The voltage-gated potassium channel KCNQ4 is found by in situ hybridisation in outer hair cells of the mouse cochlea (Kubisch et al. 1999). It has been tentatively identified with a subunit of the hair cell potassium current, termed IK,n, which is distinguished by an exceptionally negatively shifted activation curve (V1/2 = -78 mV) (Kros & Marcotti, 1999). In contrast, KCNQ4 in oocytes activates slowly at more positive potentials (V1/2 = -17.8 mV).
We used Chinese hamster ovary (CHO) cells as a mammalian expression system. The normal electrophysiological phenotype of these cells is a small leak conductance. To study the properties of KCNQ4, we have co-transfected CHO cells with the plasmids containing cDNAs of KCNQ4 (a gift from T.J. Jentsch) and green fluorescent protein (pEGFP-N1, Clonetech), both driven by a CMV promotor. Approximately 20 % of the cells were identified as showing GFP fluorescence. Of GFP-positive cells studied, over 80 % expressed a slowly developing outward current.
Using the whole-cell configuration of the patch-clamp technique, we have investigated the characteristics of these transfected cells. The average cell capacitance of the CHO cells was 22.2 ± 8.7 pF (mean ± S.D., n = 21). The outward current showed properties of KCNQ4. The current activated with a time constant of 110 ± 46 ms when the potential was stepped to 0 mV. At 0 mV the current was 2.64 ± 1.46 nA in amplitude. The current was half activated at a potential of -28.8 ± 8.0 mV. In eight cells, outward current was almost completely blocked by 200 µM linopirdine (mean inhibition 82 %). The anti-arrhythmic bepridil, a blocker of KCNQ1, applied at 10 µM, also blocked 49 % (n = 3) of the CHO outward currents.
Added to the bath, 500 µM 8-Br-cAMP activated outward current and shifted the V1/2 by approximately 30 mV (n = 5). Added to the pipette solution, 50 U ml-1 of the β-catalytic subunit of PKA also induced a similar negative shift in two cells.
The most economical interpretation, consistent with the behaviour of adult hair cells, is that the potassium channel complex may be under the control of high levels of a cAMP protein kinase. Developing around postnatal day 10 in mouse such phosphorylation would produce the activation pattern of KCNQ4 and the negative resting potential found in adult hair cells.
This work was supported by The Wellcome Trust.