Stimulus-secretion coupling in the developing exocrine pancreas: secretory responsiveness to cholecystokinin

We have studied the onset of secretory responsiveness to cholecystokinin (CCK) during development of the rat exocrine pancreas. Although acinar cells of the fetal pancreas (1 d before birth) are filled with zymogen granules containing the secretory protein, alpha- 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 (1 d after birth) was increased four- to eightfold above that of the fetal gland. The postnatal amplification of secretory responsiveness was not associated with an increase in the number or cell surface expression of 125I-CCK binding sites. When 125I-CCK-33 binding proteins were analyzed by affinity crosslinking, two proteins of Mr 210,000 and 100,000-160,000 were labeled specifically in both fetal and neonatal pancreas. To determine if cell surface receptors for CCK in the fetal pancreas are functional and able to generate a rise in the cytosolic [Ca++], we measured 45Ca++ efflux from tracer-loaded lobules. 45Ca++ efflux from both fetal and neonatal pancreas was comparably increased by CCK, indicating CCK-induced Ca++ mobilization and elevated cytosolic [Ca++]. The Ca++ ionophore A23187 also stimulated the rate of 45Ca++ extrusion from pancreas of both ages. Increased amylase secretion occurred concurrently with A23187-stimulated 45Ca++ efflux in neonatal pancreas, but not in the fetal gland. A23187 in combination with dibutyryl cAMP potentiated amylase release from the neonatal gland, but not from fetal pancreas. Similarly, the protein kinase C activator, phorbol dibutyrate, did not increase the rate of secretion from the fetal gland when added alone or in combination with A23187 or CCK. We suggest that CCK-receptor interaction in the fetal pancreas triggers intracellular Ca++ mobilization. However, one or more signal transduction events distal to Ca++ mobilization have not yet matured. The onset of secretory response to CCK that occurs postnatally may depend on amplification of these transduction events.