REGULATION OF SYNTHESIS AND SECRETION OF CHROMOGRANIN-A BY CALCIUM AND 1,25-DIHYDROXYCHOLECALCIFEROL IN CULTURED BOVINE PARATHYROID CELLS

A RIA for bovine parathyroid chromogranin-A (CgA) was developed and used to study the regulation of CgA synthesis and release in cultured bovine parathyroid cells. As previously demonstrated, an elevated medium calcium concentration (2.5 mM) led to a reduced release of CgA into the medium. In contrast, the intracellular concentration of CgA was not changed by alterations in the medium calcium concentration. 1,25-Dihydroxyvitamin D3 [1,25-(OH)2D3] stimulated a dose-dependent increase in the release of CgA, whereas intracellular CgA levels were not markedly altered. Alterations in medium calcium concentration did not affect CgA mRNA levels, whereas 1,25-(OH)2D3 caused a time- and dose-dependent increase in CgA mRNA levels. Therefore, changes in CgA release occurred in response to calcium and 1,25-(OH)2D3, with little change in the intracellular CgA concentration. This could rise by a change in the synthetic rate to match the alteration in the secretion rate and/or a change in the degradation rate. Calcium does not appear to affect CgA synthesis at a pretranslational level, whereas 1,25-(OH)2D3 stimulated a marked increase in CgA mRNA levels via an effect on CgA gene transcription. Furthermore, 1,25-(OH)2D3 had no effect on CgA mRNA and total relationship between steady state levels of CgA mRNA and total CgA levels (intracellular and extracellular) is not simple and possibly indicates that an important regulatory step for CgA synthesis occurs at the level of mRNA translation. The effect of 1,25-(OH)2D3 on CgA mRNA levels was not modulated by medium calcium concentrations. In addition, the effect of 1,25-(OH)2D3 in reducing PTH mRNA levels was not affected by medium calcium concentrations, with 1,25-(OH)2D3 and calcium acting independently to reduce PTH mRNA levels.