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

In ventricular myocytes, spontaneous release of calcium (Ca) from the sarcoplasmic reticulum via ryanodine receptors ("Ca 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 Ca 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 Ca sparks and ROS has not been investigated. We hypothesized that ischemia would enhance the increase of Ca sparks and ROS that occurs with stretch. Isolated ventricular myocytes from mice (male, C57BL/6J) were loaded with fluorescent dye to detect Ca 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). Ca spark rate and rate of ROS production were measured by confocal microscopy. Baseline Ca spark rate was greater in SI (2.54 ± 0.11 sparks·s·100 μm; = 103 cells, = 10 mice) than NT (0.29 ± 0.05 sparks·s·100 μm; = 33 cells, = 9 mice; < 0.0001). Stretch resulted in an acute increase in Ca spark rate in both SI (3.03 ± 0.13 sparks·s·100 μm; < 0.0001) and NT (0.49 ± 0.07 sparks·s·100 μm; < 0.0001), with the increase in SI being greater than NT (+0.49 ± 0.04 vs. +0.20 ± 0.04 sparks·s·100 μm; < 0.0001). Baseline rate of ROS production was also greater in SI (1.01 ± 0.01 normalized slope; = 11, = 8 mice) than NT (0.98 ± 0.01 normalized slope; = 12, = 4 mice; < 0.05), but there was an acute increase with stretch only in SI (+12.5 ± 2.6%; < 0.001). Ischemia enhances the stretch-induced increase of Ca 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.