FLASH-PHOTOLYSIS STUDIES OF THE LOCALIZATION OF CALCIUM-RELEASE SITESIN RAT PAROTID ISOLATED ACINAR-CELLS

1.The temporal relationship between cytosolic free Ca2+ concentration ([Ca2+](i)) and activation of membrane current responses in single rat parotid acinar cells has been examined. Activation of muscarinic rece ptors by carbachol (CCh) at -40 m V (midway between E(K) and E(Cl) und er our experimental conditions) frequently evoked biphasic current res ponses, application of 2 mu M CCh leading to rapid activation of an in ward current followed by a slower outward current. 2. Photochemical re lease of inositol 1,4,5-trisphosphate (InsP(3)), from 'caged' InsP(3), by a brief near-UV flash, evoked similar biphasic current responses a t -40 m V. In contrast, elevation of [Ca2+](i) by photolysis of the ca ged calcium compound nitr-5 at -40 m V activated only monophasic curre nt responses.3. These results can be explained by a model in which the InsP(3)-sensitive Ca2+ release sites are localized at the luminal pol e of the cell, combined with a relative preponderance of Ca2+-activate d Cl- channels at that pole, and a relative preponderance of Ca2+-acti vated K+ channels at the basal end.