The current-voltage (I-V) relationship of the human cystic fibrosis transmembrane conductance regulator (CFTR) Cl^– channel exhibits weak inward rectification at voltages above +50 mV. This weak inward rectification is caused by a small reduction in single-channel current amplitude and changes in gating kinetics (Cai & Sheppard, 2000). To investigate further rectification of the CFTR Cl^– channel, we studied murine CFTR. When compared with human CFTR, the murine CFTR Cl^– channel has a reduced single-channel conductance and an altered pattern of channel gating characterised by prolonged openings to a subconductance state and only brief openings to the full open state (Lansdell et al. 1998).

To investigate the effect of voltage on the full open state of murine CFTR, excised inside-out membrane patches from CHO cells expressing wild-type murine CFTR were bathed in symmetrical 147 mM Cl^– solutions and single-channel current amplitude and open probability (P_o) measured at negative and positive voltages. CFTR Cl^– channels were activated by the addition of PKA (75 nM) and ATP (1 mM) to the intracellular solution; temperature was 37 °C. Like human CFTR, the single-channel I-V relationship of murine CFTR inwardly rectified at voltages above +50 mV. For human CFTR, chord conductance decreased from 11.00 ± 0.38 pS at -100 mV to 9.47 ± 0.29 pS at +100 mV (13 ± 4 % change; mean ± S.E.M.; n = 6; P < 0.05; Student’s t test). Similarly, for murine CFTR, chord conductance decreased from 7.30 ± 0.38 pS at -100 mV to 5.30 ± 0.28 pS at +100 mV (27 ± 3 % change; n = 5; P < 0.05). Clamping membrane voltage at positive voltages caused a small reduction of P_o for human CFTR -75 mV, P_o = 0.56 ± 0.05; +75 mV, P_o = 0.53 ± 0.05; n = 6; P < 0.05). In contrast, the P_o of the full open state of murine CFTR was greatly decreased at positive voltages -80 mV, P_o = 0.10 ± 0.05; +80 mV, P_o = 0.05 ± 0.04; n = 8; P < 0.05).

These data suggest that a small reduction in chord conductance and a large change in P_o cause the inward rectification of murine CFTR. The data also suggest that the degree of rectification of murine CFTR is stronger than that of human. Future studies will examine the effect of voltage on the subconductance state of murine CFTR.

This work was supported by the CF Trust.

This work was supported by the CF Trust.

Cai, Z. & Sheppard, D.N. (2000). Pediatr. Pulmonol. suppl. 20, 180.

Lansdell, K.A., Kidd, J.F., Delaney, S.J., Wainwright, B.J. & Sheppard, D.N. (1998). J. Physiol. 512, 751-764.