Regulation of muscarinic receptor binding by guanine nucleotides and N-ethylmaleimide.

The regulation of muscarinic receptor binding by guanine nucleotides and N-ethylmaleimide (NEM) was investigated using the agonist ligand, [3H] cis methyldioxolane ([3H]CD). Characterization studies on rat forebrain homogenates show that [3H]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 [3H]CD binding in the rat brain correlated generally wit 3H3-quinuclidinyl benzilate (3HQNB) binding, although the absolute variation in binding was somewhat less. At a concentration of 100 microM, the GTP analogue, guanyl-5'-yl imidodiphosphate [Gpp(NH)p], caused a 43-77% inhibition of [3H]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 [3H]CD binding, Gpp(NH)p caused an increase in (3H](-)QNB binding in the heart heart and ileum and no change in 3HQNB binding in the corpus striatum. When measured by competitive inhibition of 3HQNB binding to the longitudinal muscle of the ileum, Gpp(NH)p (100 microM) caused an increase in the IC50 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 [3H]CD binding by NEM.