Effects of simulated ischemia on the transmural differences in the Frank-Starling relationship in isolated mouse ventricular cardiomyocytes

Ischemia Enhances the Acute Stretch-Induced Increase in Calcium Spark Rate in Ventricular Myocytes

Introduction: In ventricular myocytes, spontaneous release of calcium (Ca²⁺) from the sarcoplasmic reticulum via ryanodine receptors (“Ca²⁺ sparks”) is acutely increased by stretch, due to a stretch-induced increase of reactive oxygen species (ROS). In acute regional ischemia there is stretch of ischemic tissue, along with an increase in Ca²⁺ spark rate and ROS production, each of which has been implicated in arrhythmogenesis. Yet, whether there is an impact of ischemia on the stretch-induced increase in Ca²⁺ sparks and ROS has not been investigated. We hypothesized that ischemia would enhance the increase of Ca²⁺ sparks and ROS that occurs with stretch.

Methods: Isolated ventricular myocytes from mice (male, C57BL/6J) were loaded with fluorescent dye to detect Ca²⁺ sparks (4.6 μM Fluo-4, 10 min) or ROS (1 μM DCF, 20 min), exposed to normal Tyrode (NT) or simulated ischemia (SI) solution (hyperkalemia [15 mM potassium], acidosis [6.5 pH], and metabolic inhibition [1 mM sodium cyanide, 20 mM 2-deoxyglucose]), and subjected to sustained stretch by the carbon fiber technique (~10% increase in sarcomere length, 15 s). Ca²⁺ spark rate and rate of ROS production were measured by confocal microscopy.

Results: Baseline Ca²⁺ spark rate was greater in SI (2.54 ± 0.11 sparks·s⁻¹·100 μm⁻²; n = 103 cells, N = 10 mice) than NT (0.29 ± 0.05 sparks·s⁻¹·100 μm⁻²; n = 33 cells, N = 9 mice; p < 0.0001). Stretch resulted in an acute increase in Ca²⁺ spark rate in both SI (3.03 ± 0.13 sparks·s⁻¹·100 μm⁻²; p < 0.0001) and NT (0.49 ± 0.07 sparks·s⁻¹·100 μm⁻²; p < 0.0001), with the increase in SI being greater than NT (+0.49 ± 0.04 vs. +0.20 ± 0.04 sparks·s⁻¹·100 μm⁻²; p < 0.0001). Baseline rate of ROS production was also greater in SI (1.01 ± 0.01 normalized slope; n = 11, N = 8 mice) than NT (0.98 ± 0.01 normalized slope; n = 12, N = 4 mice; p < 0.05), but there was an acute increase with stretch only in SI (+12.5 ± 2.6%; p < 0.001).

Conclusion: Ischemia enhances the stretch-induced increase of Ca²⁺ sparks in ventricular myocytes, with an associated enhancement of stretch-induced ROS production. This effect may be important for premature excitation and/or in the development of an arrhythmogenic substrate in acute regional ischemia.

Mechanism of Action Potential Prolongation During Metabolic Inhibition in the Whole Rabbit Heart

Myocardial ischemia is associated with significant changes in action potential (AP) duration, which has a biphasic response to metabolic inhibition. Here, we investigated the mechanism of initial AP prolongation in whole Langendorff-perfused rabbit heart. We used glass microelectrodes to record APs transmurally. Simultaneously, optical AP, calcium transient (CaT), intracellular pH, and magnesium concentration changes were recorded using fluorescent dyes. The fluorescence signals were recorded using an EMCCD camera equipped with emission filters; excitation was induced by LEDs. We demonstrated that metabolic inhibition by carbonyl cyanide-p-trifluoromethoxyphenylhydrazone (FCCP) resulted in AP shortening preceded by an initial prolongation and that there were no important differences in the response throughout the wall of the heart and in the apical/basal direction. AP prolongation was reduced by blocking the I_CaL and transient outward potassium current (I_to) with diltiazem (DTZ) and 4-aminopyridine (4-AP), respectively. FCCP, an uncoupler of oxidative phosphorylation, induced reductions in CaTs and intracellular pH and increased the intracellular Mg²⁺ concentration. In addition, resting potential depolarization was observed, clearly indicating a decrease in the inward rectifier K⁺ current (I_K1) that can retard AP repolarization. Thus, we suggest that the main currents responsible for AP prolongation during metabolic inhibition are the I_CaL, I_to, and I_K1, the activities of which are modulated mainly by changes in intracellular ATP, calcium, magnesium, and pH.