STIMULUS-secretion coupling in pancreatic acinar cells is believed to be mediated by a rise in intracellular Ca2+ activity1-5. Based on the observation that physiological secretagogues trigger a rapid increase of Ca2+ efflux3,4 and stimulate enzyme release even in the absence of extracellular calcium2,4,5, it has been proposed that stimulus-secretion coupling involves a redistribution of intracellular calcium3,5,6. Since most calcium in pancreatic acinar cells is bound to membranes or is sequestered in mitochondria7, we have attempted to monitor these calcium stores during secretion by using the fluorescent probe chlorotetracycline (CTC). This probe forms highly fluorescent complexes with Ca2+ and Mg2+ in the presence of biological membranes or detergent miceles8,9 and has been successfully used in isolated mitochondria10,11 and sarcoplasmic reticulum vesicles 12 to monitor calcium movements induced by substrates and inhibitors. In this report, we show that fluorescence of CTC associated with pancreatic acinar cell membranes decreases rapidly on stimulation of enzyme secretion and that this intensity change is preferential for CTC complexed to calcium.
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