Maintenance or stimulation of steroidogenic enzymes and testosterone production in rat Leydig cells by continuous and pulsatile infusions of luteinizing hormone during passive immunization against gonadotrophin-releasing hormone
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Summary. The importance of the pulsatility of luteinizing hormone (LH) secretion in maintaining key enzymes in the testosterone biosynthetic pathway in Leydig cells was studied using rats in which LH secretion was suppressed by passive immunization against gonadotrophin-releasing hormone (GnRH) and replaced by continuous or pulsatile i.v. infusions of exogenous LH, all delivering the same daily dose of the hormone (300 ng per 100 g NIDDK-ovine LH-24). Continuous infusions (12·5 ng per 100 g h−1) were compared with infusions of 1 min pulses every 2 h (25 ng per 100 g) and every 4 h (50 ng per 100 g). After 5 days of treatment in vivo with sheep anti-GnRH serum (or normal sheep serum) and LH (or vehicle), Leydig cells were purified and assayed in vitro for maximum production of testosterone stimulated by human chorionic gonadotrophin (hCG) and supported by 25-hydroxycholesterol and for the activities of cholesterol side-chain cleavage, Δ5-3β-hydroxysteroid dehydrogenase-Δ5–4-isomerase (3β-HSD-isomerase) and 17α-hydroxylase. Relative contents of cholesterol side-chain cleavage and 17α-hydroxylase were also quantified by western and immunoblotting analysis. Activity of 3β-HSD-isomerase was reduced by about 40% by anti-GnRH treatment and was increased by all LH regimens in anti-GnRH-treated animals, with no consistent pattern in the effects of the different LH regimens. Results for testosterone-producing capacity and the other two enzymes differed in several respects. Treatment with anti-GnRH serum markedly reduced basal, hCG-stimulated and 25-hydroxycholesterol-supported testosterone production (by 80–90%) and the activities of cholesterol side-chain cleavage (about 80%) and 17α-hydroxylase (about 65%). Infusion of exogenous LH in any of the regimens tested prevented these changes or increased the activities to values greater than those in normal serum-treated controls. Differences in immunodetectable contents of the two enzymes generally paralleled those in enzyme activities. There was a consistent trend in the effects of LH replacement regimens on these parameters of steroidogenic activity: continuous infusions were more effective than pulses at 2 h intervals and these in turn were more effective than pulses at 4 h intervals, suggesting that the frequency of LH exposure is more important than the amplitude of individual exposures in maintaining Leydig cell steroidogenic function. Consistent differences among groups in hCG-stimulated relative to 25-hydroxycholesterol-supported testosterone production suggest that some constituents of Leydig cells prior to cholesterol side-chain cleavage enzyme are more sensitive to LH withdrawal and deviations from 'optimal' LH exposure than are the side-chain cleavage and subsequent enzymes in the testosterone biosynthetic pathway.
Keywords: Leydig cells; testosterone; steroidogenesis; P-450; luteinizing hormone; rat
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