Calcium dependence of proteinase-activated receptor 2 and cholecystokinin-mediated amylase secretion from pancreatic acini

Pancreatic acini secrete digestive enzymes in response to a variety of secretagogues including CCK and agonists acting via proteinase-activated receptor-2 (PAR2). We employed the CCK analog caerulein and the PAR2-activating peptide SLIGRL-NH2 to compare and contrast Ca2+ changes and amylase secretion triggered by CCK receptor and PAR2 stimulation. We found that secretion stimulated by both agonists is dependent on a rise in cytoplasmic Ca2+ concentration ([Ca2+](i)) and that this rise in [Ca2+](i) reflects both the release of Ca2+ from intracellular stores and accelerated Ca2+ influx. Both agonists, at low concentrations, elicit oscillatory [Ca2+](i) changes, and both trigger a peak plateau [Ca2+](i) change at high concentrations. Although the two agonists elicit similar rates of amylase secretion, the rise in [Ca2+](i) elicited by caerulein is greater than that elicited by SLIGRL-NH2. In Ca2+-free medium, the rise in [Ca2+](i) elicited by SLIGRL-NH2 is prevented by the prior addition of a supramaximally stimulating concentration of caerulein, but the reverse is not true; the rise elicited by caerulein is neither prevented nor reduced by prior addition of SLIGRL-NH2. Both the oscillatory and the peak plateau [Ca2+](i) changes that follow PAR2 stimulation are prevented by the phospholipase C (PLC) inhibitor U73122, but U73122 prevents only the oscillatory [Ca2+](i) changes triggered by caerulein. We conclude that 1) both PAR2 and CCK stimulation trigger amylase secretion that is dependent on a rise in [Ca2+](i) and that [Ca2+](i) rise reflects release of calcium from intracellular stores as well as accelerated influx of extracellular calcium; 2) PLC mediates both the oscillatory and the peak plateau rise in [Ca2+](i) elicited by PAR2 but only the oscillatory rise in [Ca2+](i) elicited by CCK stimulation; and 3) the rate of amylase secretion elicited by agonists acting via different types of receptors may not correlate with the magnitude of the [Ca2+](i) rise triggered by those different types of secretagogue.