Solid symbols denote presence of hypertension (left) and/or atrial fibrillation (AF) (right), and striped symbols denote individuals with uncertain phenotype. Male subjects are shown as squares and female subjects as circles. The family proband (arrow) and the presence (+) or absence (–) of the R14C KCNQ1 mutation are indicated.

(A) Typical examples of currents recorded from wild-type (WT) (KCNQ1 + KCNE1) and R14C (KCNQ1-R14C + KCNE1) I _Ks channels in Chinese hamster ovary (CHO) cells during 4-s depolarization steps from a holding potential of –80 mV to test potentials in the range –50 to +70 mV followed by repolarization to –60 mV (voltage protocol shown in inset). Scale bars are 0.5 s and 50 pA/pF. Horizontal dashed line indicates zero current level. Solid and dashed arrows indicate points where currents were measured for current-voltage relationships (upper panel to right) and tail currents for isochronal activation curves (lower panel on right), respectively. Tail current data were normalized to the maximum current value (I_max) and fitted with a Boltzmann function: I/I_max = [1 + exp((V_0.5 – V_t)/k)]^–1, where V_0.5 is the half-activation voltage, V_t is the test potential, and k is the slope factor (values for V_0.5 and k are summarized in [Appendix]). (B) Typical examples of currents recorded during 4-s depolarization steps to voltages in the range 0 to +60 mV (voltage protocol shown in inset). Scale bars as for panel A. I _Ks channels undergo transitions through multiple pre-open closed states before finally opening, as indicated by the sigmoidal shape of activation time courses. Rates of activation were estimated by fitting an exponential function (dashed lines) to the second half of the activation time course. Lower panel summarizes the time constants for activation at voltages in the range +20 to +60 mV for WT (red) and R14C (blue) I _Ks channels. (C) Typical examples of tail currents recorded at voltages in the range –60 mV to –120 mV (voltage protocol shown in inset). Rates of deactivation were obtained by fitting a single exponential to tail current recordings. Lower panel summarizes time constants for deactivation in the range –120 mV to –60 mV for WT (red) and R14C (blue) I _Ks channels.

(A) Forskolin (Fsk), 1 µM, resulted in an increase in current amplitude (green) for both wild-type (WT) and R14C I _Ks channels. (B) The Fsk resulted in an 60% increase in current recorded at the end of a 1-s depolarization pulse to +20 mV in both WT (red) and R14C (blue) I _Ks channels. (C) The Fsk caused a small increase in current density measured after 4-s depolarization steps (protocol as shown in Figure 2A) for both WT and R14C I_Ks channels. (D) The Fsk caused a leftward shift in the V_0.5 of the voltage-dependence of activation for WT (red dashed line = control, red boxes = + Fsk) and R14C (blue dashed line = control, blue boxes = + Fsk) I _Ks channels. Both shifts were statistically significant (p < 0.05). The Fsk caused an acceleration of the rates of activation (E) and a deceleration of the rates of deactivation (F) for both WT and R14C I _Ks channels. Symbols in panels E and F are the same as for panel D.

(A) Hypotonic solution resulted in an increase in current amplitude (green) for both wild-type (WT) and R14C I _Ks channels. (B) After 5-min exposure to hypotonic solution, currents recorded at the end of a 1-s depolarization pulse to +20 mV increased to a greater extent in R14C (blue, 215 ± 34%) compared with WT (red, 96 ± 34%) I _Ks channels (WT vs. R14C, p < 0.05). (C) Hypotonic solution caused a greater increase in current density at all voltages for R14C compared with WT I _Ks channels. (D) Hypotonic solution caused a greater leftward shift in the V_0.5 of the voltage dependence of activation for R14C than for WT I _Ks channels (values for V_0.5 and k obtained from Boltzmann fits to the steady-state activation curves are summarized in ). Hypotonic solution caused an acceleration of the rates of activation (E) and deceleration of the rates of deactivation (F) for both WT and R14C I _Ks channels. However, the changes in values for _deact were more marked for R14C than for WT I _Ks channels. Symbols in panels D to F are the same as in Figure 3.

Modeled atrial action potentials using the (i) Nygren (13) and (ii) Courtemanche (14) human atrial action potential models. Results shown were obtained after stimulation for 6 min at 1 Hz (A), 2 Hz (B), and 4 Hz (C). Control data for WT and R14C were indistinguishable (black). Cell swelling caused more action potential shortening in models with R14C (blue) compared with WT I _Ks channels. The heterozygous phenotype (WT/R14C, dashed blue) was intermediate between the WT and R14C phenotypes but is only shown for the Nygren model, panel (i), where the differences can be seen clearly.