The correlation between calcium outflow and growth hormone release in perifused rat somatotrophs

The time-relationships between GH secretion and 45Ca2+ efflux in response to human GRF (hGRF)-(1-29), hGRF-(1-44), 3-isobutyl-1-methylxanthine (IBMX), (Bu)2-cAMP, and high extracellular K+ were studied in perifused cultured rat somatotrophs. In cells exposed to 1-10 nM hGRF-(1-29) or -(1-44), GH release and 45Ca2+ efflux increased during the first 15 sec and reached peak values within 75 sec. At lower GRF concentrations, 45Ca2+ efflux still increased within 15 sec while GH secretion commenced 15-30 sec later. hGRF-(1-29) increased GH release and 45Ca2+ efflux also after 30 min preperifusion in a calcium-depleted medium with 0.1 mM EGTA added during the last 5 min of preperifusion. However, the magnitude of the stimulation was lower than in the presence of calcium. IBMX increased GH release and 45Ca2+ efflux within 15 sec and peak values were reached within 60 sec. (Bu)2cAMP increased GH release both in the presence and absence of extracellular calcium although the magnitude of stimulation was less in the calcium-depleted medium. Efflux of 45Ca2+ was stimulated by (Bu)2cAMP, independently of extracellular calcium. When exposed to 50 mM K+, both GH release and 45Ca2+ efflux increased within 15 sec and reached high peak values within 60 sec, an effect blocked by removal of extracellular calcium. We conclude that GRF and three other GH secretagogues, (Bu)2cAMP, IBMX, and a high extracellular K+ concentration, rapidly increase 45Ca2+ efflux. GH release and 45Ca2+ efflux appear to be tightly coupled with the calcium response perhaps slightly preceding GH release. This tight coupling strengthens the hypothesis that increased Ca2+ activity is directly involved in exocytosis. GRF and (Bu)2cAMP stimulate GH release and 45Ca2+ efflux also in a calcium-free medium, suggesting that mobilization of intracellular calcium is involved in the action of these secretagogues.