Altered K+ current profiles underlie cardiac action potential shortening in hyperkalemia and β-adrenergic stimulation

Abstract

Hyperkalemia is known to develop in various conditions including vigorous physical exercise. In the heart, hyperkalemia is associated with action potential (AP) shortening that was attributed to altered gating of K⁺ channels. However, it remains unknown how hyperkalemia changes the profiles of each K⁺ current under a cardiac AP. Therefore, we recorded the major K⁺ currents (inward rectifier K⁺ current, I_K1; rapid and slow delayed rectifier K⁺ currents, I_Kr and I_Ks, respectively) using AP-clamp in rabbit ventricular myocytes. As K⁺ may accumulate at rapid heart rates during sympathetic stimulation, we also examined the effect of isoproterenol on these K⁺ currents. We found that I_K1 was significantly increased in hyperkalemia, whereas the reduction of driving force for K⁺ efflux dominated over the altered channel gating in case of I_Kr and I_Ks. Overall, the markedly increased I_K1 in hyperkalemia overcame the relative decreases of I_Kr and I_Ks during AP, resulting in an increased net repolarizing current during AP phase 3. β-Adrenergic stimulation of I_Ks also provided further repolarizing power during sympathetic activation, although hyperkalemia limited I_Ks upregulation. These results indicate that facilitation of I_K1 in hyperkalemia and β-adrenergic stimulation of I_Ks represent important compensatory mechanisms against AP prolongation and arrhythmia susceptibility.

Keywords: action potential voltage-clamp; arrhythmia; arythmie; canaux potassiques; cellular electrophysiology; cœur; exercice physique; heart; hyperkalemia; hyperkaliémie; physical exercise; potassium channels; stimulation sympathique; sympathetic stimulation; voltage-clamp du potentiel d’action; électrophysiologie cellulaire.

Fig. 1.

Shortening of ventricular action potential (AP) in hyperkalemia. (A) Representative ventricular APs at 1 Hz pacing in either 4 or 8 mM [K⁺]_o. Pipette solution contained 10 mM BAPTA. AP upstroke and its derivative (dV/dt) are enlarged in insets. (B) AP duration measured at 90% repolarization (APD₉₀) was significantly decreased in hyperkalemia in a reverse-rate dependent manner. (C) Depolarization of resting membrane potential (V_rest) followed the change in Nernst-potential for K⁺, which led to a decrease in peak voltage of the AP overshoot (V_peak). (D) Maximal upstroke velocity (dV/dt_max) significantly decreased, while maximal rate of repolarization (−dV/dt_max) significantly increased in hyperkalemia. Columns/symbols and bars represent mean ± SEM. n = 11 cells from 4 animals. Student’s paired t test. ***, p < 0.001. [Color online.]

Fig. 2.

Outward I_K1 is significantly increased in hyperkalemia. (A) Representative traces and I–V relationships of 100 μM Ba²⁺-sensitive inward rectifier K⁺ current (I_K1) in a rabbit ventricular myocyte (applied voltage protocol is shown in the inset). [Ca²⁺]_i was buffered to nominally zero by 10 mM BAPTA in the pipette solution. I_Kr, I_Ks, L-type Ca²⁺, and voltage-gated Na⁺ currents were inhibited using 1 μM E-4031, 1 μM HMR-1556, 10 μM nifedipine, and 10 μM tetrodotoxin, respectively. (B) Hyperkalemia increased the peak outward current. (C) The shift in reversal potential followed the change in the Nernst-potential for K⁺. The voltage where the outward current reached its peak density was more positive in high [K⁺]_o. (D) Both inward and outward I_K1 (measured at −160 and −50 mV, respectively) were significantly increased in hyperkalemia. Columns and bars represent mean ± SEM. n = 18 cells from 6 animals. Student’s paired t test. ***, p < 0.001. [Color online.]

Fig. 3.

Altered K⁺ current profiles under action potential (AP)-clamp in hyperkalemia and β-adrenergic stimulation. (A) I_K1 traces (mean ± SEM) recorded under AP-clamp in physiological (4 mM) and elevated (8 mM) [K⁺]_o. A prerecorded, typical rabbit ventricular AP (shown above) was used as voltage command in all AP-clamp experiments (canonical AP-clamp) at 2 Hz pacing frequency. I_K1 was measured as 100 μM Ba²⁺-sensitive current. [Ca²⁺]_i was buffered using 10 mM BAPTA in the pipette, whereas Ca²⁺ and Na⁺ channels were inhibited using 10 μM nifedipine and 10 μM tetrodotoxin, respectively. Diastolic I_K1 in 8 mM [K⁺]_o is out of range. (B) I_Kr traces (mean ± SEM) were recorded under AP-clamp using 1 μM E-4031. (C) I_Ks traces (mean ± SEM) recorded under AP-clamp using 1 μM HMR-1556. (D–F) I_K1, I_Kr, and I_Ks traces recorded (mean ± SEM) in the presence of 10 nM isoproterenol (ISO) in 4 and 8 mM [K⁺]_o. (G–I) Peak current densities. I_K1 peak density was significantly increased in 8 mM [K⁺]_o both in basal conditions and following ISO stimulation. I_Kr was decreased in 8 mM [K⁺]_o and its density was unchanged by ISO stimulation. I_Ks was significantly upregulated following ISO stimulation; however, the increase in I_Ks amplitude was attenuated in 8 mM [K⁺]_o. Columns and bars represent mean ± SEM. n refers to cells/animals measured in each group. Two-way ANOVA with Bonferroni post hoc test. *, p < 0.05; ***, p < 0.001; NS, not significant. [Color online.]

Fig. 4.

Relative contributions of each K⁺ current in adaptation to hyperkalemia and β-adrenergic stimulation. (A) Net charges of I_Kr, I_Ks, and I_K1 in 4 and 8 mM [K⁺]_o under control and in the presence of 10 nM isoproterenol (ISO). (B) Relative contributions and magnitudes of the main K⁺ currents during action potential (AP) phase 3 are compared in different phase of the repolarization process in 4 and 8 mM [K⁺]_o. Upregulation of I_K1 overcomes the decreases of I_Kr and I_Ks during AP phase 3 measured at −20 and −60 mV. (C) β-Adrenergic stimulation significantly increased net repolarizing current via I_Ks upregulation; however, the relative contribution of I_K1 was still dominant in 8 mM [K⁺]_o vs. 4 mM [K⁺]_o. Columns represent mean current densities. [Color online.]