Actions of rat growth hormone-releasing factor and norepinephrine on cytosolic free calcium and inositol trisphosphate in rat C-cells.

Rat growth hormone-releasing factor (rGRF) and norepinephrine (NE) stimulate secretion of calcitonin (CT) and neurotensin (NT) from cultured C-cells. The mechanism by which these agents cause secretion has not been well studied. We have examined the actions of the CT and NT secretagogues rGRF and NE on cytosolic free calcium concentrations ([Ca2+]i) in the rat C-cell line rMTC 44-2. Because inositol trisphosphate (IP3) has been shown to cause release of intracellular calcium stores in several cell types, we have also examined the effects of rat GRF, NE, and increases in extracellular calcium on IP3 accumulation in rMTC 44-2 cells. Stimulation by 10(-6) M rGRF caused a biphasic response in [Ca2+]i consisting of a rapid spike to 136 +/- 4% (mean +/- SE) of basal [Ca2+]i. This increase in [Ca2+]i decayed to base line and then gradually increased to 173 +/- 13% of basal [Ca2+]i. Stimulation by 10(-6) M NE gave a similar biphasic increase in [Ca2+]i. The increases in [Ca2+]i induced by both rGRF and NE were inhibited by pretreatment with EGTA or verapamil. rGRF, NE, and increasing concentrations of extracellular calcium, which all caused rapid increases in [Ca2+]i, failed to increase IP3 accumulation in rMTC 44-2 cells. These results suggest that rGRF- and NE-induced secretion in C-cells are mediated by changes in [Ca2+]i. These increases in [Ca2+]i appear to be generated by extracellular calcium influx rather than by release of intracellular calcium stores.