The Effects of Calcium and Magnesium on the Secretion of Parathormone and Parathyroid Secretory Protein by Isolated Porcine Parathyroid Cells*

Abstract

The preparation of dispersed parathyroid cells by collagenase digestion of porcine parathyroid glands is described. The cells secrete parathormone [PTH] linearly for at least 4 h of incubation and rapidly respond in inverse fashion to changes in the medium Ca and Mg concentrations over the range 0.5-3.0 mM. In terms of inhibition of secretion, either ion was more effective in the presence of a minimum concentration of the other, indicating that Ca and Mg affect separate cellular sites. PTH was identified both by immunoassay of the whole incubation medium and by its separation by polyacrylamide gels and carboxymethylcellulose chromatography. When the cells were incubated with radioactive amino acids and both the medium and cells were subsequently analyzed on gels, it was found that PTH secretory protein as well as PTH and some immunoactive fragments were present. Analysis of the radioactive protein contained in the cells at high and low Cal concentrations revealed that Cal decreased the formation of the secretory protein by approximately 40% without appreciably affecting the formation of proparathormone or PTH. The secretion of parathyroid secretory protein and PTH were inversely proportional to the concentrations of medium Cal or Mg. The secretion of the latter was more sensitive (95% inhibition) than PTH (40-60% inhibition) to changes in medium divalent cations. The synthesis, intracellular processing, or secretion of PTH and parathyroid secretory protein utilize independent Ca- and Mg-regulated pathways.