Fluorescent probe detects redistribution of cell calcium during stimulus-secretion coupling

STIMULUS-secretion coupling in pancreatic acinar cells is believed to be mediated by a rise in intracellular Ca²⁺ activity^1-5. Based on the observation that physiological secretagogues trigger a rapid increase of Ca²⁺ efflux^3,4 and stimulate enzyme release even in the absence of extracellular calcium^2,4,5, it has been proposed that stimulus-secretion coupling involves a redistribution of intracellular calcium^3,5,6. 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 miceles^8,9 and has been successfully used in isolated mitochondria^10,11 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.