Stimulation-induced changes in cytosolic calcium in rat parotid acini.

The concentration of cytosolic, free calcium (Ca2+i) in collagenase-isolated rat parotid acini was measured by use of the fluorescent dye fura-2 (G. Grynkiewicz, M. Poenie, and R. Y. Tsien, J. Biol. Chem. 260: 3440-3450, 1985). Ca2+i was measured in unstimulated acini and in acini stimulated with cholinergic, alpha-adrenergic, and beta-adrenergic agonists, respectively. In unstimulated acini suspended in 1 mM Ca media, Ca2+i amounted to 155 +/- 38 nM (mean +/- SD, n = 14). After stimulation with the cholinergic agonist carbachol, Ca2+i increased transiently with a peak value of 640 +/- 90 nM (n = 3) obtained 4-6 s after stimulation. The peak value of Ca2+i was reduced when the extracellular calcium concentration decreased, and at 2 nM external calcium, Ca2+i amounted to 50% of the peak value observed at 1 mM external calcium. It was calculated that at least 50% of the rise in Ca2+i observed under physiological conditions (1 mM external calcium) after cholinergic stimulation is due to calcium mobilizations from intracellular pools. Stimulation with the nonspecific adrenergic agonist adrenaline caused a rise in Ca2+i of a similar magnitude as the rise observed after cholinergic stimulation. However, stimulations with the alpha-adrenergic agonist phenylephrine and the beta-adrenergic agonist isoproterenol caused a rise in Ca2+i amounting to 60 and 10%, respectively, of the rise observed after cholinergic or adrenergic stimulation.