The interaction of nicotinic receptor antagonists at muscarinic receptors

Allosteric and competitive interactions of 2 alpha-(2',2'-disubstituted- hydroxy-ethoxy)tropane and its optical isomers at rat central muscarinic acetylcholine receptors

Both allosteric and competitive interactions of 2 alpha-(2',2'-disubstituted-hydroxy-ethoxy)tropane (2 alpha-DHET) and its four optical isomers with muscarinic acetylcholine receptors in rat cerebral cortex were investigated. 2 alpha-DHET and its optical isomers allosterically decelerated the dissociation of bound [3H]N-methylscopolamine in a concentration-dependent manner, but these compounds failed to decelerate the dissociation of bound [3H]quinuclidinyl benzilate. The potencies of 2 alpha-DHET and its optical isomers to decelerate the dissociation of bound [3H]N-methylscopolamine were not differed significantly, but the competitive binding potencies of these compounds were significantly different. The order of potencies of 2 alpha-DHET and its four optical isomers to inhibit the binding of [3H]quinuclidinyl benzilate to central muscarinic acetylcholine receptors was 1S-2 alpha-2'R > 1R-2 alpha-2'R > 2 alpha-DHET > 1S-2 alpha-2'S > 1R-2 alpha-2'S the isomer with 1S-2 alpha-2'R configuration was about two orders of magnitude more potent than the isomer with 1R-2 alpha-2'S configuration. The allosteric potencies were found not to be correlated with their competitive binding potencies.

Competitive and allosteric binding of 2 alpha-DHET and its optical isomers to rat cardiac muscarinic receptors

The possibility of both competitive and allosteric interactions of 2 alpha-(2',2'-disubstituted-hydroxy-ethoxy)tropane (2 alpha-DHET) and its four optical isomers with rat cardiac muscarinic receptors was studied. 2 alpha-DHET and its optical isomers competitively inhibited the binding of [3H] quinuclidinyl benzilate to rat cardiac muscarinic receptors and allosterically decelerated the dissociation of bound [3H] quinuclidinyl benzilate in a concentration-dependent manner. The rank order of potencies to displace [3H] quinuclidinyl benzilate binding was 1R-2 alpha-2'R > 1S-2 alpha-2'R > 2 alpha-DHET > 1S-2 alpha-2'S > 1R-2 alpha-2'S, while their potencies to decelerate the dissociation of bound [3H] quinuclidinyl benzilate were not significantly different. The allosteric potencies were found not to be correlated with their binding potencies.

Mixed competitive and allosteric antagonism by gallamine of muscarinic receptor-mediated second messenger responses in N1E-115 neuroblastoma cells

The antagonistic effects of gallamine on muscarinic receptor-linked responses were investigated in N1E-115 neuroblastoma cells. M1 muscarinic receptor-mediated phosphoinositide hydrolysis induced by carbamylcholine was antagonized by gallamine, with a Ki value of 33 microM. By comparison, gallamine was four- to fivefold less potent in blocking noncardiac M2 muscarinic receptor-mediated inhibition of cyclic AMP formation, with a Ki value of 144 microM. The resulting Arunlakshana-Schild plots of the antagonism of both responses by gallamine were linear and exhibited slopes not differing from 1, a result indicative of a competitive mechanism. To elucidate further the nature of gallamine's inhibitory actions, experiments were performed where the effects of gallamine in combination with the known competitive muscarinic antagonist, N-methylscopolamine (NMS), were studied. In the presence of both antagonists, a supraadditive shift in the carbamylcholine dose-response curve was demonstrated for the two responses, a result suggestive of an allosteric mode of interaction between gallamine and NMS binding sites. Confirmation that gallamine allosterically modifies the muscarinic receptor was provided by radioligand binding studies. Gallamine competition curves with either [N-methyl-3H]scopolamine methyl chloride ([3H]NMS) or [N-methyl-3H]quinuclidinyl benzilate methyl chloride ([3H]NMeQNB) were unusually shallow. Furthermore, gallamine decelerated the rate of dissociation of receptor-bound [3H]NMS greater than [3H]NMeQNB in a dose-dependent manner. The present study demonstrates that whereas gallamine antagonizes carbamylcholine-mediated responses in N1E-115 cells in a competitive manner, an allosteric component of its action is revealed in the presence of muscarinic antagonists such as NMS.

Muscarinic M1- and M2-receptors mediating opposite effects on neuromuscular transmission in rabbit vas deferens

Twitch contractions of the rabbit vas deferens elicited by electrical field stimulation were inhibited by tetrodotoxin, guanethidine, bretylium and alpha,beta-methylene-ATP but were unaffected by hexamethonium, physostigmine, 1,1-dimethyl-4-phenylpiperazinium and prazosin, suggesting that they resulted from ATP released following postganglionic sympathetic nerve stimulation. McN-A-343 inhibited but carbachol and several other muscarinic agonists potentiated the twitch contractions; these effects were not modified by hexamethonium or physostigmine. Muscarinic agonists had no effect on the tension in unstimulated organs whereas contractions elicited by ATP, noradrenaline and KCl were potentiated by carbachol but remained unaffected by McN-A-343. The responses of the twitch contractions to McN-A-343 and carbachol were inhibited to different degrees by antimuscarinic drugs: the affinity (pA2) of atropine, secoverine and himbacine against McN-A-343 and carbachol was similar. However, pirenzepine, telenzepine, trihexyphenidyl, dicyclomine and hexahydro-sila-difenidol displayed preferential antagonism of the responses to McN-A-343 whereas the converse was true for AF-DX 116 and gallamine. The highly significant correlation between the pA2 values obtained for 10 antagonists against carbachol responses in rabbit vas deferens and rat left atrium suggests that the receptors may be similar. The data support the presence of a presynaptic M1-receptor mediating inhibition and a postsynaptic, cardiac-like M2-receptor responsible for enhancing neurogenic contractions in rabbit vas deferens.