Identification of testicular cell plasma membrane receptors for calcitonin

Recent studies have suggested a role for CT in the serum and tissue zinc homeostatic mechanism. The effects of CT on zinc transport appeared to be most pronounced in testis and thymus. If these effects are physiologically important, a cell membrane CT receptor, similar to those described for other peptide hormones, ought to be demonstrable. Rat testicular and hepatic cell membranes were isolated on a linear 15% to 50% sucrose density gradient. Specific hormone binding was assessed by Scatchard analysis of the binding of a constant amount of radiolabeled hormone in the presence of known variable amounts of unlabeled hormone. Nonspecific binding was assessed by measurement of radiolabeled hormone binding in the presence of 10³ molar excess of unlabeled hormone. Hepatic cell membrane demonstrated no specific binding of either ¹²⁵I-synthetic human CT or ¹²⁵I-synthetic 1–34 bovine PTH. Testicular cell membranes demonstrated no specific binding for PTH. CT, however, was specifically bound with a mean K_D of 3.2 ± 0.8 × 10⁻⁸ (S.E.M.) mol/L (M). Specificity studies demonstrated no inhibition of CT binding of 10² molar excess of FSH, LH, 1–34 PTH, or insulin. These data suggest the presence of a specific plasma membrane receptor for CT in rat testes. The presence of a membrane receptor in this tissue is consistent with previous observations that CT can alter the zinc content of rat testis in vivo.