Stimulation of 45Ca influx in rat parotid gland by carbachol

Secretion of alpha-amylase by rat parotid glands was stimulated by 10(-5) M carbachol when Ca was present in the bathing medium. The optimal Ca concentration was 1.0 mM in the absence of Mg ion. Lowering the extracellular Na to near one-half and one-quarter of normal levels produced a significant increase in carbachol-induced secretion. The metabolism of 45Ca by the slices could be described by three components of exchange. The faster two, with time constants of 4 and 16 minutes, each appeared to represent a mixture of free extracellular Ca and rapidly exchanging superficially bound Ca. Influx into the slower (tau = 78 minutes) component (apparently intracellular) was enhanced by 10(-5) M carbachol. This was apparent by standard flux techniques only in low (65 mM) Na solutions, but was readily discernible in normal Na (125 mM) with the "lanthanum-residual" technique. The increase in 45Ca influx due to carbachol resembles the secretory response to carbachol in being potentiated by low Na, blocked by 1.0 mM La, and blocked by 10(-4) M atropine. It was concluded on the basis of this and other evidence that the primary step in the induction of exocytosis by carbachol in the parotid gland is an increased influx of Ca.