Modulation of 2-[125I]iodomelatonin binding in the guinea pig spleen by guanine nucleotides and cations

Abstract

The effect of guanine nucleotides on the binding of 2-[125I]iodomelatonin in membrane preparations of guinea pig spleen was studied. The GTP analogues guanosine 5'-O-(3-thiophosphate) and 5'-guanylimidodiphosphate dose-dependently inhibited the binding. Saturation studies revealed that the presence of GTP analogues either increased the equilibrium dissociation constant (K(d)) alone or both increased the K(d) and decreased the binding site density (B(max)). Our results suggest that melatonin receptors in the guinea pig spleen, similar to those in the neural and retinal tissues, are coupled to a G protein. This study showed that N+ and Li+ dose-dependently inhibited 2-[125I]iodomelatonin binding while Mg2+ potentiated the binding. K+ and choline were without significant effects. Low Ca2+ concentrations (1-5 mmol/l) potentiated the radioligand binding while higher concentrations were inhibitory. Saturation studies demonstrated that 125 mmol/l N+ decreased the B(max) while 2.4 mmol/l Ca2+ increased the B(max) and 40 mmol/l Ca2+ increased the K(d). Our results suggest that physiological concentrations of N+ and Ca2+ may play an important modulatory role on melatonin binding to its receptors in the spleen.