Simulation of Ca-calmodulin-dependent protein kinase II on rabbit ventricular myocyte ion currents and action potentials

Abstract

Ca-calmodulin-dependent protein kinase II (CaMKII) was recently shown to alter Na(+) channel gating and recapitulate a human Na(+) channel genetic mutation that causes an unusual combined arrhythmogenic phenotype in patients: simultaneous long QT syndrome and Brugada syndrome. CaMKII is upregulated in heart failure where arrhythmias are common, and CaMKII inhibition can reduce arrhythmias. Thus, CaMKII-dependent channel modulation may contribute to acquired arrhythmic disease. We developed a Markovian Na(+) channel model including CaMKII-dependent changes, and incorporated it into a comprehensive myocyte action potential (AP) model with Na(+) and Ca(2+) transport. CaMKII shifts Na(+) current (I(Na)) availability to more negative voltage, enhances intermediate inactivation, and slows recovery from inactivation (all loss-of-function effects), but also enhances late noninactivating I(Na) (gain of function). At slow heart rates, with long diastolic time for I(Na) recovery, late I(Na) is the predominant effect, leading to AP prolongation (long QT syndrome). At fast heart rates, where recovery time is limited and APs are shorter, there is little effect on AP duration, but reduced availability decreases I(Na), AP upstroke velocity, and conduction (Brugada syndrome). CaMKII also increases cardiac Ca(2+) and K(+) currents (I(Ca) and I(to)), complicating CaMKII-dependent AP changes. Incorporating I(Ca) and I(to) effects individually prolongs and shortens AP duration. Combining I(Na), I(Ca), and I(to) effects results in shortening of AP duration with CaMKII. With transmural heterogeneity of I(to) and I(to) downregulation in heart failure, CaMKII may accentuate dispersion of repolarization. This provides a useful initial framework to consider pathways by which CaMKII may contribute to arrhythmogenesis.

FIGURE 1

Markov model of the cardiac Na⁺ channel. The channel model contains background (upper nine states) and burst (lower four states) gating modes. The burst mode reflects a population of channels that transiently fail to inactivate and accounts for a sustained inward current (or late current).

FIGURE 2

Experimental (squares) and simulated (solid lines) voltage-clamp protocols for βGal- (black) and CaMKII-modulated (gray) Na⁺ channels (data at 140 mM external Na⁺ concentration). (A) Steady-state inactivation. CaMKIIδc overexpression shifts the availability curve toward negative potentials. (B) Activation. Na⁺ channel activation is not affected by CaMKIIδc overexpression (traces are superimposed). (C and D) CaMKIIδc slows the recovery from inactivation (C) and enhances the intermediate inactivation (D). (E) Late I_Na is enhanced by CaMKIIδc overexpression. Simulated traces are shown at left, and bar graphs of simulated and experimental normalized current integrals at right. (F) Fast and slow time constants of I_Na decay. Acute CaMKIIδc overexpression slows the late component of fast I_Na inactivation. Experimental data are from Wagner et al. (15).

FIGURE 3

CaMKII effects on Na⁺ channel gating affect the AP in a rate-dependent manner. (A–C) Simulations show that at slower heart rates the enhanced late I_Na prolongs the AP (B and C); this effect is completely blunted at faster rates (A), where the reduced channel availability slows down the AP upstroke (A, inset). (D) The upstroke velocity (dV_m/dt) is reduced upon CaMKII overexpression in a rate-dependent fashion: the faster the pacing rate, the more significant the decrease in maximum dV_m/dt. (E) Quantitative summary of the APD₉₀ dependence on the pacing rate.

FIGURE 4

Experimental (left) and simulated (right) CaMKII effects on [Na⁺]_i induced by I_Na alterations. (Left) The dotted line shows the shifted experimental CaMKII data to equalize [Na⁺]_i at 2 Hz. (Right) The dashed line represents the [Na⁺]_i at different frequencies when I_Na, I_Ca, and I_to alterations due to CaMKII are incorporated into the model.

FIGURE 5

CaMKII effects on I_Ca. (Left) Experimental (A) and simulated (C) traces recorded upon a depolarization pulse. (Right) Experimental (B) and simulated (D) I/V relations. I_Ca is significantly increased in CaMKIIδc compared to control (LacZ). Inactivation is slowed by CaMKIIδc. Experimental data (upper panels) are from Kohlhaas et al. (8).

FIGURE 6

CaMKII effects on I_to. Experimental (left) and simulated (middle) I/V relations for total (A and D), slow (B and E), and fast (C and F) I_to. CaMKIIδc-mediated increase of the total current (solid symbols) is mainly due to CaMKIIδc effects on the slow component. (G) Representative current traces during voltage-clamp activation protocol for experimental (upper) and simulated (lower) CaMKII-mediated I_to. (H) Recovery from inactivation was investigated using a 500-ms depolarization pulse (from −80 mV holding potential to +50 mV) followed by recovery intervals of increasing duration and a subsequent test pulse. Recovery from inactivation was significantly increased by CaMKIIδc. Symbols represent experimental data, and lines indicate simulation results for βGal (black) and CaMKII (gray). Data are from Wagner et al. (15).

FIGURE 7

Simulated APs at 1-Hz pacing rate (A) and APD-frequency relations (B) in βGal and CaMKIIδc-overexpressing cardiac myocytes. The separate effects of each CaMKII-induced current alteration, as well as their combined impact, have been considered. CaMKII alteration of I_Na prolongs the AP at slow heart rate, with no effects at a 2-Hz pacing. The APD is reduced by I_to and increased by I_Ca at all frequencies. The predicted overall effect of CaMKII is AP shortening.

FIGURE 8

Experimental (A and B) and simulated (C and D) APs and APD-frequency relations in βGal and CaMKIIδc-overexpressing cardiac myocytes. Experimental data are from Wagner et al. (15). CaMKII shortens the AP in the entire range of pacing rates. At 0.1 Hz, where no experimental data are available, the difference between βGal and CaMKIIδc APs is negligible.

FIGURE 9

APD₉₀ sensitivity to the late Na⁺ current amplitude. APs were simulated by varying the amount of late Na⁺ current in a range between 0.02% and 0.2% of the peak current in βGal (open symbols) and between 0.17% and 0.35% in CaMKII (solid symbols), thus keeping constant the differential current amplitude. APD changes significantly with the late current amplitude, and the sensitivity increases with the lowering of the pacing frequency (compare 0.5 and 1 Hz). Note that the results of AP shortening upon CaMKII overexpression are confirmed for any value of late Na⁺ current amplitudes in the explored range.

FIGURE 10

Simulated APs at 1 Hz in βGal and CaMKIIδc-overexpressing cardiac myocytes in the presence of (A) 100%, (B) 25%, and (C) 10% I_to. (D) Ratio of CaMKII to βGal APDs for different levels of I_to. When I_to is fully expressed (A, epi), CaMKII shortens the AP, whereas the AP is prolonged due to I_to downregulation (current expression <60% (D), e.g., endo (B and C)). This could amplify transmural dispersion of repolarization (endo-epi: 66 and 78 ms for βGal versus 152 and 183 ms for CaMKII), which may predispose cells to reentry phenomena.