Dixon CJ, Bowler WB, Walsh CA, Gallagher JA (1997) Effects of extracellular nucleotides on single cells and populations of human osteoblasts: Contribution of cell heterogeneity to relative potencies. Brit. J. Pharmacol. 120:777-780.

Human osteoblasts responded to the application of extracellular nucleotides, acting at P-2-receptors, with increases in cytosolic free calcium concentration ([Ca2+](i)). 2 In populations of human osteoblasts, adenosine 5'-diphosphate (ADP) evoked a rise in [Ca2+](i) with less than 40% of the amplitude of that induced by adenosine 5'-triphosphate (ATP). 3 ATP and uridine 5'-triphosphate (UTP) were applied to single human osteoblasts and induced [Ca2+](i) rises of comparable amplitude in every cell tested. 4 However, from the results of single cell studies with ADP (and 2- methylthioATP (2-meSATP)) two groups of cells were delineated: one group responded to ADP (or 2-meSATP) with a rise in [Ca2+](i) indistinguishable from that evoked by ATP; whereas the second group failed completely to respond to ADP (or 2-meSATP). 5 Therefore heterogeneity of receptor expression exists within this population of human osteoblasts. The limited distribution of the ADP- responsive receptor underlies the small response to ADP, compared with ATP, recorded in populations of human osteoblasts. This heterogeneity may reflect differences in the differentiation status of individual cells.

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