Guanosine 5'-triphosphate converts some populations of propylbenzilylcholine mustard-sensitive muscarinic cholinoceptor sites to sites resistant to the drug in intestinal smooth muscle.
From functional studies with propylbenzilylcholine mustard (PrBCM), we reported that there coexist PrBCM-sensitive and PrBCM-resistant muscarinic cholinoceptor mechanisms in guinea pig taenia caecum. We investigated the interrelationship between these two cholinoceptor mechanisms using an in vitro receptor binding assay with [3H]quinuclidinyl benzilate (QNB) and [3H]PrBCM. Pretreatment of the muscle strips with 300 nM PrBCM (in vivo alkylation) for 10-50 min resulted in progressive decreases of the number of the maximum [3H]QNB binding sites. However, a prolongation of the period of in vivo alkylation up to 90 min was accompanied with no further loss in the binding sites. Under these conditions, there is no significant change in the affinity of [3H]QNB for the binding sites. The concentration of carbachol required to displace 50% of the bound [3H]QNB was larger in membranes obtained from the tissues that had been alkylated in vivo with PrBCM for 50 min than that from control strips, but was not altered when the pretreatment with the drug was carried out after homogenization (in vitro alkylation). When GTP was added during in vitro alkylation, the affinity of carbachol was lower than that in control membranes, as observed when in vivo alkylation was carried out. In the presence of guanine nucleotide, PrBCM thus appears to recognize two distinct populations or states of muscarinic receptors.