Ontogeny of the GH response to phorbol ester and phospholipase C in rat pituitary cells

Abstract

Abstract GH secretory patterns undergo marked change during early mammalian development. The factors that underlie these changes and the major components of signal transduction in the immature somatotrophs are not fully understood. Increasing evidence suggests that protein kinase C (PKC) plays a central role in perinatal organ differentiation and function. To evaluate the possible role of PKC as a mediator of GH secretion from immature pituitaries, we tested the effects of the PKC activating phorbol ester 12-0-tetradecanoylphorbol-13-acetate (TPA), alone or together with GH-releasing factor (GRF), somatostatin (SRIF), and Ca2+ modifying agents; an inactive phorbol analogue (4α-12–13-didecanoate; 4α-PDD), and phospholipase C on GH release from pituitary cell cultures from perinatal and mature rats. Pituitary primary cell cultures were prepared from fetal (day 20 of 21·5 days of gestation), 2-day-old, 12-day-old, and adult male (2- to 4-month-old) rats. Each experiment was performed on at least three separate occasions. The magnitude of TPA (0·15–150 nm)-induced GH release was markedly age-dependent, fractional GH release being greatest from pituitaries of fetal and newborn rats, and least from those of adults (P<0·001). Further, the minimum dose of TPA required to stimulate GH release over basal levels was tenfold higher for adult pituitaries (15 nm) than for perinatal pituitaries (1·5 nm). Phospholipase C (1 and 10 U/ml) also caused greater fractional GH release from neonatal pituitaries than from adult pituitaries (P<0·01). By contrast, the inactive phorbol ester 4αPDD did not stimulate GH release from pituitary cells of 2-day-old, 12-day-old or adult rats. SRIF (0·1–10 nm) inhibited TPA (150 nm)-induced GH release in a dose-dependent manner, but the degree of inhibition was significantly less in perinatal than in mature pituitaries (P<0·001). Combined treatment with near maximal stimulatory doses of GRF (1 nm) and TPA (150 nm) resulted in a partially additive GH response. Inhibitors of Ca2+ mobilization reduced TPA-induced GH release in all age groups, although GH suppression was least in pituitaries of immature rats. These findings indicate that PKC activators such as TPA and phospholipase C are potent secretagogues of GH in perinatal pituitaries. The action of phorbol ester was mediated by Ca2+-dependent pathways in both immature and mature pituitaries. Somatotroph responsiveness to TPA and phospholipase C was strikingly age-dependent, with greatest GH response in the perinatal period and declining response with advancing age after birth. These data suggest that immature somatotrophs possess functional PKC-dependent signal transduction pathways whose relative contribution declines during postnatal development. Journal of Endocrinology (1995) 145, 307–314