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UTP inhibits store-activated Ca²⁺ influx in HT29 cells

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.