CALCIUM-MOBILIZING PURINE RECEPTORS ON THE SURFACE OF MAMMALIAN ARTICULAR CHONDROCYTES

If we are to fully understand mechanisms of cartilage homeostasis, it is essential that we know the full catalogue of receptors present on t he surface of a chondrocyte and the pathways regulated by ligands that bind to these receptors. In this study, we describe chondrocyte respo nses to adenosine 5'-triphosphate and related molecules. Adenosine 5'- triphosphate stimulated a statistically significant, dose-dependent, t ransient rise in the concentration of calcium ions in Fura 2-loaded, d ifferentiated, primary chondrocytes. The increase occurred in the abse nce of extracellular calcium, indicating a mobilization from intracell ular stores. The increase in concentration of cytoplasmic calcium ions induced by adenosine 5'-triphosphate was mimicked by uridine 5'-triph osphate but not by 2-methylthioadenosine 5'-triphosphate, cytidine 5'- triphosphate, or adenosine. Heterologous desensitization experiments d emonstrated that chondrocytes showed no subsequent response to uridine 5'-triphosphate after initial stimulation with adenosine 5'-triphosph ate nor did they respond to adenosine 5'-triphosphate in inverse condi tions, thereby indicating competition for the same receptor site. Toge ther, these results are consistent with the presence of a P-2U recepto r on the cell surface of chondrocytes. Purine-induced calcium mobiliza tion in passaged chondrocytes showed the same pharmacological profile with respect to agonist sensitivity, but responses were of greater mag nitude than responses in primary differentiated chondrocytes, suggesti ng upregulation of the receptor with time in culture. Adenosine 5'-tri phosphate and uridine 5'-triphosphate (1-100 mu M) did not alter carti lage matrix synthesis as measured by rate of incorporation of [S-35]su lfate into glycosaminoglycan by cartilage explants or primary chondroc ytes. Matrix degradation, measured by release of glycosaminoglycan fro m cartilage explants, was also unaltered by adenosine 5'-triphosphate or uridine 5'-triphosphate (1-100 mu M). Production of prostaglandin E -2 was upregulated by incubation with either adenosine 5'-triphosphate or uridine 5'-triphosphate. These data demonstrate the presence of a functional P-2U-like purine receptor on the surface of primary articul ar chondrocytes and support the hypothesis that altered concentrations of extracellular purines may influence chondrocyte metabolism.