Cytosolic Ca2+ and Ca2+-activated Cl- current dynamics:: Insights from two functionally distinct mouse exocrine cells

The dynamics of Ca2+ release and Ca2+-activated Cl- currents in two related, but functionally distinct exocrine cells, were studied to gain insight into how the molecular specialization of Ca2+ signalling machinery are utilized to produce different physiological endpoints: in this case, fluid or exocytotic secretion. Digital imaging and patch-clamp methods were used to monitor the temporal and spatial properties of changes in cytosolic Ca2+ concentration ([Ca2+](c)) and Cl- currents following the controlled photolytic release of caged-InsP(3) or caged-Ca2+. In parotid and pancreatic acinar cells, changes in [Ca2+](c) and activation of a Ca2+-activated Cl- current occurred with close temporal coincidence. In parotid, a rapid global Ca2+ signal was invariably induced, even with low-level photolytic release of threshold amounts of InsP(3). In pancreas, threshold stimulation generated an apically delimited [Ca2+](c) signal, while a stronger stimulus induced a global [Ca2+](c) signal which exhibited characteristics of a propagating wave. InsP(3) was more effective in parotid, where [Ca2+](c) signals initiated with shorter latency and exhibited a faster time-to-peak than in pancreas. The increased potency of InsP(3) in parotid probably results from a four-fold higher number of InsP(3) receptors as measured by radiolabelled InsP(3) binding and western blot analysis. The Ca2+ sensitivity of the Cl- channels in parotid and pancreas was determined from the [Ca2+]-current relationship measured during a dynamic 'Ca2+ ramp' produced by the continuous, low-level photolysis of caged-Ca2+. In addition to a greater number of InsP(3) receptors, the Cl- current density of parotid acinar cells was more than four-fold greater than that of pancreatic cells. Whereas activation of the current was tightly coupled to increases in Ca2+ in both cell types, local Ca2+ clearance was found to contribute substantially to the deactivation of the current in parotid. These data reveal specializations of common modules of Ca2+-release machinery and subsequent effector activation that are specifically suited to the distinct functional roles of these two related cell types.