Basal and physiological Ca(2+) leak from the endoplasmic reticulum of pancreatic acinar cells. Second messenger-activated channels and translocons

Abstract

We have studied the Ca(2+) leak pathways in the endoplasmic reticulum of pancreatic acinar cells by directly measuring Ca(2+) in the endoplasmic reticulum ([Ca(2+)](ER)). Cytosolic Ca(2+) ([Ca(2+)](C)) was clamped to the resting level by a BAPTA-Ca(2+) mixture. Administration of cholecystokinin within the physiological concentration range caused a graded decrease of [Ca(2+)](ER), and the rate of Ca(2+) release generated by 10 pm cholecystokinin is at least 3x as fast as the basal Ca(2+) leak revealed by inhibition of the endoplasmic reticulum Ca(2+)-ATPase. Acetylcholine also evokes a dose-dependent decrease of [Ca(2+)](ER), with an EC(50) of 0.98 +/- 0.06 microm. Inhibition of receptors for inositol 1,4,5-trisphosphate (IP(3)) by heparin or flunarizine blocks the effect of acetylcholine but only partly blocks the effect of cholecystokinin. 8-NH(2) cyclic ADP-ribose (20 microm) inhibits the action of cholecystokinin, but not of acetylcholine(.) The basal Ca(2+) leak from the endoplasmic reticulum is not blocked by antagonists of the IP(3) 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 Ca(2+) leak from the endoplasmic reticulum by a mechanism independent of the endoplasmic reticulum Ca(2+) pumps and of the receptors for IP(3) or ryanodine.