Intracellular divalent cation release in pancreatic acinar cells during stimulus-secretion coupling. II. Subcellular localization of the fluorescent probe chlorotetracycline

Subcellular distribution of the divalent cation-sensitive probe chlorotetracycline (CTC) was observed by fluorescence microscopy in isolated pancreatic acinar cells, dissociated hepatocytes, rod photoreceptors, and erythrocytes. In each cell type, areas containing membranes fluoresced intensely while areas containing no membranes (nuclei and zymogen granules) were not fluorescent. Cell compartments packed with rough endoplasmic reticulum or Golgi vesicles (acinar cells) or plasma membrane-derived membranes (rod outer segments) exhibited a uniform fluorescence. In contrast, cell compartments having large numbers of mitochondria (hepatocytes and the rod inner segment) exhibited a punctate fluorescence. Punctate fluorescence was prominent in the perinuclear and peri-granular areas of isolated acinar cells during CTC efflux, suggesting that under these conditions mitochondrial fluorescence may account for a large portion of acinar cell fluorescence. Fluorometry of dissociated pancreatic acini, preloaded with CTC, showed that application of the mitochondrial inhibitors antimycin A, NaCN, rotenone, or C1CCP, or of the divalent cation ionophore A23187 (all agents known to release mitochondrial calcium) rapidly decreased the fluorescence of acini. In the case of mitochondrial inhibitors, this response could be elicited before but not following the loss of CTC fluorescence induced by bethanechol stimulation. Removal of extracellular Ca2+ and Mg2+ or addition of EDTA also decreased fluorescence but did not prevent secretagogues or mitochondrial inhibitors from eliciting a further response. These data suggest that bethanechol acts to decrease CTC fluorescence at the same intracellular site as do mitochondrial inhibitors. This could be due to release of calcium from either mitochondria or another organelle that requires ATP to sequester calcium.