroxylase/C-17,20 lyase (P450(17 alpha)) and a 65% higher (P < 0.05) relative amount of mRNA for cytochrome P450 cholesterol side-chain cleavage enzyme complex (P450scc) in the testis. Of the steroidogenic enzyme activi ties examined, only that for 17 beta-hydroxysteroid dehydrogenase (17 beta- HSD) tended to be increased by oestradiol immunization. Blood concentration s of cholesterol lipoproteins and expression of the testicular low density lipoprotein receptor were not affected by season or immunization. The amoun t of steroidogenic acute regulatory protein (StAR) mRNA was 65% higher (P < 0.01) in the breeding season and 20% higher (P < 0.01) in immunized rams v ersus controls. These results indicate that greater LH stimulation may incr ease testosterone biosynthesis in the breeding season by increasing StAR mR NA (and presumably delivery of cholesterol to P450scc) and the activity of P450(17 alpha), and possibly that of P450scc (activity not measured). More moderate increases in StAR mRNA and 17 beta-HSD activity may explain, in pa rt, the increases in testosterone secretion with oestradiol immunization.