The role of stochastic and modal gating of cardiac L-type Ca2+ channels on early after-depolarizations

Abstract

Certain signaling events that promote L-type Ca2+ channel (LCC) phosphorylation, such as beta-adrenergic stimulation or an increased expression of Ca(2+)/calmodulin-dependent protein kinase II, promote mode 2 gating of LCCs. Experimental data suggest the hypothesis that these events increase the likelihood of early after-depolarizations (EADs). We test this hypothesis using an ionic model of the canine ventricular myocyte incorporating stochastic gating of LCCs and ryanodine-sensitive calcium release channels. The model is extended to describe myocyte responses to the beta-adrenergic agonist isoproterenol. Results demonstrate that in the presence of isoproterenol the random opening of a small number of LCCs gating in mode 2 during the plateau phase of the action potential (AP) can trigger EADs. EADs occur randomly, where the likelihood of these events increases as a function of the fraction of LCCs gating in mode 2. Fluctuations of the L-type Ca2+ current during the AP plateau lead to variability in AP duration. Consequently, prolonged APs are occasionally observed and exhibit an increased likelihood of EAD formation. These results suggest a novel stochastic mechanism, whereby phosphorylation-induced changes in LCC gating properties contribute to EAD generation.

FIGURE 1

Markov model describing L-type Ca²⁺ channel. Top row describes Mode Normal; lower row, Mode Ca. States O and O_Ca are open states; all other states are closed.

FIGURE 2

Model and experimental APs and Ca²⁺ transients for control (black) and β-AR stimulation (shaded). (A) Representative APs measured in isolated canine myocytes in control bath (black) and ISO (shaded). (B) Indo-1 fluorescence ratio measured simultaneously with the APs in panel A (Greenstein et al., 2004). (C) Control (black) and β-AR stimulated (shaded) model APs. (D) Model cytosolic Ca²⁺ concentration (μM) corresponding to the APs in panel C. Results in panels C and D were performed with a model including only 2000 simulated CaRUs.

FIGURE 3

(A) Membrane potential (ordinate, mV) as a function of time (abscissa, s) for the baseline β-AR-stimulated model in response to 1 Hz pacing. APD distribution (ordinate) in simulations of 150 APs with 0% (B), 15% (C), or 25% (D) of LCCs gating in mode 2 (abscissa, 10-ms bins).

FIGURE 4

Total I_CaL (ordinate, pA/pF) during a normal AP with 0% (black), 15% (light shaded), and 25% (dark shaded) of LCCs gating in mode 2, plotted against time (abscissa, ms). (A) Total I_CaL during an AP demonstrating little effect of modal fraction on peak current amplitude. (B) Blow-up of boxed area in panel A during the AP plateau. (C) Total I_CaL averaged over 10 APs during the AP plateau.

FIGURE 5

(A) Membrane potential (ordinate, mV) as a function of time (abscissa, ms) for two AP simulations in which model parameters, state variable initial conditions, and initial states of each channel are identical, where only the random number generator seeds are initialized to different values. (B) Membrane potential (ordinate, mV) as a function of time (abscissa, ms) for three AP simulations in which the random number generator seeds are reinitialized from the single-EAD AP (solid shaded line) to different values at 140 ms (dashed shaded line) and 170 ms (solid black line) after the stimulus, resulting in a non-EAD AP and a two-EAD AP, respectively. (C) L-type Ca²⁺ currents corresponding to the three APs shown in panel B. (D) Ratio of LCCs (ordinate) that have not inactivated via the voltage-dependent (dash-dotted line), the Ca²⁺-dependent (dashed line), or either (solid line) mechanism, plotted against time (abscissa, ms).

FIGURE 6

(A) Total Ca²⁺ current through LCCs (ordinate, pA/pF) during an EAD (black), compared with currents from a non-EAD AP (shaded) plotted against time (abscissa, ms). (B) Mode 1 and (C) mode 2 LCC current components (ordinate, pA/pF) of the currents in panel A, plotted against time (abscissa, ms).

FIGURE 7

(A) Power spectra (ordinate, dB/Hz) of detrended total I_CaL with 0% (solid black), 25% (dark shaded), and 50% (light shaded) of LCCs gating in mode 2, plotted against frequency (abscissa, Hz). (B) Average power spectra (ordinate, dB/Hz) of voltage fluctuations averaged over 10 APs with 0% (solid black) and 25% (dark shaded) of LCCs gating in mode 2, plotted against frequency (abscissa, Hz).