Oscillations of free cytosolic calcium evoked by cholinergic and catecholaminergic agonists in rat parotid acinar cells

Abstract

1. In single, dissociated, rat parotid acinar cells the muscarinic agonist carbachol evokes a rapid rise in cytosolic free calcium [( Ca2+]i), from near 100 nM to peak levels of up to 1 microM. In the continued presence of the agonist the response decays to a lower, maintained, level. 2. In most cells, at 22 degrees C, oscillations, with a mean frequency of 0.19 Hz, are superimposed upon this elevation of [Ca2+]i. In voltage-clamped cells oscillations of current occur in phase with the oscillations of [Ca2+]i. 3. The oscillations occur in voltage-clamped cells, and in the absence of extracellular Ca2+, indicating that neither voltage-gated processes, or an influx of Ca2+ is involved. 4. Oscillation frequency is independent of carbachol concentration, in the range 100 nM to 250 microM, and furthermore, shows no relationship to the mean level of [Ca2+]i during the oscillations. 5. Stimulation with the alpha-adrenergic agonist noradrenaline, in the presence of the beta-blocker propanolol, evokes oscillations having the same frequency as those evoked by carbachol. 6. The oscillations show a strong temperature dependence, the frequency increasing with a Q10 of 2.8. In contrast, the amplitude of the oscillations drops from a mean of 33% of the response amplitude at 22 degrees C, and below, to 6% at 33 degrees C. Above the latter temperature oscillations are not resolvable. 7. The phorbol esters, 12-O-tetradecanoyl-phorbol-13-acetate and 12,13-phorbol dibutyrate (1 microM), do not affect the response to carbachol at 22 degrees C, at which temperature the oscillations are of maximum amplitude. Diacylglycerol is, therefore, unlikely to be involved in oscillation generation in these cells. 8. These observations are consistent with a model in which a negative feed-back loop links [Ca2+]i to the mechanisms of Ca2+ elevation, possibly to the inositol 1,4,5-trisphosphate-sensitive Ca2+ release mechanism of the endoplasmic reticulum. If the feed-back path involved an enzymatic step, the slowing of this step at lowered temperatures could give rise to oscillations. At body temperature such a mechanism would act to ensure that [Ca2+]i was elevated in a regulated and dose-dependent manner.