Pepsinogen secretion: coupling of exocytosis visualized by video microscopy and [Ca]i in single cells

Tao, Chie, Masao Yamamoto, Hiroshi Mieno, Masaki Inoue, Tsutomu Masujima, and Goro Kajiyama. Pepsinogen secretion: coupling of exocytosis visualized by video microscopy and [Ca]i in single cells. Am. J. Physiol. 274 (Gastrointest. Liver Physiol. 37): G1166–G1177, 1998.— Conventional in vitro studies of pepsinogen secretion have measured secretion into the bulk medium and have demonstrated the critical role of Ca21 in the process. The present study was undertaken to obtain further details of the process of secretion and its relation to Ca21 changes over very short time periods. The relation between Ca21 mobilization and exocytosis in an isolated individual peptic cell of the bullfrog was investigated by a method to measure both intracellular Ca21 ([Ca]i), using a fluorescent Ca21 indicator, fura 2, and exocytosis from single cells using a video microscope analyzing system. Bombesin (3.2 3 1027 M) and bethanechol (3.2 3 1024 M) caused a rapid increase in [Ca]i (initial peak) and a corresponding high frequency of initial exocytosis. After the initial peak, [Ca]i was maintained at a somewhat elevated level over the baseline (sustained phase), with a corresponding low frequency of exocytosis. Both the sustained phase of elevated [Ca]i and the related exocytosis were eliminated by the depletion of extracellular Ca21. Low concentrations of bombesin (3.2 3 10210 M) and bethanechol (3.2 3 1027 M) caused sustained low-amplitude Ca21 oscillations with correspondingly low frequencies but also caused sustained exocytosis. These data show that 1) cellular response differs between high and low concentrations of stimulus, 2) there is a close relation between [Ca]i and exocytosis, 3) exocytosis follows elevation of [Ca]i by 14–45 s (n 5 6), and 4) there is a significant positive correlation between the peak [Ca]i and the number of exocytoses.