EXTRACELLULAR ATP MODULATES [CA2-TREATED EMBRYONIC CHONDROCYTES(](I) IN RETINOIC ACID)

When treated with low doses of retinoic acid (RA), cephalic chondrocyt es of the chick embryonic sternum mature and express phenotypic charac teristics of postmitotic hypertrophic cells. In concert with these mat uration-dependent changes, cells release adenine nucleotides into the culture medium. To ascertain if these compounds modulate chondrocyte f unction, we challenged chondrocytes with nucleotides and measured one determinant of the signal transduction pathway, intracellular Ca2+ con centration ([Ca2+](i)). In the presence of micromolar concentrations o f ATP, there was a dose-dependent elevation in chondrocyte [Ca2+](i); ADP caused a small but significant rise in the peak [Ca2+](i) response . We found that the change in the [Ca2+](i) response is linked to reti noid-dependent maturation of chondrocytes. Thus the [Ca2+](i) rise was dependent on the RA concentration and treatment time. Immature caudal chondrocytes, cells that were not affected by RA, were used as contro l cells for this study. When treated with ATP, these cells did not exh ibit a [Ca2+](i) response. Although the purinergic subtype receptor wa s not characterized, the observation that cells responded to ATP and A DP but were refractory to AMP and adenosine suggested that P-2 purinoc eptors were expressed by chondrocytes. Because, during the same cultur e period, chondrocytes exhibited many of the unique characteristics of the terminally differentiated cell, the acquisition of purinergic rec eptors represents a new feature associated with expression of the matu re phenotype. Finally, to ascertain if the ATP-dependent response was due to release of Ca2+ from intracellular stores, cells were treated w ith thapsigargin. Since this compound significantly reduced the [Ca2+] (i) signal, we concluded that the ATP response is mediated by release of cations from the endoplasmic reticulum.