Release of Growth Hormone from Purified Somatotrophs: Use of Perifusion System to Elucidate Interrelations among Ca++, Adenosine 3',5'-Monophosphate, and Somatostatin*

Abstract

In order to define the roles of intracellular Ca⁺⁺ and cAMP in the mechanisms governing GH release, we have carried out perifusion studies using a purified preparation of acutely dispersed somatotrophs obtained from rat adenohypophyses. Two groups of secretagogues were used: those that act by increasing intracellular cAMP [(Bu)₂cAMP], the phosphodiesterase inhibitor 3-isobutyl-l-methylxanthine, and prostaglandin E₂; and those thought to act primarily by increasing Ca⁺⁺ influx into the cells (high extracellular K⁺ and the Ca⁺⁺ ionophore A23187). The release of GH induced by the three secretagogues that increase cAMP levels is instantaneous and maintained during perifusion, whereas the release induced by high K⁺ and A23187, unlike the cAMP-induced release, reaches a peak within 2 min and then falls rapidly back to baseline levels, even though the secretagogues are still present. After GH release has returned to baseline levels but while high K⁺ is maintained, the somatotrophs respond briskly to (Bu)₂cAMP, 3-isobutyl-l-methylxanthine, prostaglandin E₂, and A23187. The administration of SRIF results in an immediate and complete cessation of the augmented release of GH induced by all of the secretagogues except for high K⁺, for which the inhibition of release is incomplete. These results are consistent with a model in which an increase in free cytosol Ca⁺⁺ will result in GH release. Thus, 1) a sustained release of GH is initiated by an increase in intracellular cAMP, which increases free cytosol Ca₊₊ derived from intracellular stored sites; 2) a transient release of GH is initiated by high K⁺⁺ and A23187, which increase Ca⁺⁺ influx into the cells possibly through transient potential-sensitive Ca⁺⁺ channels; and 3) SRIF blocks GH release by blocking the expression of action of cAMP and by decreasing Ca⁺⁺ influx into the cells.