Selective masking of M1-receptors in calf retina membranes by the venom of the marine snail Conus tessulatus.

The non discriminatory antagonist [3H]QNB labels M1- and M2-muscarinic receptors in calf retina membranes. Crude venom from the marine gastropod Conus tessulatus produces a partial decrease in [3H]QNB binding. The total number of sites (560 +/- 13 fmol/mg protein in control experiments) decreases to 370 +/- 10 fmol/mg protein whereas the affinity of the radioligand is unaffected (KD = 0.42 +/- 0.01 nM and 0.46 +/- 0.02 nM, respectively). This process is venom concentration-dependent, quasi-irreversible, and calcium-dependent. Proteolytic activity can not be detected. The partial effect of the venom is related to preferential masking of the M1-receptors. Competition curves of the M1-selective antagonist pirenzepine are shallow in control experiments: 45% of the receptors are of the M1-type (Ki = 45 +/- 6 nM) while the remaining are of the M2-type (Ki = 1.0 +/- 0.2 microM). In venom-treated membranes, only a low affinity site (M2-receptors, Ki = 1.5 +/- 0.4 microM) 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 [3H]pirenzepine without affecting its KD (23 +/- 4 nM and 20 +/- 6 nM in control- and venom-treated membranes respectively). The venom produces a leftward shift of the carbachol/[3H]QNB competition binding curve, but the ability of 0.1 mM GTP to confer a rightward shift of the competition curve is not affected.(ABSTRACT TRUNCATED AT 250 WORDS)