Properties of parathyroid hormone receptors on circulating bovine lymphocytes

Binding of parathyroid hormone (PTH) to circulating bovine lymphocytes was studied using, as the radioligand, a synthetic sulfur-free analog of bovine PTH, [Nle8,Nle18,Tyr34]bPTH-(1–34)amide, which was labeled to high specific activity with 125I and was purified by reverse-phase high-performance liquid chromatography. Binding of PTH to lymphocytes satisfies several criteria indicative of a specific interaction between the hormone and its receptor. Specific binding is saturable at 3.3 fmoles of radioligand bound per 107 cells, occurs more rapidly at 37°C than at lower temperatures, and reaches equilibrium within 2 hr at 15°C. Inhibition of specific binding occurs with intact PTH, with biologically active PTH analog or fragment, and with synthetic PTH antagonists, but not with biologically inactive PTH fragments, or peptide hormones unrelated to PTH. There is an excellent correlation between the binding affinities of PTH agonists with receptors on lymphocytes and those previously reported with receptors in canine renal membranes, and on rat osteosarcoma cells. The dissociation constant (Kd) is approximately 10−9M, as calculated from the association and dissociation rate constants. This correlates very closely both with the apparent Kd, as estimated from Scatchard analysis of radioligand saturation and competition studies, and with previously reported Kd of PTH receptors in canine renal membranes and on intact rat osteosarcoma and opossum kidney cells. In addition, the relative binding affinity of intact hormone and a synthetic PTH agonist to receptors on lymphocytes correlates closely with the relative biologic potency of these peptides in stimulating adenylate cyclase in membranes from these cells. The presence of PTH specific receptors on lymphocytes is consistent with evidence suggesting that PTH-lymphocyte interactions may modulate bone resorption in vitro. Also, the ready accessibility of circulating lymphocytes may serve as a useful model system for examining PTH-receptor regulation in vivo.