Modulation of the Ca2+-sensing function of parathyroid cells in vitro and in hyperparathyroidism

When raising the extracellular Ca2+ concentration stepwise from 0.5 to 3.0 mM, bovine parathyroid cells reacted with initial transient and sustained elevations of the cytoplasmic Ca2+ concentration (Ca2+i), as well as more than 50% inhibition of parathyroid hormone (PTH) release. Human parathyroid adenoma cells and bovine cells cultured for 1 day or exposed to a low concentration of a monoclonal antiparathyroid antibody exhibited right-shifted dependencies of PTH release and Ca2+i on extracellular Ca2+ and reduced Ca2+i transients. The protein kinase C activator 12-O-tetradecanoylphorbol-13-acetate (TPA) further right-shifted the dose response relationship for Ca2+ regulated Ca2+i of the adenoma cells, whereas the protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7) tended to normalize it, without affecting Ca2+i of normal bovine cells. In cells from an oxyphil adenoma and a parathyroid carcinoma as well as in bovine cells cultured 4 days or exposed to a high concentration of the antiparathyroid antibody, there were no Ca2+i transients, very small increases in steady-state Ca2+i and nonsuppressible PTH release. The results suggest that reduced availability of a putative Ca2+-receptor and increased protein kinase C activity may be important factors in the decreased Ca2+ sensitivity of abnormal parathyroid cells.