Positive allosteric action of alcuronium on solubilized cardiac muscarinic receptors.
Alcuronium was found earlier to increase the binding of (3H)methyl-N-scopolamine [(3H)NMS] to muscarinic receptors in membranes of rat heart atria by increasing the receptors' affinity for (3H)NMS, without altering the number of (3H)NMS binding sites. We have now investigated how the interaction of alcuronium with muscarinic receptors is affected by solubilization. In experiments on pig heart atria, alcuronium had a positive allosteric effect on (3H)NMS binding both to unsolubilized receptors and to receptors solubilized by digitonin and deoxycholate. The cooperativity coefficient alpha was 0.39 +/- 0.01 for unsolubilized and 0.57 +/- 0.01 for solubilized receptors. Under the conditions used for solubilization, muscarinic receptors did not interact with G proteins, as indicated by experiments with the binding of carbachol in the absence and presence of a stable GTP analogue. Alcuronium slowed down the rates of (3H)NMS association with, and dissociation from, solubilized receptors; the dissociation rate constant was diminished more than 300 times by 30 microM alcuronium. The results show that the positive allosteric action of alcuronium on cardiac muscarinic receptors and its effect on the kinetics of radioligand binding are preserved after receptor solubilization and do not depend on the interaction of receptors with G proteins.
Musílková
J
J
Institute of Physiology, Academy of Sciences of the Czech Republic, Prague, Czech Republic.
Tucek
S
S
D000443:Alcuronium / Q000494:pharmacology*; D000494:Allosteric Regulation; D000818:Animals; D001667:Binding, Competitive; D002217:Carbachol / Q000378:metabolism; D005260:Female; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D008297:Male; D009206:Myocardium / Q000378:metabolism*; D019832:N-Methylscopolamine; D009994:Osmolar Concentration; D011976:Receptors, Muscarinic / Q000187:drug effects* / Q000378:metabolism; D012602:Scopolamine Derivatives / Q000378:metabolism; D012995:Solubility; D013552:Swine
D011976:Receptors, Muscarinic; D012602:Scopolamine Derivatives; D006165:Guanylyl Imidodiphosphate; D002217:Carbachol; D000443:Alcuronium; D019832:N-Methylscopolamine