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 cae ci was produced with 10(-4) M carbachol for 15 s, 30 s, 1 min, 2 min a nd 30 min, and the time course of developing desensitization was studi ed by measuring the muscle contractility and the binding characteristi cs of muscarinic receptors. 2 The contractile response to carbachol wa s analyzed using dose-response curves. The response to 10(-7) M carbac hol was reduced by treatment for 15 s with 10(-4) M carbachol (fast de sensitization), but recovered partially after 30 s treatment and compl etely after 1 min treatment (resensitization). Contractility was reduc ed again after 2 min and 30 min treatment (re-desensitization). 3 The affinity of carbachol for muscarinic receptors was changed by the carb achol treatment in a manner similar to the contractility. Thus, the af finity was reduced at 15 s, restored slightly at 30 s and completely a t 1 and 2 min, and was reduced again at 30 min. 4 5'-Guanylylimidodiph osphate (GppNHp), a non-hydrolysable analogue of guanosine triphosphat e (GTP) reduced the affinity of muscarinic receptors for carbachol via guanine nucleotide-binding regulatory proteins (G proteins). A simila r effect was observed in tissues desensitized by 15 s carbachol treatm ent. This effect disappeared after 30 s, recovered completely after 1 and 2 min, and disappeared again after 30 min carbachol treatment. 5 N either the dissociation constant (K(d) value) nor the maximal binding (B(max)) of [H-3]-quinuclidinyl benzilate ([H-3-QNB) to muscarinic rec eptors were changed by the carbachol treatment. 6 These results indica te that the whole process of desensitization, resensitization and re-d esensitization are related to changes in the binding ability of muscar inic receptors, in their coupling with G proteins and in the post-rece ptor steps of the signal transduction. We emphasize that the desensiti zing 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 re sensitization mechanism may have some physiological significance for c ellular homoeostasis by modulating cellular responsiveness transiently or even in an oscillatory manner during the process of desensitizatio n.