1. The whole-cell configuration of the patch-clamp technique was used to determine if P-glycoprotein (Pgp) is a swelling-activated Cl- channel. 2. Hamster pgp1 cDNA was transfected into a mouse fibroblast cell line resulting in expression of functional Pgp in the plasma membrane. This cell line was obtained without exposure to chemotherapeutic agents. 3. Swelling-activated whole-cell Cl- current (I(Cl,swell) was elicited by lowering the bath osmolality. I(Cl,swell) was characterized in detail in the pgp1-transfected mouse cell line and compared with that of its parental cell line. Expression of Pgp did not modify the magnitude or properties of I(Cl,swell), except that addition of the anti-Pgp antibody C219 to the pipette solution inhibited this current 75% only in the Pgp-expressing cells. 4. I(Cl,swell) in the mouse Pgp-expressing cell line was compared with that in a Pgp-expressing hamster fibroblast cell line. The characteristics of I(Cl,swell) (voltage dependence, blocker sensitivity, anion selectivity sequence, requirement for hydrolysable ATP) in Pgp-expressing cells were different between the two cell lines. These results suggest that the channel(s) responsible for I(Cl,swell) are different between the two cell lines. In addition, C219 inhibited I(Cl,swell) in both Pgp-expressing cell lines, even though they seem to express different swelling-activated Cl- channels. 5. We conclude that firstly, Pgp is not a swelling-activated Cl- channel; secondly, it possibly functions as a Cl- channel regulator; and thirdly, I(Cl,swell) is underlined by different Cl- channels in different cells.
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