In both glands, low doses of either guanabenz (10 g/kg) or guanfacine (100 g/kg) markedly inhibited the secretory responses induced by noradrenaline, methacholine and substance P, but not that induced by isoprenaline. The inhibition caused by the ₂-agonists was greater for noradrenaline than for either methacholine or substance P. Blockade of ₂-adrenoceptors with yohimbine (300 g/kg) did not modify the response to noradrenaline, methacholine or substance P in either gland. However, the same dose of yohimbine injected 5 min before the ₂-agonists prevented the inhibitory effects of guanfacine and guanabenz on the response induced by either one of the three sialagogic agents. Guanabenz (10 g/kg) did not modify the increase in mean blood pressure observed after the different doses of noradrenaline employed to induce salivary secretion. Guanabenz (10 g/kg) and guanfacine (100 g/kg) did not change the time course of the secretion elicited by either noradrenaline, methacholine or substance P, since the degree of inhibition was of similar magnitude at all the periods of time analyzed.

The results obtained give further support to the hypothesis that activation of ₂-adrenoceptors in the submaxillary as well as parotid gland of the rat inhibits secretory responses which are mediated by either muscarine, substance P and ₁-receptors and not those elicited by -adrenoceptors.