Growth hormone (GH)-releasing factor differentially activates cyclic adenosine 3 ',5 '-monophosphate- and inositol phosphate-dependent pathways to stimulate GH release in two porcine somatotrope subpopulations

Somatotropes comprise two morphologically and functionally distinct subpopu lations of low (LD) and high (HD) density cells. We recently reported that GRF induces different patterns of increase in the cytosolic free Ca2+ conce ntration in single porcine LD and HD somatotropes, which for LD cells requi red not only Ca2+ influx but also intracellular Ca2+ mobilization. This sug gested that GRF may activate multiple signaling pathways in pig LD and KD s omatotropes to stimulate GH secretion. To address this question, we first a ssessed the direct GRF effect on second messenger activation in cultures of LD and HD cells by measuring cAMP levels and [H-3]myo-inositol incorporati on. Secondly, to determine the relative importance of cAMP- and inositol ph osphate (IP)-dependent pathways, and of intra- and extracellular Ca2+, GRF- induced GH release from cultured LD and HD somatotropes was measured in the presence of specific blockers. GRF increased cAMP levels in both subpopula tions, whereas it only augmented IP turnover in LD cells. Accordingly, aden ylate cyclase inhibition by MDL-12,330A abolished GRF-stimulated GH release in both subpopulations, whereas phospholipase C inhibition by U-73122 only reduced this effect partially in LD cells. Likewise, blockade of Ca2+ infl ux with Cl2Co reduced GRF-stimulated GH secretion in both LD and HD somatot ropes, whereas depletion of thapsigargin-sensitive intracellular Ca2+ store s only decreased the secretory response to GRF in LD cells. These results d emonstrate that GRF specifically and differentially activates multiple sign aling pathways in two somatotrope subpopulations to stimulate GH release. T hus, although the prevailing signaling cascade employed by GRF in both subp opulations is adenylate cyclase/cAMP/extracellular Ca2+, the peptide also r equires activation of the phospholipase C/IP/intracellular Ca2+ pathway to exert its full effect in porcine LD somatotropes.