PINEALECTOMY INCREASES OUABAIN HIGH-AFFINITY BINDING-SITES AND DISSOCIATION-CONSTANT IN RAT CEREBRAL-CORTEX

The effect of the pineal gland on the ouabain high-affinity binding sites (K(d) = 3.1 +/- 0.4 nM, B(max) = 246.4 +/- 18.4 fmol/mg protein) in rat cerebral cortex was studied. Pinealectomy increased B(max) (940.7 +/- 42.8 fmol/mg protein) and K(d) (7.6 +/- 1.5 nM) while melatonin injection (100-mu-g/kg b.wt.) counteracted these effects, restoring kinetic parameters (K(d) = 1.9 +/- 0.05 nM; B(max) = 262.2 +/- 29.6 fmol/mg prot) to control values. Melatonin activity on ouabain binding in vitro did not depend upon a direct effect on the binding sites themselves. However, in competition experiments, melatonin increased binding affinity of ouabain as shown by the decreased IC50 values.