Beta-adrenergic stimulation reverses the I Kr-I Ks dominant pattern during cardiac action potential

Abstract

β-Adrenergic stimulation differentially modulates different K(+) channels and thus fine-tunes cardiac action potential (AP) repolarization. However, it remains unclear how the proportion of I Ks, I Kr, and I K1 currents in the same cell would be altered by β-adrenergic stimulation, which would change the relative contribution of individual K(+) current to the total repolarization reserve. In this study, we used an innovative AP-clamp sequential dissection technique to directly record the dynamic I Ks, I Kr, and I K1 currents during the AP in guinea pig ventricular myocytes under physiologically relevant conditions. Our data provide quantitative measures of the magnitude and time course of I Ks, I Kr, and I K1 currents in the same cell under its own steady-state AP, in a physiological milieu, and with preserved Ca(2+) homeostasis. We found that isoproterenol treatment significantly enhanced I Ks, moderately increased I K1, but slightly decreased I Kr in a dose-dependent manner. The dominance pattern of the K(+) currents was I Kr > I K1 > I Ks at the control condition, but reversed to I Kr < I K1 < I Ks following β-adrenergic stimulation. We systematically determined the changes in the relative contribution of I Ks, I Kr, and I K1 to cardiac repolarization during AP at different adrenergic states. In conclusion, the β-adrenergic stimulation fine-tunes the cardiac AP morphology by shifting the power of different K(+) currents in a dose-dependent manner. This knowledge is important for designing antiarrhythmic drug strategies to treat hearts exposed to various sympathetic tones.

Figure 1

β-adrenergic stimulation effects on the AP, the three K⁺ currents, and the Ca²⁺ transient. Panel A and B show the AP-clamp Sequential Dissection experiments to directly record the steady-state AP (upper panel) at 1 Hz pacing rate, the three K⁺ currents (mid panel) in the same cell, and the Ca²⁺ transients (lower panel) under physiological condition (CTRL) and following 30 nM isoproterenol (ISO) treatment. Note that the Ca²⁺ transient during the AP is preserved by having the endogenous Ca²⁺ buffers and without adding any exogenous Ca²⁺ buffer (No EGTA) in pipette solution. Panel C and D demonstrate that the show that the Ca²⁺ transients were largely eliminated by using 1 mM EGTA in the pipette solution under both the control condition and following ISO treatment.

Figure 2

Dose-dependent β-adrenergic tuning of delayed rectifier K⁺ currents during the AP. Panel A and B show the I_Kr recorded during the cell’s own AP before and after β-adrenergic stimulation using 30 nM ISO. In each panel the upper trace shows the AP and the lower trace shows the corresponding current. ISO had little effect on the amplitude of I_Kr at lower concentrations but slightly reduced I_Kr at 30 nM (Panel C). Panel D and E show the AP and I_Ks current traces before and after 30 nM ISO treatment, respectively. ISO increased I_Ks during AP in dose-dependent manner (Panel F). Each point on panel C and F represents averaged current values and standard error from 7–15 cells isolated from 7 hearts. Student’s t-test p values: p<0.05*, p<0.01**, p<0.001***.

Figure 3

Dose-dependent β-adrenergic tuning of I_K1 current. Representative traces were recorded in the absence (panel A) and presence (panel B) of 30 nM ISO. Dose response curves for current values at different membrane potentials and total charge movement during AP are shown in panel C and D respectively. ISO increased I_K1 during AP plateau, but left diastolic current value unaltered. n=7–15 cells from 7 animals.

Figure 4

β-adrenergic stimulation shifts the relative contribution of individual K⁺ currents to the total repolarizing reserve. The three K⁺ currents measured in the same cell were summed, and then each current was normalized to this sum. ISO dose-response curves for normalized values measured at +20 mV and −20 mV are shown in panel A and B respectively. Panel C shows the total amount of K⁺ charge movement for the individual K⁺ current and the sum of the currents during the AP. n=7–15 cells from 7 animals.

Figure 5

β-adrenergic state alters the effects of specific K⁺ channel inhibitors on modifying the AP. The AP lengthening effect of using 1 μM Chromanol-293B to block I_Ks is moderate under control condition (Panel A) but became prominent after 30 nM ISO treatment. The AP lengthening effect of using 1 μM E-4031 to block I_Kr is similar under control conditions (Panel C) and after ISO treatment (Panel D).