Drug Receptor Mechanisms in Smooth Muscle: β-Chloroethylamine-Sensitive and -Resistant Receptor Mechanisms

Both α1-adrenoceptors and M3-cholinoceptors can be divided into two subtypes discriminated by the β-chloroethylamines, chloroethylclonidine and propylbenzilylcholine mustard (PrBCM), only in the presence of GTP. The full agonists interact with both subtypes to induce responses. The partial agonists activate one of them to induce responses but behave as competitive antagonists when they interact with the other. The responses mediated through the receptors that are activated by the partial agonists are resistant to myosin light chain kinase inhibitors, while the response through the activation of the other receptors are suppressed by the inhibitors. The receptor stimulations through α1A-adrenoceptor and PrBCM-sensitive M3-cholinoceptor subtypes mainly activate the myosin light chain-phosphorylationindependent pathway mediated through protein kinase C and low molecular weight GTP-binding protein, whereas the stimulations through α1B-adrenoceptors and the PrBCM-phosphorylation-dependent pathway are directly related to Ca2+ calmodulin.