Effect of phospholipase A2 on the binding of a specific blocker [H-3]quinuclidinyl benzilate to M-cholinoceptors of rat cerebral cortex membranes

Major parameters of the binding of a specific blocker [H-3]quinuclidinyl benzilate are obtained for M-cholinoceptors of rat cerebral cortex membranes. It was shown that one receptor binds two molecules of the ligand. Two discrete receptor pools were revealed, which differed in the sensitivity (K-dl = 0,35 +/- +/-0,03, K-d2 = 1,79 +/- 0,21 nM) and the number on the membrane ligand binding sites (B-1 = 468 +/- 54, B-2 = 864 +/- 81 fmol/mg protein). Phospholipase A(2) from Naja naja venom in the concentration of 0.5 units/ml did not change the general pattern of [H-3]quinuclidinyl benzilate binding. The dissociation constants of the high and low-affinity pools did not differ from the control. The numbers of the high-affinity and low-affinity reseptors were decreased by 94 and 87%, respectively. The maximum number of cholinoceptors was reduced by 89%. Phospholipase A(2) decreased the concentration of the ligand binding sites following the pattern of the noncompetitive inhibition with K-i = 0,059 units/ml.