Background: 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) production. In acute regional ischemia there is stretch of ischemic tissue, along with an increase in Ca²⁺ sparks 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 production is unknown.
Objective: Determine if ischemia alters the effect of stretch on Ca²⁺ sparks and ROS production in ventricular myocytes.
Methods: Isolated ventricular myocytes from mice (male, C57BL/6J) were loaded with fluorescent dye to detect Ca²⁺ sparks (4.6 µM Fluo-4) or ROS (1 µM DCF), exposed to physiologic or simulated ischemia solution (hyperkalemia [15 mM K⁺], acidosis [6.5 pH], and metabolic inhibition [1 mM sodium cyanide, 20 mM 2-deoxyglucose]), and subjected to sustained stretch by the carbon fibre technique (20 µm, 15 s). Rate of Ca²⁺ sparks or ROS production were measured by confocal microscopy before and during stretch.
Results: Baseline Ca²⁺ spark rate was greater in ischemic (53±2 sparks/s; n=121 cells, N=10 mice) than physiologic conditions (7±1 sparks/s; n=41, N=10; p<0.0001). Stretch resulted in an acute increase in Ca²⁺ spark rate in both conditions, however the increase was greater in ischemia (+10±1 vs +4±1 sparks/s; p<0.0001). Similarly, baseline rate of ROS production was greater in ischemic (1.01±0.01 normalized slope; n=11, N=8) than physiologic conditions (0.98±0.01 normalized slope; n=12, N=4; p<0.05) and the acute increase with stretch was also greater in ischemia (+12.5±2.6 vs +2.1±2.7%; p<0.05).
Conclusion: Ischemia enhances the stretch-induced increase of Ca²⁺ sparks and ROS production in ventricular myocytes. This effect may contribute to premature excitation and/or the development of an arrhythmogenic substrate in acute regional ischemia