On the interaction of gallamine with muscarinic receptor subtypes

Abstract

The interaction of gallamine with muscarinic receptor subtypes was examined using radioligand binding studies. In competition studies using [ 3H]N-methylscopolamine ([ 3H]NMS), gallamine displayed high affinity for the rat cardiac and guinea-pig uterine M 2 muscarinic receptors and for the atypical muscarinic receptor present in chicken heart. Gallamine displayed low affinity for rat glandular and human 1321 N1 astrocytoma cell M 3 receptors and also for the M 4 receptors of NG108-15 and PC12 cells. The compound displayed intermediate affinity for M 1 receptors of rat cortex labeled using [ 3H]pirenzepine. The interaction of gallamine with the M 1 and M 2 receptors appeared to be competitive at the low concentrations required to determine affinity estimates. Thus, gallamine inhibited the binding of [ 3H]pirenzepine to M 1 receptors and [ 3H]NMS to M 2 receptors at concentrations that were 263- and 23-fold lower, respectively, than those required to decrease radioligand dissociation kinetics. Furthermore, gallamine, at a concentration that inhibited between 63 and 71% of specific radioligand binding, had no effect on the observed rate of association of the radioligand with either the M 1 or the M 2 receptor. At the M 3 glandular receptor, there was little separation between the concentrations of gallamine that produced inhibition of binding and those that decreased the association and dissociation rates of [ 3H]NMS. It is therefore difficult to determine if the inhibition of binding seen in competition studies on the M 3 receptor was produced through a competitive or an allosteric mechanism. Despite its possible allosteric properties at the M 3 receptor, gallamine can be used to detect heterogeneity of muscarinic receptor subtypes in several tissues and therefore represents a useful tool for defining muscarinic receptor subtypes.