Failure of P‐glycoprotein (MDR1) expressed in Xenopus oocytes to produce swelling‐activated chloride channel activity.

Abstract

1. P-glycoprotein, the protein product of the multidrug resistance (MDR1) gene, has ATPdependent transporter activity. It has been suggested that P-glycoprotein may also function as a volume-regulated chloride channel or chloride channel regulator. To assess the chloride channel function of P-glycoprotein, we examined swelling-activated chloride conductances in Xenopusoocytes injected with human MDR1 cRNA. 2. Functional expression of P-glycoprotein in Xenopus oocytes was confirmed using Western blot analysis and by assessing transport of the P-glycoprotein substrate, calcein AM. 3. Endogenous, swelling-activated chloride conductances were virtually absent by the time P-glycoprotein expression was confirmed. Thus, this expression system afforded the advantage of assessing putative MDR1 -associated chloride currents in the absence of background currents. 4. The currents activated by hypotonic shock (50 %) in both MDR1 -injected and control (waterinjected) oocytes were not significantly different. The swelling response was due in part to the activation of a potassium-selective conductance which could be inhibited by barium. No chloride-selective currents were activated by hypotonic shock in the presence or absence of barium. Therefore, we conclude that P-glycoprotein expression does not produce a swellingactivated chloride conductance in the Xenopus oocyte expression system.

Bestrophin 1 is indispensable for volume regulation in human retinal pigment epithelium cells

“…Defolliculated oocytes were thoroughly monitored for endogenous swelling-activated whole-cell currents as reported earlier (33). Three days after injection, oocyte rupture and volume-activated whole-cell currents were virtually absent in waterinjected control oocytes, similar to earlier studies (34). In AQP1-injected oocytes, the hypotonic bath solution activated only a small endogenous whole-cell current before membrane bursting (Fig.…”

Functional Characterization of Human Breast Cancer Resistance Protein (BCRP, ABCG2) Expressed in the Oocytes of Xenopus laevis

“…Previously, human MDR1 (Morin et al, 1995) and mouse MDR1b (Castillo et al, 1990) have been expressed functionally in X. laevis oocytes. Here we present the first demonstration of the functional expression of glycosylated BCRP in X. laevis oocytes and provide evidence that the functional form of BCRP in these oocytes is a homodimer with complex substrate interactions.…”

NH 4 + conductance in Xenopus laevis oocytes

“…Ackerman et al [1] reported a 24±2 mV shift in membrane potential when bath chloride was decreased from 103.6 mmol/l to 55.6 mmol/l during hypoosmolality. Whereas their finding suggests the presence of a volume-sensitive Cl -conductance, Morin et al [15] found no shift in zero-current potential under comparable experimental conditions. Again, interindividual differences may account for these conflicting results, showing hyperpolarization (our results), depolarization [1] or no changes [15] upon hypoosmolal challenge of the oocytes.…”

“…With regard to X. laevis oocytes, conflicting results exist. Whereas Ackerman et al [1] and Arellano and Miledi [2] described an endogenous Cl -current activated by hypoosmolality, no such current was detected by Morin et al [15]. In contrast, these authors found that a K + conductance contributes to oocyte volume regulation, which others could not find [1,2,6].…”