Vasopressin receptors in adrenal cortex of sheep: does autoradiography indicate an irreversible binding of the ligand?

Tritiated arginine vasopressin ([3H]-AVP) labelled specific loci of murine renal medulla and ovine adrenal cortex in thin sections of an autoradiographic experiment. The label was fully displaced by 2 x 10(-6) M cold ligand in the case of renal, but not of adrenal sections. 10 and 100 microM AVP, however, partially displaced the radioactivity also from labelled adrenal sections. At room temperature, the half maximal blackening of the film occurred at a concentration of 26 +/- 0.9 microM. In binding experiments employing AVP and adrenocortical cell membranes, the model assuming two saturable binding sites yielded a significantly better fit than the one-site model. The equilibrium dissociation constants of ice-cold membrane preparations were 8.67 nmol/l for the high affinity site and 3.16 mumol/l for the low affinity binding site. It is concluded that the low affinity binding is governed by laws of chemical equilibrium, rather than by surface adsorption or similar "nonspecific" phenomena. When such low affinity sites are present in a tissue, higher concentrations of cold ligand ought to be used before a nondisplaceable binding is ascribed as "non-specific" or "irreversible".