SIGNAL-TRANSDUCTION EVENTS INVOLVED IN BOMBESIN-STIMULATED GASTRIN-RELEASE FROM HUMAN G-CELLS IN CULTURE

The mechanism of action of bombesin on human antral gastrin cells in culture was evaluated by modulating internal and external calcium levels and intracellular enzyme activities. Increasing extracellular calcium increased basal gastrin release and had an additive effect on bombesin-stimulated gastrin release. Removing extracellular calcium had no effect on bombesin-stimulated gastrin release. Inhibiting the activities of phospholipase C by neomycin and protein kinase C by staurosporine had no effect on basal release but decreased bombesin-stimulated gastrin release by up to 50%. Chelating intracellular calcium with BAPTA-AM also decreased bombesin-stimulated release by up to 50%. Increasing intracellular calcium levels with thapsigargin did not alter basal gastrin release and had no effect on bombesin-stimulated release. The preparation utilized was a mixed, primary cell culture. To demonstrate direct activation of gastrin cells, alterations in internal calcium levels were monitored by dual-excitation microfluorometry of fura 2-AM loaded cells. The individual cells were subsequently identified by immunocytochemistry, confirming that bombesin directly increases calcium levels in the G cells. The data indicated that bombesin acting directly on the G cells activated both arms of the phosphoinositol signalling pathway and that both activities were required for optimal gastrin release.