Effect of purification followed by solubilization of receptor material on quantitative receptor assays for anticholinergic drugs.

In order to optimize quantitative receptor assays for anticholinergics, the different receptor preparations resulting from the purification and the solubilization of the P2 pellet from the calf striatum were evaluated. The dissociation constants for two chemically different anticholinergics, the tertiary amine scopolamine and the quaternary amine oxyphenonium, were calculated from inhibition studies of 3H-NMS binding in buffer and plasma. The Kd values for both anticholinergics were similar for all the membrane-bound receptor preparations (unpurified and the purified P2 pellet) either in buffer or in plasma. More pronounced differences were observed between the membrane-bound and solubilized receptors. By introducing the solubilized receptor as well, differences between the individual anticholinergics appeared. On the one hand, for scopolamine, a gain in sensitivity of 1.5-2.8 in plasma was observed for the solubilized receptor. On the other hand, in the case of oxyphenonium, a dramatic loss in sensitivity (by a factor of about 24) was observed with the solubilized receptor, as compared to the membrane-bound receptor, in buffer. Very interestingly, however, when the solubilized receptor was used in plasma, a lowering of the Kd value was found for both anticholinergics, i.e. the assays became more sensitive. Such an effect (not observed for the membrane-bound receptor) could be obtained only when the percentage of digitonin present in the assay was at least 0.12% (w/v) or higher.