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Potentiation of isoproterenol-induced salivation by labetalol.

The authors previously reported that low doses of labetalol potentiated isoproterenol (ISO)-induced salivation in mice. The present study was carried out to ascertain the potentiating effect of labetalol in in vitro experiments using rat parotid tissue slices and their isolated cells. ISO-induced amylase release was enhanced by low concentrations of labetalol (less than 10(-6) M), while it was inhibited by high doses (more than 10(-5) M). This potentiating effect of labetalol did not occur in the Ca-free medium used for incubation or in experiments using the dispersed parotid cells or the parotid tissues which were obtained from 6-hydroxy-dopamine- or reserpine-treated rats. The effect of ISO on cyclic AMP accumulation in parotid tissue was completely blocked by the addition of labetalol in concentrations which were sufficient to increase the ISO-induced amylase release. Labetalol also inhibited the ISO-induced reduction of potassium release in parotid tissue. From these results, it is concluded that increases in the secretion of amylase and potassium from salivary glands due to the stimulation of glandular alpha-adrenoceptors by endogenous norepinephrine may play an important role in the potentiating effect of labetalol on ISO-induced salivation in mice.