SELECTIVE MASKING OF M1-RECEPTORS IN CALF RETINA MEMBRANES BY THE VENOM OF THE MARINE SNAIL CONUS-TESSULATUS

The non discriminatory antagonist [H-3]QNB labels M1- and M2-muscarini c receptors in calf retina membranes. Crude venom from the marine gast ropod Conus tessulatus produces a partial decrease in [H-3]QNB binding . The total number of sites (560 +/- 13 fmol/mg protein in control exp eriments) decreases to 370 +/- 10 fmol/mg protein whereas the affinity of the radioligand is unaffected (K(D) = 0.42 +/- 0.01 nM and 0.46 +/ - 0.02 nM, respectively). This process is venom concentration-dependen t, quasi-irreversible, and calcium-dependent. Proteolytic activity can not be detected. The partial effect of the venom is related to prefer ential masking of the M1-receptors. Competition curves of the M1-selec tive antagonist pirenzepine are shallow in control experiments: 45 % o f the receptors are of the M1-type (K(i) = 45 +/- 6 nM) while the rema ining are of the M2-type (K(i) = 1.0 +/- 0.2 muM). In venom-treated me mbranes, only a low affinity site (M2-receptors, K(i) = 1.5 +/- 0.4 mu M) is detected by pirenzepine competition binding. Saturation binding experiments reveal that the venom causes a substantial decrease in the number of high affinity sites for [H-3]pirenzepine without affecting its K(D) (23 +/- 4 nM and 20 +/- 6 nM in control- and venom-treated me mbranes respectively). The venom produces a leftward shift of the carb achol/[H-3]QNB competition binding curve, but the ability of 0.1 mM GT P to confer a rightward shift of the competition curve is not affected . These data suggest that the venom from Conus tessulatus contains one or more components which specifically shield part of the M1-receptors (75 % in calf retina membranes), leaving the M2-receptors and their f unctional coupling to G proteins unaffected.