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Novel regulation of muscarinic receptors and their coupling with G proteins in smooth muscle: transient resensitization during desensitizing process

1 Muscarinic stimulation of the smooth muscle of guinea‐pig taenia caeci was produced with 10−4 m carbachol for 15 s, 30 s, 1 min, 2 min and 30 min, and the time course of developing desensitization was studied by measuring the muscle contractility and the binding characteristics of muscarinic receptors. 2 The contractile response to carbachol was analyzed using dose‐response curves. The response to 10−7 m carbachol was reduced by treatment for 15 s with 10−4 m carbachol (fast desensitization), but recovered partially after 30 s treatment and completely after 1 min treatment (resensitization). Contractility was reduced again after 2 min and 30 min treatment (re‐desensitization). 3 The affinity of carbachol for muscarinic receptors was changed by the carbachol treatment in a manner similar to the contractility. Thus, the affinity was reduced at 15 s, restored slightly at 30 s and completely at 1 and 2 min, and was reduced again at 30 min. 4 5′‐Guanylylimidodiphosphate (GppNHp), a non‐hydrolysable analogue of guanosine triphosphate (GTP) reduced the affinity of muscarinic receptors for carbachol via guanine nucleotide‐binding regulatory proteins (G proteins). A similar effect was observed in tissues desensitized by 15 s carbachol treatment. This effect disappeared after 30 s, recovered completely after 1 and 2 min, and disappeared again after 30 min carbachol treatment. 5 Neither the dissociation constant (Kd value) nor the maximal binding (Bmax) of [3H]‐quinuclidinyl benzilate ([3H]‐QNB) to muscarinic receptors were changed by the carbachol treatment. 6 These results indicate that the whole process of desensitization, resensitization and re‐desensitization are related to changes in the binding ability of muscarinic receptors, in their coupling with G proteins and in the post‐receptor steps of the signal transduction. We emphasize that the desensitizing process involves an early transient phase of resensitization that could be caused by restoration of both the affinity of carbachol for muscarinic receptors and their coupling with G proteins. This novel resensitization mechanism may have some physiological significance for cellular homoeostasis by modulating cellular responsiveness transiently or even in an oscillatory manner during the process of desensitization.