Complex allosteric modulation of cardiac muscarinic receptors by protamine: potential model for putative endogenous ligands.

A large number of diverse pharmacological agents bind to a secondary domain on the muscarinic receptor, to influence allosterically the interaction of ligands at the primary binding site. Based on common structural features of these antagonists, we examined the interaction of protamine, an endogenous polycationic peptide, and of polyamines with muscarinic receptors in rat heart. Our results provide several lines of qualitative evidence that protamine allosterically modulates the conformation of muscarinic receptors, in a marked negatively cooperative manner. It decelerated the dissociation of N-[3H]methylscopolamine ([3H] NMS) initiated by atropine, in a concentration-dependent fashion. Inhibition by protamine of [3H]NMS binding at equilibrium showed a distinct plateau, which increased in magnitude at higher ligand concentrations. Scatchard analysis of saturation isotherms of [3H]NMS binding in the absence and presence of protamine indicated that protamine did not alter Bmax in a statistically significant fashion, although there was a trend of a concentration-dependent increase in this parameter. On the other hand, it caused a marked concentration-dependent decrease in the affinity of [3H]NMS, and this effect reached a ceiling limit. However, there were marked quantitative deviations of the interaction of protamine from a simple ternary allosteric model. Some of these discrepancies could be explained by the tendency of protamine to increase Bmax. The allosteric actions of protamine demonstrated in kinetic and equilibrium experiments were selective for m1 and m2 muscarinic receptors, compared with m3, m4, and m5 receptors, as studied in Chinese hamster ovary cells transfected with the genes of the different muscarinic receptors. Arginine residues play an important role in the allosteric interaction of protamine, inasmuch as poly-L-arginine qualitatively mimicked the effects of protamine. In contrast, no effects of the polyamines spermine, spermidine, and putrescine were observed on [3H]NMS binding. This is the first report on the allosteric modulation of muscarinic receptors by an endogenous peptide.