Basal and Physiological Ca2+ Leak from the Endoplasmic Reticulum of Pancreatic Acinar Cells

Abstract

We have studied the Ca2+ leak pathways in the endoplasmic reticulum of pancreatic acinar cells by directly measuring Ca2+ in the endoplasmic reticulum ([Ca2+] ER ). Cytosolic Ca2+ ([Ca2+] C ) was clamped to the resting level by a BAPTA-Ca2+ mixture. Administration of cholecystokinin within the physiological concentration range caused a graded decrease of [Ca2+] ER , and the rate of Ca2+ release generated by 10 pm cholecystokinin is at least 3× as fast as the basal Ca2+ leak revealed by inhibition of the endoplasmic reticulum Ca2+-ATPase. Acetylcholine also evokes a dose-dependent decrease of [Ca2+] ER , with an EC50 of 0.98 ± 0.06 μm. Inhibition of receptors for inositol 1,4,5-trisphosphate (IP3) by heparin or flunarizine blocks the effect of acetylcholine but only partly blocks the effect of cholecystokinin. 8-NH2 cyclic ADP-ribose (20 μm) inhibits the action of cholecystokinin, but not of acetylcholine. The basal Ca2+ leak from the endoplasmic reticulum is not blocked by antagonists of the IP3 receptor, the ryanodine receptor, or the receptor for nicotinic acid adenine dinucleotide phosphate. However, treatment with puromycin (0.1–1 mm) to remove nascent polypeptides from ribosomes increases Ca2+leak from the endoplasmic reticulum by a mechanism independent of the endoplasmic reticulum Ca2+ pumps and of the receptors for IP3 or ryanodine.