Proceedings of The Physiological Society
University of Edinburgh (2007) Proc Physiol Soc 6, C1
Regulation of Electrophysiology and Pharmacology of Voltage-Gated Potassium Channels by Ancillary Subunits Found in the Pregnant Uterus
S. Abdul Sahib2, M. E. Duffey2, G. C. Bett1
1. Gynecology-Obstetrics, State University of New York, University at Buffalo, Buffalo, NY, USA. 2. Physiology and Biophysics, State University of New York, University at Buffalo, Buffalo, NY, USA.
The electrical profile of the myometrium undergoes dramatic changes during pregnancy, as the myometrial smooth muscle changes from being largely quiescent to exhibiting well coordinated excitation-contraction coupling. Voltage-gated potassium channel currents contribute to repolarization, and therefore play a key role in determining myometrial excitability. KCNE1 is a small protein (130 amino acids) which forms a single transmembrane alpha helix. The expression of KCNE1 in the uterus is strongly and rapidly mediated by estrogen. KCNE1 does not form a functional channel by itself, but acts as an ancillary subunit to the KCNQ1 voltage-gated ion channel. KCNQ1 (KvLQT1 or Kv7.1) in the uterus is thought to contribute to repolarization of the myometrial action potential. Changes in the biophysical and pharmacological behavior of KCNQ1 by KCNE1 will likely contribute to changes in the electrical and pharmacological profile during pregnancy. We used two-electrode voltage clamp to study the effect of KCNE1 on KCNQ1 when cloned KCNQ1 (P51787) and KCNE1 (NP_000210) were heterologously expressed in Xenopus oocytes. Oocytes were injected with 50 ng mRNA for KCNQ1, and co-injected with KCNE1 as noted, in a 1:1 ratio. We used the KCNQ1-specific open channel blocker chromanol 293B and the sodium/potassium channel blocker quinidine as pharmacological probes. KCNE1 significantly slows KCNQ1 activation, resulting in a sigmoidal onset of activation. The KCNQ1/KCNE1 current continues to increase, even at the end of a 3 s depolarizing pulse. KCNE1 also slows deactivation, and removes voltage-dependent inactivation. When a 500 ms depolarizing pulse from -90 to +50 mV was applied with a 500 ms inter-pulse interval there was a potentiation of KCNQ1/KCNE1 current. The increase was well described by two exponentials, τfast = 1.07 ± 0.04 s, and τslow = 7.66 ± 0.43 s (n = 4). The fast time constant dominates: Afast/Aslow = 4.78 ± 0.16 (n = 4). There was a ~4 fold increase in the pharmacological sensitivity of KCNQ1 to Chromanol 293B when it was co-expressed with KCNE1. IC50 for KCNQ1 alone was 65.4 ± 1.7 μM in contrast to the IC50 for KCNQ1/KCNE1, which was only 15.1 ± 3.3 μM. KCNE1 had a similar effect on KCNQ1 affinity for quinidine. Application of 400 μM Quinidine reduced KCNQ1 current by 8.0 ± 4.5 % (n = 5), whereas KCNQ1/KCNE1 was reduced by 30.3 ± 4.0 % (n = 4). These data suggest that the estrogen-dependent KCNE1 ancillary subunit strongly modulates voltage-gated KCNQ1 ion channel physiology and pharmacology. Understanding the contribution of KCNQ1 to the uterine electrical and pharmacological profile therefore requires an understanding of how KCNE1 subunits modulate KCNQ1.
Where applicable, experiments conform with Society ethical requirements