Rapid increases in cytosolic free calcium in response to muscarinic stimulation of rat parotid acinar cells.

Carbachol-evoked rises in [Ca2+]i were measured in fura-2-loaded, rat parotid acinar cells. In suspensions of dissociated cells examined by dual wavelength excitation fluorimetry, a maximally effective concentration of carbachol produced a measured peak [Ca2+]i of 780 +/- 60 nM followed by a maintained elevation in the presence of 1 mM external Ca2+, and a peak of 630 +/- 95 nM followed by a return to resting values in the absence of external Ca2+. Stopped-flow, single wavelength fluorimetry was used to resolve the rising phase of the response. There was a dose-dependent lag of 70-220 ms before [Ca2+]i started to increase, and [Ca2+]i was maximal by 800-900 ms. These times were similar in the presence or absence of external Ca2+, although the initial rate of rise was faster in the presence of external Ca2+. These kinetics are consistent with a biochemical event, possibly phosphatidylinositol bisphosphate hydrolysis, mediating both internal release and Ca2+ entry, with a component of the initial rise being due to Ca2+ entry.