Potentiation of isoproterenol-induced salivation by labetalol.

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Abstract
Low doses of labetalol potentiated isoproterenol (ISO)-induced salivation in mice were previously reported. The potentiating effect of labetalol in in vitro experiments using rat parotid tissue slices and their isolated cells was studied. ISO-induced amylase release was enhanced by low concentrations of labetalol (< 10-6 M), while it was inhibited by high doses (> 10-5 M). This potentiating effect of labetalol did not occur on 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 cAMP 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 K release in parotid tissue. Increases in the secretion of amylase and K 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.