Characterization of M1- and M2-muscarinic receptors in calf retina membranes.

Muscarinic receptors are identified in bovine retina membranes by the specific binding of 1-[benzilic-4,4'-3H]-quinuclidinyl benzilate [3H]-QNB. Binding occurs to one population of non-cooperative binding sites: KD = 0.11 +/- 0.02 nM and Bmax = 0.61 +/- 0.07 pmol/mg protein. Competition binding curves of the M1-selective antagonist pirenzepine are shallow. Computer-analysis reveals the presence of 45 +/- 1% M1-receptors (high affinity sites for pirenzepine, Ki = 31 +/- 10 nM). The remaining low affinity sites (Ki = 1.0 +/- 0.3 microM) are denoted as M2-receptors. Competition binding curves with the agonist carbachol are shallow as well. 1 mM GTP causes a rightward shift and a steepening of the carbachol curve, whereas 1 mM N-ethylmaleimide (NEM) provokes a leftward shift and also a steepening of the curve. The GTP effect is abolished by NEM. Binding of the antagonists [3H]-QNB, atropine or pirenzepine is not modulated by GTP nor by NEM.