The effects of age on the postreceptor regulation of luteinizing hormone secretion by gonadotropin-releasing hormone

Abstract

We studied the effects of age on the roles of phosphoinositide (PI) and protein kinase C (PKC) in luteinizing hormone (LH) release by gonadotropin-releasing hormone from mouse pituitaries. Pituitary cells from intact and 14-day ovariectomized (OVX) mice aged 4–8 months, 10–12 months and 14–18 months were cultured at a dilution of 3×10 5 cells/ml of M199-bovine serum albumin medium for 3 days prior to stimulation with either buserelin or phorbol ester (phorbol myristate acetate, PMA), while LH was assayed by radioimmunoassay using anti-rat LH antibody (NIDDK-5-10). In intact young mice, buserelin and PMA specifically induced time- and dose-dependent increases in LH release with specific mean ED 50 of 0.82×10 −11 M (buserelin) and of 1.6×10 −8 M (PMA) and a maximal LH release of 138±15 ng/10 6 cells after a 3 h stimulation period. Age did not affect the ED 50 of either agonist but significantly reduced their ability to release LH. This reduction was more pronounced for buserelin than for PMA and was evident as early as middle-age. OVX resulted in a significant increase in both basal and stimulated LH release, but did not affect the age-related reduced secretion rate of LH by either agonist. Buserelin stimulated the incorporation of [ 3H]inositol into [ 3H]inositol phosphates (IP) in a dose-dependent manner, which was unaffected by either age or OVX. We conclude that, with aging, there occurs a reduced LH release rate to both buserelin and PKC stimulations, uncoupled to changes in PI-IP cycle. Furthermore our results suggest that these cellular defects are first manifested at middle-age and are unaffected by the status of the gonad.