Abstract
The regulation of muscarinic receptor binding by guanine nucleotides and Nโethylmaleimide (NEM) was investigated using the agonist ligand, [^3^H] cis methyldioxolane ([^3^H] CD). Characterization studies on rat forebrain homogenates showed that [^3^H] CD binding was linear with tissue concentration and was unaffected by a change in pH from 5.5 to 8.0. The regional variation in [^3^H] CD binding in the rat brain correlated generally with [^3^H] (โ)3โquinuclidinyl benzilate ([^3^H] (โ)QNB) binding, although the absolute variation in binding was somewhat less. At a concentration of 100 ฮผM, the GTP analogue, guanylโ5โฒโyl imidodiphosphate [Gpp(NH)p], caused a 43โ77% inhibition of [^3^H] CD binding in the corpus striatum, ileum, and heart. The results of binding studies using several Gpp(NH)p concentrations demonstrated that the potency of this guanine nucleotide for inhibition of [^3^H] CD binding was greater in the heart than in the ileum. In contrast to its effects on [^3^H] CD binding, Gpp(NH)p caused an increase in [^3^H] (โ)QNB binding in the heart heart and ileum and no change in [^3^H] (โ)QNB binding in the corpus striatum. When measured by competitive inhibition of [^3^H] (โ)QNB binding to the longitudinal muscle of the ileum, Gpp(NH)p (100 ฮผM) caused an increase in the IC~50~ values of a series of agonists in a manner that was correlated with the efficacy of these compounds. The results of binding studies on NEM treated forebrain homogenates revealed an enhancement of [^3^H] CD binding by NEM.