P‐glycoprotein is not a swelling‐activated Cl⁻ channel; possible role as a Cl⁻ channel regulator

Abstract

The whole‐cell configuration of the patch‐clamp technique was used to determine if P‐glycoprotein (Pgp) is a swelling‐activated Cl− channel. 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. Swelling‐activated whole‐cell Cl− current (Icl,swell) was elicited by lowering the bath osmolality. Icl,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 Icl,swell, except that addition of the anti‐Pgp antibody C219 to the pipette solution inhibited this current by 75% only in the Pgp‐expressing cells. 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 ICl,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 ICl,swell are different between the two cell lines. In addition, C219 inhibited ICl,swell in both Pgp‐expressing cell lines, even though they seem to express different swelling‐activated Cl− channels. We conclude that firstly, Pgp is not a swelling‐activated Cl− channel; secondly, it possibly functions as a Cl− channel regulator; and thirdly, ICl,swell is underlined by different Cl− channels in different cells.

Biophysical Properties of ClC-3 Differentiate It from Swelling-activated Chloride Channels in Chinese Hamster Ovary-K1 Cells

“…For two other putative ion channels pICln and P-glycoprotein, the initial reports of channel activity were later shown to result from the protein in question activating endogenous chloride channels (4,10). Nonetheless, there are several facts that suggest we are observing the current flow through the expressed channel.…”

“…Accordingly, great efforts have been focused on determining the molecular identity of this channel. Several candidates have been proposed, such as pICln (7), P-glycoprotein (8), and ClC-2 (9), but in each case further investigation has shown that the proposed channel was not responsible for the observed currents (4,10).…”

Physiology of Cell Volume Regulation in Vertebrates

“…P-glycoprotein is withdrawn as a candidate for the channel per se (see Refs. 716,741), but several groups have reported evidence for Pglycoprotein being a regulator of VRAC (655,1022,1027). With respect to the possible role of I Cln , the discussion can be followed in References 99, 267, 306, 550, 777, and 1040.…”

Fundamental role of ClC-3 in volume-sensitive Cl- channel function and cell volume regulation in AGS cells

“…Instead it was suggested that pICln is a regulator for a volume-sensitive chloride channel (13). P-glycoprotein (P-gp), the product of the multidrug resistance (MDR1) gene, proposed as a candidate for the swelling-activated chloride channels, was subsequently shown to be a regulator of endogenous channel activity (2,24,25). Another swelling-activated chloride channel expressed in many cell types, ClC-2, has also been suggested as contributing to RVD (29).…”