Biochemical and pharmacological studies on the opiate receptors from human placenta.

The effect of different enzymatic treatments on the specific binding of 3H-Etorphine to the membrane bound opiate receptors from human placenta was studied. Phospholipase A2 inhibited etorphine binding, suggesting either that phospholipids are essential for ligand binding or that products of the phospholipase A2 reaction, either lysophospholipids or fatty acids, disrupt the membrane integrity because of their detergent like properties. Trypsin and alpha-chymotrypsin had no effect, suggesting that the receptors binding site was not accessible to these proteolytic enzymes. We also find that the placental villus upon in vitro stimulation with potassium releases acetylcholine. The inhibitory effect of morphine on acetylcholine release was investigated.