Modeling regulation of cardiac KATP and L-type Ca2+ currents by ATP, ADP, and Mg2+

Abstract

Changes in cytosolic free Mg(2+) and adenosine nucleotide phosphates affect cardiac excitability and contractility. To investigate how modulation by Mg(2+), ATP, and ADP of K(ATP) and L-type Ca(2+) channels influences excitation-contraction coupling, we incorporated equations for intracellular ATP and MgADP regulation of the K(ATP) current and MgATP regulation of the L-type Ca(2+) current in an ionic-metabolic model of the canine ventricular myocyte. The new model: 1), quantitatively reproduces a dose-response relationship for the effects of changes in ATP on K(ATP) current, 2), simulates effects of ADP in modulating ATP sensitivity of K(ATP) channel, 3), predicts activation of Ca(2+) current during rapid increase in MgATP, and 4), demonstrates that decreased ATP/ADP ratio with normal total Mg(2+) or increased free Mg(2+) with normal ATP and ADP activate K(ATP) current, shorten action potential, and alter ionic currents and intracellular Ca(2+) signals. The model predictions are in agreement with experimental data measured under normal and a variety of pathological conditions.

FIGURE 1

Schematic diagram illustrating Ca²⁺ and Mg²⁺ buffering and transport by ATP and ADP, adenine nucleotides regulation of ionic channels and pump, and electrophysiology in ventricular myocyte. See Appendix II for the notations of the parameters used throughout the study.

FIGURE 2

Schematic representation of Kir6.2 and SUR2A subunit stoichiometry in cardiac octameric K_ATP channel (A). It is assumed that the single ATP molecule (•) is sufficient to close the channel and that the binding of 2MgADP molecules (▴) to one SUR2A subunit increases the channel open probability. Panel B shows and action-potential time courses (9–10 s; 1-Hz frequency) generated with original ionic-metabolic model (dashed lines) and with reduced-order model of the L-type Ca²⁺ channel (solid lines). Normal condition (1) is 5 mM 200 μM 0.5 mM ; metabolic inhibition (2) is 3 mM 3 mM 0.68 mM

FIGURE 3

Effects of ATP on cardiac EC coupling. (A) Dose-response relationship for the effects of free ATP (or total ATP from 0 to 10 mM) on relative K_ATP channel current (). The solid line was fitted to experimental data by Nichols et al. (1991a) (▪) with = 600 μM, = 400 μM, = 0.05 mS/μF, = 0.08, = 0.89. (B–F) Time courses of and action potential. Letters (i–v) correspond to 5, 4, 3, 2, and 0.5 mM respectively. Insets show experimental recordings (Fig. 4 B in Nichols et al., 1991a) of superimposed (panel B), twitches (panel E), and action potentials (panel F) for the effects of injection of stimulated K_ATP current. Letters (i–v) in insets correspond to digitized experimental plots (i, ii, vii, x, xi). Simulations are generated in response to 1-Hz pulse and model outputs at the tenth stimulus are shown only. 200 μM, 4.84 mM, 4 mM, 138 mM, 2 mM.

FIGURE 4

Dependence of K_ATP channel activity on ADP in the presence of Mg²⁺. Calculated ATP-sensitive K⁺ currents (5–10 s) in response to 1-Hz stimuli are shown. (A) Top, middle, and bottom traces show the shift in the sensitivity of K_ATP channels to ATP caused by increases in free ADP. (B) Effects of 1 mM increase or decrease in total ATP from 5 mM in the absence of ADP on K_ATP current. (C) Effects of 100 μM increase or decrease in total ADP from 200 μM in the absence of ATP on K_ATP current. 4.84 mM, 4 mM, 138 mM, 2 mM.

FIGURE 5

Absolute levels of ATP and ADP regulate cardiac EC coupling independently of ATP/ADP ratio. Panels A–D show effects of 50% simultaneous increase or decrease in total ATP and total ADP with unchanged on and action potential (9–10 s) in response to 1-Hz periodic pulse. (Dashed line) 7.5 mM, 300 μM. (Solid line) 5 mM, 200 μM. (Dashed-dotted line) 2.5 mM, 100 μM. Total Mg²⁺ ∼ 4.84 mM, ATP/ADP = 25.

FIGURE 6

Effects of cytosolic Mg²⁺ on cardiac EC coupling. Panels A–D show effects of changes in cytosolic Mg²⁺ on and action potential (9–10 s) in response to 1-Hz periodic pulse. (Dashed-dotted line) 0.2 mM, total Mg²⁺ 3.73 mM. (Solid line) 0.5 mM, total Mg²⁺ 4.84 mM. (Dashed line) 1.8 mM, total Mg²⁺ 6.7 mM. Total ATP 5 mM, total ADP 200 μM. Bottom trace in panel A shows an expanded view of the peak of L-type Ca²⁺ current. Inset in panel D is from Agus et al. (1989) and shows the suppressive effect of elevated Mg²⁺ levels on the experimentally recorded action potentials. The first, second, and last experimental plots (right to left) are digitized.

FIGURE 7

Panels A–H show model outputs in response to 1-Hz periodic pulse (9–10 s). Bottom trace in panel E shows an expanded view of the peak of L-type Ca²⁺ current. Normal conditions are 6.8 mM, 15 μM, 2 mM (solid line). Ischemia (40 s) is 5.4 mM, 30 μM, 3.165 mM (dashed line). Ischemia (10 min) is 4.6 mM, 99 μM, 3.55 mM (dashed-dotted line). Total Mg²⁺ 8.56 mM.