Calcium distribution and mobilization in single rat parotid acinar cells investigated by digital imaging microscopy

The intracellular distribution and mobilization of cytosolic free calcium in single rat parotid acinar cells was analyzed by a digital imaging microscope equipped with a microspectrofluorometer, using calcium-sensitive dye fura-2. In the resting state, intracellular distribution of cytosolic free calcium concentration ([Ca2+]i) was heterogeneous: [Ca2+]i in the nuclear and perinuclear region was usually higher than that in the cytoplasm. By Ca(2+)-ionophore ionomycin and muscarinic agonist carbachol stimulation in the presence of 1 mM extracellular Ca2+, [Ca2+]i increased markedly and the gradient of [Ca2+]i between the nuclear region and the cytoplasm decreased. In ionomycin stimulation, [Ca2+]i increased homogeneously and this homogeneous increase was irreversible. In carbachol stimulation the gradient of [Ca2+]i between the nuclear region and the cytoplasm obviously reappeared within 2 min. By carbachol stimulation in the absence of extracellular Ca2+ (added 1 mM EGTA), [Ca2+]i returned to the prestimulation level after the initial transient increase. The distribution of [Ca2+]i also returned approximately to the prestimulation state. The gradient of [Ca2+]i between the nuclear region and the cytoplasm did not disappear even when [Ca2+]i elevated at the peak value.