Positive regulation of capacitative Ca2+ entry by intracellular Ca2+ in Xenopus oocytes expressing rat TRP4.

We have investigated the role of intracellular Ca2+ in the opening of capacitative Ca2+ entry (CCE) channels formed with rat TRP4 (rTRP4) using Xenopus oocytes. In rTRP4-expressing oocytes pretreated with thapsigargin, perfusion with A23187, a Ca2+ ionophore, significantly potentiated the delayed phase of the CCE-mediated Cl- current response evoked by extracellular perfusion with Ca2+, without affecting the transient phase of CCE response. In control oocytes, the potentiation of delayed CCE response by A23187 was not significant. Using cut-open recording in combination with artificial intracellular perfusion of oocytes, CCE-mediated Cl- response was recorded at controlled cytosolic Ca2+ concentrations. Intracellular perfusion with a Ca2+ free solution containing 10 mM EGTA abolished most of the CCE responses of both non-injected and rTRP4-expressing oocytes. The native CCE response was not fully recovered by subsequent increases in the intracellular Ca2+ concentration up to 300 nM. However, CCE response of the rTRP4-expressing oocytes was restored at an internal Ca2+ concentration of 110 nM. Blockade of endogenous Cl- channels with anion channel blocker isolated Ca2+ current flowing through CCE channels and clarified the difference in the sensitivity to an internal Ca2+ concentration. These findings indicate that recombinant CCE channels formed with rTRP4 are positively regulated by cytosolic Ca2+ at higher sensitivity compared to oocyte-endogenous CCE channels.