M. Koolpe et Hp. Benton, CALCIUM-MOBILIZING PURINE RECEPTORS ON THE SURFACE OF MAMMALIAN ARTICULAR CHONDROCYTES, Journal of orthopaedic research, 15(2), 1997, pp. 204-212
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.