Characterization of endogenous volume-sensitive ion currents and ATP release in Xenopus laevis oocytes

Abstract

The Xenopus laevis oocyte is a well-known heterologous expression system. Several endogenous ion channels and transporters of Xenopus laevis oocytes have been well characterized, but the identification of volume-sensitive endogenous channels is still a matter of debate. In previous communications we showed that, following shrinkage, oocytes trigger an endogenous ion conductance denoted as Gshrink with simultaneous ATP release. Since shrinkage depends on particular features of aquaporins mediating transmembrane water transport, and these may modulate ion transport by physical interactions, we now evaluated Gshrink using two electrode voltage clamp (TEVC) when either aquaporin-1 (AQP1) or -4 (AQP4) were expressed. In addition, we studied the possible role of the plasma membrane voltage dependent anion channel-1 (pl-VDAC-1) mediating Gshrink. For this purpose, we tested the effect of a hypertonic stimulus Gshrink using oocytes injected with an antisense oligonucleotide of pl-VDAC-1, a channel reported to act as a volume-sensitive channel, permeable to Cl- and other anions such as ATP. Expression pl-VDAC was reported for Xenopus laevis oocytes, but the associated currents were not functionally characterized. Our results indicate that Gshrink is increased in both AQP1 and AQP4 injected oocytes, suggesting the effect of AQPs is not specific. Interestingly, injection of pl-VDAC-1 oligonucleotide antisense reduced Gshrink from 3.7 ± 0.4 to 1.2 ± 0.2 µS (p < 0.05, n=6) in the absence of any aquaporin expression, suggesting a possible role of this channel as direct or indirect mediator Gshrink