Figure 4. Swelling-activated chloride current correlates with P-gp expression.

A, mean current following hypotonic activation. The mean current in response to a voltage clamp step of +80 mV was measured 7 min following the latency period; the latency was taken as the time for the current to activate following hypotonic exposure. The graph shows the mean current following activation in cells incubated for 48 h in different concentrations of either anti-mouse mdr (dotted lines) or anti-human MDR1 antisense oligonucleotide (continuous lines). Anti-mouse mdr had no inhibitory effect on the mean current at any concentration, whereas anti-human MDR1 decreased the mean current significantly at 100 and 200 μg ml⁻¹ (n= 14 and 27, respectively; ** P < 0.01). B, the correlation between mean hypotonic-activated current and human MDR1 antisense oligonucleotide uptake (fluorescence). The mean current was measured as in A in cells that had been incubated for 48 h in lipofectin alone (control), and lipofectin with 50, 100 and 200 μg ml⁻¹ human MDR1 antisense oligonucleotide. These current values were then compared with the mean grey level for each of these 4 treatment groups as measured by confocal microscopy (see legend to Fig. 1C). There is an inverse correlation between antisense oligonucleotide concentration and mean current indicating that increasing levels of antisense oligonucleotide cause an increasing block of the hypotonic-activated current. The dashed line is a linear regression fit to the data with a correlation coefficient of 0.99 (P= 0.0045). C, the correlation between mean hypotonic-activated current and P-gp immunofluorescence. The mean current, as above, and the level of P-gp fluorescence associated with the different concentrations of MDR1 antisense oligonucleotide (from Fig. 2D) are plotted. The dashed line represents a linear regression fit to the data (r= 0.95, P= 0.012).