THE REDUCED RESPONSIVENESS OF CULTURED BOVINE PARATHYROID CELLS TO EXTRACELLULAR CA2+ IS ASSOCIATED WITH MARKED REDUCTION IN THE EXPRESSION OF EXTRACELLULAR CA2+-SENSING RECEPTOR MESSENGER-RIBONUCLEIC-ACID AND PROTEIN

PTH secretion from dispersed bovine parathyroid cells maintained in culture becomes progressively less responsive to changes in the extracellular Ca2+ concentration (Ca-0(2+)) over several days. We have now investigated whether this change in secretory control is associated with alterations in the expression of the Ca-0(2+)-sensing receptor (BoPCaR) recently cloned from bovine parathyroid, which plays a central role in Ca-0(2+)-regulated PTH secretion. BoPCaR messenger RNA levels dropped rapidly in cultured bovine parathyroid cells, as assessed by Northern analysis, decreasing by 78% within 18 h and remaining low for at least 4 days. The level of receptor protein decreased to a comparable extent (similar to 72-82%) after 3-4 days in culture, as determined by immunocytochemistry with specific antibodies directed at the extracellular domain of the receptor. The half-time for the reduction in receptor protein (similar to 2 days) was considerably longer, however, than that for BoPCaR messenger RNA, but was comparable to that for the loss of sensitivity of PTH secretion to Ca-0(2+). Indeed, there was a close linear correlation between maximal suppressibility of PTH secretion and the intensity of staining for the receptor protein (r = 0.88; P = 0.004). We conclude that alterations in the expression of BoPCaR could explain much of the reduced responsiveness of cultured bovine parathyroid cells to Ca-0(2+).