Alpha-1 adrenoceptors mediate secretory responses to norepinephrine in innervated and denervated rat submaxillary...

The alpha adrenoceptor that mediates the secretory responses to norepinephrine in submaxillary glands was characterized. All three alpha antagonists employed produced in innervated glands parallel displacements of the dose-response curves to norepinephrine, prazosin being 30 and 100 times more potent than phentolamine and yohimbine, respectively. Chronic sympathetic denervation produced supersensitivity to norepinephrine. Under these conditions prazosin also was more potent than yohimbine but the shifts of the dose-response curves to norepinephrine were nonparallel. Potentiation of norepinephrine by cocaine did not modify the effects of prazosin in innervated glands. The nonparallel displacements observed in denervated glands were related to the development of postjunctional supersensitivity of the beta adrenoceptor-mediated responses because after administration of propranolol the displacements produced by prazosin were parallel. Responses to methoxamine were not potentiated by denervation and were competitively antagonized by prazosin in both innervated and denervated glands. Specific [3H]clonidine binding to membranes of submaxillary glands showed at least two independent sites with a KD1 of 0.37 nM and a KD2 of 65 nM. The number of sites of each component was doubled after chronic sympathetic denervation. Binding of [3H]prazosin (KD, 0.43 nM; maximum binding sites, 78.5 fmol/mg of protein) was not affected by denervation. The present findings show that sialagogue responses to sympathomimetic agents are mediated by alpha-1 adrenoceptor activation in both innervated and denervated submaxillary glands. Chronic sympathetic denervation does not produce postjunctional supersensitivity of the alpha-1-mediated responses and the supersensitivity of the beta-mediated responses interferes with determinations of dose-response curves to the sympathetic neurotransmitter.