Stimulus-secretion coupling in the developing exocrine pancreas

Acinar cells of the embryonic pancreas are filled with zymogen granules containing, among others, the secretory protein, cholecystokinin (CCK) α-amylase, the rate of amylase secretion from pancreatic lobules incubated in vitro was not increased in response to CCK. In contrast, the rate of CCK-stimulated amylase discharge from the neonatal pancreas was increased 4- to 8-fold above that seen in the embryonic gland. The postnatal amplification of secretory responsiveness was not associated with an increase in the level of 125I-CCK octapeptide specifically bound/cell equivalent or a change in the affinity of binding. Light microscopic autoradiography revealed a similar 125I-CCK-33 labeling pattern in pancreatic lobules from both ages with autoradiographic grains specifically localized at the periphery of acinar cells. In order to determine whether CCK binding is coupled to a rise in the cytosolic Ca++concentration, [Ca++]c, in the embryonic pancreas, 45Ca++ efflux from tracer-loaded lobules was measured. Efflux of 45Ca++ from both embryonic and neonatal pancreas was comparably increased in the presence of CCK