Promiscuous or heterogeneous muscarinic receptors in rat atria? II. Antagonism of responses to carbachol by pirenzepine

Abstract

Carbachol produces both negative and positive inotropy in rat left atria. It is not clear whether these two effects are mediated by two separate cell surface muscarinic receptors or a single receptor interacting with two coupling proteins in the cell membrane. Pirenzepine, known to selectively block some biochemical muscarinic responses, was used in this study to block the biphasic response to carbachol in rat left atria. The negative inotropy to carbachol was blocked by pirenzepine, and Schild analysis indicated a -log dissociation constant (pK b) for the pirenzepine-receptor complex of 6.2. However, the Schild analysis may have been complicated by positive inotropy observed with pirenzepine. This positive inotropic effect was sensitive to blockade by other muscarinic antagonists. In atria from rats pretreated with pertussis toxin, carbachol produced a positive inotropic effect. Schild analysis with pirenzepine for antagonism of this response indicated a -log equilibrium dissociation constant for the pirenzepine-receptor complex of 6.7, significantly different from that for antagonism of negative inotropy. This ostensibly suggested a difference in the receptors mediating these responses. In view of the possible complicating effects of the positive inotropic effects of pirenzepine in this assay, an alternative method for the measurement of pirenzepine affinity was utilized. Resultant analysis was used to measure the pK b for pirenzepine antagonism of negative inotropy to carbachol. This method had the advantage of cancelling the positive inotropy to pirenzepine. Under these circumstances, pirenzepine had a pK b of 6.9, a value not significantly different from for antagonism of the positive inotropy to carbachol. The relevance of these findings is discussed in terms of a single promiscuous muscarinic receptor or heterogeneous receptors in this tissue. These data do not support the hypothesis that two separate receptors mediate these two effects.