Thymulin stimulates prolactin and thyrotropin release in an age-related manner

Thymulin is a Zn-bound nonapeptide produced by the thymic epithelial cells (TEC) whose secretion is modulated by prolactin (PRL) and thyroid hormones, among other hormones. We assessed the ability of thymulin to influence the release of PRL and thyroid stimulating hormone (TSH) from dispersed anterior pituitary (AP) cells from young, middle-aged and senescent Sprague-Dawley female rats. Perifused and incubated AP cells were used in different sets of experiments and PRL and TSH release was measured by radioimmunoassay. Perifusion of young and senescent AP cells with thymulin doses ranging from 10(-8) to 10(-5) M gave a logarithmic dose response pattern for both hormones. Supernatants from TEC lines also showed PRL and TSH secretagogue activity. Hormone release was always lower in the senescent cells. AP cells incubated with 10(-8) to 10(-3) M thymulin showed a time- and dose-dependent response for both hormones, the latter being bell-shaped with a maximum at 10(-7) M thymulin. Preincubation of thymulin with an anti-thymulin serum completely quenched the secretagogue activity of the thymic hormone. Coincubation of thymulin with TRH revealed a synergistic release of PRL and TSH in AP cells from all age groups. The calcium chelator EGTA but not the calcium ionophore A23187, blocked the hormone-releasing activity of thymulin in AP cells. The cAMP enhancers, caffeine, NaF and forskolin, significantly increased the thymulin-stimulated release of PRL and TSH, while trifluoperazine, a protein kinase C inhibitor, had no effect. The inositol phosphate enhancer LiCl, potentiated the action of thymulin on PRL and TSH. The present results suggest that the TRH-like activity documented here for thymulin is a receptor-mediated effect which appears to involve calcium, cAMP and inositol phosphates. Senescence but not middle age, appears to impair AP cell responsiveness to thymulin.