PEPSINOGEN SECRETION - COUPLING OF EXOCYTOSIS VISUALIZED BY VIDEO MICROSCOPY AND [CA2+](I) IN SINGLE CELLS

Conventional in vitro studies of pepsinogen secretion have measured se cretion into the bulk medium and have demonstrated the critical role o f Ca2+ in the process. The present study was undertaken to obtain furt her details of the process of secretion and its relation to Ca2+ chang es over very short time periods. The relation between Ca2+ mobilizatio n and exocytosis in an isolated individual peptic cell of the bullfrog was investigated by a method to measure both intracellular Ca2+ ([Ca2 +](i)), using a fluorescent Ca2+ indicator, fura 2, and exocytosis fro m single cells using a video microscope analyzing system. Bombesin (3. 2 x 10(-7) M) and bethanechol (3.2 x 10(-4) M) caused a rapid increase in [Ca2+](i) (initial peak) and a corresponding high frequency of ini tial exocytosis. After the initial peak, [Ca2+](i) was maintained at a somewhat elevated level over the baseline (sustained phase), with a c orresponding low frequency of exocytosis. Both the sustained phase of elevated [Ca2+](i) and the related exocytosis were eliminated by the d epletion of extracellular Ca2+. Low concentrations of bombesin (3.2 x 10(-10) M) and bethanechol (3.2 x 10(-7) M) caused sustained low-ampli tude Ca2+ oscillations with correspondingly low frequencies but also c aused sustained exocytosis. These data show that 1) cellular response differs between high and low concentrations of stimulus, 2) there is a close relation between [Ca2+](i) and exocytosis, 3) exocytosis follow s elevation of [Ca2+](i) by 14-45 s (n = 6), and 4) there is a signifi cant positive correlation between the peak [Ca2+](i) and the number of exocytoses.