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UTP inhibits store-activated Ca2+ influx in HT29 cells

H. Hu1, P. Poronnik2 D.I. Cook1, 1Department of Physiology, University of Sydney, NSW, Australia, 2School of Biomedical Sciences, University of Queensland, QLD, Australia

In HT29 human colonic epithelial carcinoma cells, the activation of the M3 muscarinic receptor by carbachol (CCh) leads to a Ca2+ response with a characteristic prolonged plateau phase due to Ca2+ influx following Ca2+ store activation.  The activation of the P2Y2 purinergic receptor by UTP, however, does not show this plateau phase.  The lack of a plateau phase during P2Y2 stimulation may result from the inhibition of Ca2+ influx.  Hence, the aim of this study was to investigate whether UTP inhibited store-operated Ca2+ influx. 

Fura-2 imaging techniques were used to monitor changes in intracellular Ca2+ concentration in HT29 cells.

We first used the rate of quenching of intracellular fura-2 by exogenous Mn2+ to estimate the activity of the store-operated Ca2+ channels. We found that the rate of Mn2+ influx during prolonged UTP stimulation was 34% lower than during prolonged CCh stimulation, consistent with UTP inhibiting the activity of the Ca2+ influx channels. We then estimated the activity of these channels by using the rate of increase of intracellular Ca2+ concentration during re-admission of extracellular Ca2+ to cells in which the intracellular Ca2+ stores had been depleted by exposure to thapsigargin in Ca2+-free medium. We found that UTP reduced the rate of Ca2+ influx under these conditions by 45% compared to the rate of Ca2+ influx during re-admission of Ca2+ alone, and by 34% compared to the rate observed during the re-admission of Ca2+ in the presence of CCh. This also suggested that UTP inhibits store-operated Ca2+ influx.

We conclude that UTP inhibits store-activated Ca2+ influx channels.