Inhibitory Effect of Carbachol on Isoproterenol-Induced Amylase Release from Isolated Rat Parotid Cells

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Abstract
The effect of carbachol (CCh) on isoproterenol (ISP)-induced amylase release was investigated using isolated rat parotid cells. CCh (1-100 .mu.M) inhibited ISP (1 .mu.M)-induced amylase release in a dose-dependent manner, whereas CCh alone had a slightly increasing effect on amylase release. Both inhibitory and stimulatory effects of CCh were blocked by atropine (10 .mu.M) and they also disappeared in Ca-free (1 mM EGTA [ethyleneglycol-bis(.beta.-aminoethylether)-N,N''-tetraacetic acid]) medium. CCh (10 .mu.M) did not change cAMP levels induced by ISP (1 .mu.M), but significantly inhibited dibutyryl cAMP (1 .mu.M)-induced amylase release. CCh and ISP increased 45Ca2+ uptake in 30 min. 45Ca2+ uptake in the presence of CCh plus ISP increased almost additively. These increasing effects of CCh were abolished by atropine. Calcium ionophore A23187 (calcimycin, 10 .mu.M) inhibited ISP-induced amylase release to the level of the release by A23187 alone and considerably increased 45Ca2+ uptake. CCh increased both control and ISP-stimulated effluxes of 45Ca2+ in Ca-free (1 mM EGTA) medium from parotid cells. CCh evidently produces a potent increase in Ca2+ influx from the extracellular medium into parotid cells; this increase may result in a higher level of cytosolic free Ca2+ which inhibits the ISP-induced amylase release.