Dual effects of clonidine on amylase secretory responses of rat parotid tissue induced by beta- and alpha 1-agonists.

The effects of clonidine on the amylase secretory responses of rat parotid tissue were examined in vitro. Clonidine increased amylase secretion induced by the beta-adrenergic agonist isoproterenol and decreased that induced by the alpha 1-adrenergic agonist methoxamine or phenylephrine with propranolol. Clonidine itself had no significant effect on the secretion. Yohimbine prevented the effects of clonidine on the secretions induced by both adrenergic agonists. Clonidine did not have any significant effect on cyclic AMP accumulation in the tissues induced by the beta-adrenergic agonist. The parotid tissue after reserpine treatment exhibited supersensitivity to beta- and alpha 1-adrenergic agonists. In reserpine treated tissues, the stimulatory effect of clonidine on beta-agonist-induced secretion disappeared, but the inhibitory effect on alpha 1-agonist-induced secretion remained unchanged. Similar results were obtained after sympathectomy of the gland. These data suggest that the inhibitory effect of clonidine on alpha 1-agonist-induced secretion is related to a postsynaptic site, whereas its effect on beta-agonist-induced secretion is still unknown.