MODULATION OF CALCITONIN BINDING BY CALCIUM - DIFFERENTIAL-EFFECTS OFDIVALENT-CATIONS

Binding of salmon calcitonin to bovine hypothalamic membranes is enhan ced about 25% by calcium with a half-maximal effect at 15 mM calcium. In contrast, membranes prepared from a cell line expressing a recombin ant human calcitonin receptor show no effect of calcium under similar conditions. The hypothalamic calcitonin receptor solubilized with CHAP S detergent retains an apparent Kd of 0.3 nM for salmon calcitonin; ho wever, binding of calcitonin to the detergent-solubilized receptor com plex can be inhibited by divalent cations in order of potency Mn>Ca al most-equal-to Sr almost-equal-to Mg>>NaCl with Mn and Ca having appare nt Ki's of 5 mM and 20 mM respectively. Dixon and Scatchard plots of M n and Ca inhibition of binding to the soluble receptor complex suggest a noncompetitive mechanism of inhibition. Calcium also inhibits calci tonin binding to a detergent-solubilized recombinant human calcitonin receptor. Inhibition of calcitonin binding is observed using two indep endent methods for determining soluble receptor-hormone complex and in hibition is reversed by EDTA. These data suggest that a low-affinity d ivalent-cation binding site exists on the calcitonin receptor complex with a binding preference for calcium and manganese. Binding of calciu m to this site can perturb the binding of calcitonin. This divalent-ca tion site may be an important structural component of the calcitonin r eceptor complex and have a potential physiological role in regions of high extracellular calcium such as at sites of bone resorption.