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The secretion of parathyroid hormone (PTH) is inversely related to the extracellular Ca²⁺ concentration (Ca_e²⁺). 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 Ca_e²⁺ 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 Ca_e²⁺/1 but not at 0·5 mmol Ca_e²⁺/1. In trypsinized cells, however, TPA enhanced PTH secretion at both 0·5 and 2·0 mmol Ca_e²⁺/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 (Ca_i²⁺), as measured with fura-2, was raised within seconds in response to increasing Ca_e²⁺ 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 Ca_i²⁺.

Journal of Endocrinology (1989) 122, 213–218