Molecular Pharmacology, Vol 13, 1048-1059, Copyright © 1977 by the American Society for Pharmacology and Experimental Therapeutics

Refractoriness to Muscarinic and Adrenergic Agonists in the Rat Parotid: Responses of Adenosine and Guanosine Cyclic 3',5'-Monophosphates

¹ Department of Pharmacology, University of Virginia School of Medicine, Charlottesville, Virginia 22903

Cyclic 3',5'-AMP and cyclic 3',5'-GMP elevations in response to various receptor agonists are reduced upon repeated stimulation in rat parotid. The concentration of cyclic AMP in rat parotid is elevated upon stimulation with either beta adrenergic or muscarinic receptor agonists, although the cholinergic receptor agonist carbachol produces only 1% of the maximal response to the beta adrenergic receptor agonist isoproterenol. In both cases the cyclic AMP elevation is transient, and readdition of the agent used to produce the stimulation does not lead to additional cyclic AMP accumulation, showing that the tissue has become refractory. Removal of extracellular calcium does not prevent elevations of cyclic AMP or refractoriness after stimulation with either norepinephrine or carbachol. The simultaneous addition of carbachol and norepinephrine in the presence and absence of calcium does not produce as great an elevation of cyclic AMP as can norepinephrine alone. Carbachol also produces a time- and calcium-dependent, nonspecific refractoriness to norepinephrine-mediated cyclic AMP accumulation. Cyclic AMP accumulation in the parotid therefore shows receptor-nonspecific refractoriness, since stimulation by any agent capable of elevating cyclic AMP leads to cyclic AMP accumulation refractory to all agonists. Cyclic GMP refractoriness can also be observed in the parotid. Carbachol and norepinephrine are able to stimulate the accumulation of cyclic GMP through muscarinic and alpha adrenergic receptors, respectively, while sodium azide stimulates its accumulation through a different mechanism. Extracellular calcium is required for carbachol or norepinephrine to stimulate cyclic GMP accumulation, although azide is effective in its absence. Unlike the interaction of carbachol and norepinephrine toward cyclic AMP concentration, the simultaneous addition of these drugs produces a cyclic GMP accumulation no different from the maximal response to either agent alone. Both carbachol and norepinephrine produce refractoriness to readdition of the same agent, but neither is capable of producing that state in response to drugs acting through receptors not originally involved. Extracellular calcium is not required for the induction of cyclic GMP refractoriness, and thus refractoriness is not triggered by the accumulation of cyclic GMP. Cyclic GMP refractoriness is receptor-specific in the same tissue in which cyclic AMP elevation exhibits a receptor-nonspecific refractory pattern.

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ACKNOWLEDGMENTS The authors wish to thank Ms. Celinda Johnson, whose excellent assistance in the preparation of the manuscript was invaluable.