Regulation of parathyroid hormone secretion in vitro by divalent cations and cellular metabolism

Parathyroid hormone (PTH) secretion was investigated in intact cells isolated in vitro. Parathyroid cells from bovine parathyroid glands were obtained through tissue dispersion and cell purification through isotonic Percoll gradients, a newly developed protocol enhancing cell homogeneity and viability. Isolated cells maintained both metabolic viability and plasma membrane intactness for over 3 h at 37 degrees C, as shown by the large ATP/ADP ratios and the high intracellular K+ content (ouabain-sensitive) measured. The rate of PTH secretion was inversely related to the Ca2+ concentrations in the medium; secretion was 54 and 18 ng PTH/mg of protein/min when free Ca2+ in the buffer was 0.8 and 2 mM, respectively. At either Ca2+ concentration, PTH secretion was strongly dependent on cell metabolism; it was inhibited by 80-85% within 10 min when cells were suspended in glucose-free buffer containing either cyanide or oxidative phosphorylation uncoupler. Under these conditions, both cellular ATP production and calcium-dependent PTH release could be partially restored by addition of 5 mM glucose. La3+, Mn2+, Sr2+, Ba2+, and Mg2+ were each tested in a range of 0.5-2.5 mM for their effects in suppressing low calcium-stimulated secretion. La3+ and Mn2+ were about twice as effective as Ca2+ on a molar basis, Sr2+ was similar to Ca2+, Mg2+ was about half as effective, and Ba2+ had almost no effect. These results suggest that the Ca2+-dependent stimulus-secretion coupling of these cells is largely different from that established in other secretory cells and provide an in vitro system to further investigate the regulation of PTH secretion.