Calcitonin Receptors on Circulating Normal Human Lymphocytes*

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Abstract
Circulating human lymphocytes possess specific and functional receptors for calcitriol and PTH. We sought to determine if they also possess receptors for calcitonin (CT), the third classical calciotropic hormone. We isolated blood mononuclear cells from healthy volunteers to separate monocytes, total lymphocytes, and T-lymphocytes; the purity of the three cell populations was more than 90%, 95%, and 85%, respectively. Salmon CT (sCT) was labeled by the chloramine-T method (SA, 254 .mu.Ci/.mu.g) without loss of biological activity. We found saturable (16 h at 8 C), specific, high affinity binding sites for [125I]sCT on unstimulated lymphocytes. As for CT receptors on other cells, binding of [125I]sCT was poorly reversible. Binding specificity was demonstrated by the total absence of competing effect of several unrelated hormones; human CT and CT gene-related peptide competed much less efficiently than sCT for the binding sites, whereas PDN-21 had no effect. When plotted according to the method of Scatchard, binding data on the mixed population of T- and B-lymphocytes showed an apparent Kd (mean .+-. SD) of 2.9 .+-. 1.0 .times. 10-10 M (n = 34), with an estimation of 91-8338 (median, 1971) binding sites/cell. The data were repeatedly compatible with an aspect of positive cooperativity between the binding sites, as confirmed by a Hill coefficient greater than 1 (1.18 .+-. 0.13). However, this aspect of positive cooperativity in CT binding was not observed on isolated T-lymphocytes (Hill coefficient, 0.96 .+-. 0.08; n = 9; P < 0.001 vs. the mixed population of lymphocytes). CT did not induce a significant increase in cAMP levels, but regulation of receptor concentration was demonstrated by the finding of down-regulation of CT-binding sites after sCT or human CT preincubation. In summary, we have found saturable, specific, high affinity receptors for CT on unstimulated normal human T-lymphocytes, which could, thus, be target sites for CT action on bone metabolism or on the immune system.

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