Effect of clonidine on secretion of fluid and ions by the parotid and submandibular glands of the rat.

Secretion in response to this alpha 2-adrenergic agonist was evaluated in the presence and absence of several adrenergic antagonists, reserpine and sympathectomy (Sx). In both glands, the response was qualitatively but not quantitatively similar to that induced by the alpha 1-adrenergic agonist, phenylephrine, in the presence of propranolol. With clonidine, the volume of submandibular saliva was much higher but the Ca concentration was 3-4 times lower than that of the parotid; both salivas had low Na but high K concentrations. Clonidine-induced secretion was almost completely blocked by the alpha 1-adrenergic antagonist, prazosin and a mixed alpha-adrenergic antagonist, phentolamine, and markedly reduced by the alpha 2-adrenergic antagonist, yohimbine, but unaffected by the beta-adrenergic antagonist, propranolol. Reserpine reduced the parotid, but enhanced the submandibular secretory response to clonidine. Results in Sx glands were similar. Thus, in the rat glands clonidine may activate alpha 1-rather than alpha 2-adrenoceptors, which appear to play a part similar to alpha 1-adrenoceptors only after reserpine or Sx.