Iso Stimulation of GH and cAMP: Comparison of β-Adrenergic- to GRF-Stimulated GH Release and cAMP Accumulation in Monolayer Cultures of Anterior Pituitary Cells in vitro

Abstract

Growth hormone (GH) release and cAMP content were measured in monolayer cultures of anterior pituitary cells after β-adrenergic and GH-releasing factor (GRF) receptor activation. Isoproterenol (Iso, ED50 – 20 nM) was less potent than GRF (ED50 – 20 p M) in stimulating GH release. Iso caused a rapid stimulation of GH release that was maximal after 15 min and declined thereafter, while GRF caused a more gradual increase in GH secretion that was maximal after 30 min and remained elevated after 3 h. Both Iso- and GRF-stimulated GH release were preceded by an increase in cAMP content in the pituitary cells. Further, the addition of 3-isobutyl-1-methylxanthine (IBMX) to the medium enhanced the GH-stimulatory and cAMP-accumulating effects of both secretagogues. Experiments performed with native catecholamines and synthetic catecholamine agonists and antagonists indicated that the GH-stimulatory effect of Iso was mediated by a mixed population of β₁-adrenergic and β₂-adrenergic receptors. Additionally, experiments performed with cultured Gα tumor cells, found that incubation with GRF, Iso, vasoactive intestinal polypeptide, forskolin, or cholera toxin caused an increase in cAMP content in the cells. However, compared to the responses observed in primary pituitary cultures the GH secretory response to these agents was comparatively small. Together, these studies suggest that a mixed population of β₁-adrenergic and β₂-adrenergic receptors may act, at least in part, on somatotrophs in the anterior pituitary to stimulate GH release. Although both GRF and β₂-adrenergic receptor agents affect GH release through a common second messenger system, their differing pharmacokinetic properties suggest distinct intracellular mechanisms.