The secretion of parathyroid hormone (PTH) is inversely related to the extracellular Ca2+ concentration (Cae 2+). To test the hypothesis that a Ca2+ sensor on the surface of parathyroid cells is involved in Ca2+-regulated PTH secretion, limited trypsinization of bovine parathyroid cells was carried out. Treatment with trypsin (1·1–10 mg/ml) inhibited, in a dose-dependent manner, PTH secretion stimulated by lowering Cae 2+ from 2·0 to 0·5 mmol/l. In control cells, activation of protein kinase C with 12-O-tetradecanoylphorbol-13-acetate (TPA) enhanced PTH secretion at 2·0 mmol Cae 2+/1 but not at 0·5 mmol Cae 2+/1. In trypsinized cells, however, TPA enhanced PTH secretion at both 0·5 and 2·0 mmol Cae 2+/1. Isoproterenol-stimulated PTH secretion was maintained in trypsinized cells, but reduced cyclic AMP production revealed that some β-adrenergic receptors were destroyed. The cytosolic free Ca2+ concentration (Cai 2+), as measured with fura-2, was raised within seconds in response to increasing Cae 2+ from 0·5 to 2·0 mmol/l and was then lowered within 1 min to a sustained plateau; the changes were the same in trypsinized and control cells. In conclusion, trypsinization of parathyroid cells abolished Ca2+-regulated PTH secretion without affecting Cai 2+.
Journal of Endocrinology (1989) 122, 213–218