Calcium entry in rat parotid acini: activation by carbachol and aluminum fluoride.
Entry of extracellular Ca2+ into the cytosol of exocrine cells plays an important role in the process of fluid transport, especially during periods of prolonged secretion. However, in parotid acinar cells, the process of Ca2+ entry and the identity of factors which regulate it remain obscure. In this report, we demonstrate that AlF-4, like carbachol, activates Ca2+ entry into dispersed rat parotid acini. In physiological Ca2(+)-containing (1.28 mM) medium, both agents elicit three phases of cytosolic Ca2+ change, an initial transient increase (intracellular Ca2+ dependent) followed sequentially by a decrease (intra- and extracellular Ca2+ dependent) and a small sustained increase (extracellular Ca2+ dependent). Cytosolic Ca2+ concentration ([Ca2+]i) during the last two phases is influenced by variations in extracellular [Ca2+]. Elevation of extracellular [Ca2+], at any time after the initial transient increase, results in a rise of cytosolic [Ca2+], thus demonstrating the existence of a Ca2+ entry pathway during the two later phases. These data suggest the likelihood that in parotid acini, G protein activation is involved in stimulating this Ca2+ entry pathway. Because in AlF-4-treated acini entry into the cytosol is detectable only after the initial intracellular Ca2+ release phase, we suggest that this Ca2+ entry process does not accompany initial intracellular Ca2+ mobilization. Furthermore, the sustained cytosolic [Ca2+] elevation which can be observed 15-30 min after initial stimulation of acini is likely determined by this Ca2+ entry process which, in physiological conditions, could support sustained fluid secretion.