Mechanisms involved in the effects of TRH on GHRH-stimulated growth hormone release from ovine and bovine pituitary cells

Abstract

Incubation of cultured ovine pituitary cells with growth hormone-releasing hormone (GHRH) (10 −12−10 −7 M) stimulated growth hormone secretion up to 3-fold. At a maximal stimulatory concentration of GHRH (10 −10 M), thyrotropin-releasing hormone (TRH) (10 −7 M) caused an inhibition of growth hormone release to approx. 50% of the response obtained with GHRH alone (during a 15 min incubation period). TRH also caused a small inhibition of the GHRH-stimulated cellular cyclic AMP level but this effect was only significant at a relatively high concentration of GHRH (10 −9 M). Incubation of cultured bovine pituitary cells with GHRH (10 −11−10 −8 M) plus TRH (10 −7 M) caused a significant stimulation of growth hormone release by up to 40%, compared with the response obtained with GHRH alone (at all concentrations of GHRH). TRH (10 −7 M) had no effect on GHRH (10 −8 M)-stimulated cellular cyclic AMP levels in a partially purified bovine pituitary cell preparation. The effects of varying extracellular [Ca 2+] (0.1–10 mM) on intracellular [Ca 2+] and on the responsiveness to releasing hormones were also determined using ovine pituitary cells. GHRH (10 −10 M)-stimulated growth hormone release was inhibited when cells were incubated at both high (10 mM) and low (0.1 mM) [Ca 2+] (compared with 1 mM or 3 mM Ca 2+) with or without TRH (10 −7 M). At 1 mM Ca 2+, TRH produced a synergistic effect with GHRH to stimulate growth hormone release. However, at 3 mM Ca 2+ TRH inhibited GHRH-stimulated growth hormone release. A model, to explain the differences between the effects of TRH on GHRH-stimulated growth hormone release from bovine and ovine pituitary cells is presented, based on evidence suggesting that the effects are Ca 2+-mediated.