The effects of endurance run training on Na⁺-dependent Ca²⁺ regulation in rat left ventricular myocytes were examined. Myocytes were isolated from sedentary and trained rats and loaded with fura 2. Contractile dynamics and fluorescence ratio transients were recorded during electrical pacing at 0.5 Hz, 2 mM extracellular Ca²⁺ concentration, and 29°C. Resting and peak cytosolic Ca²⁺ concentration ([Ca²⁺]_c) did not change with exercise training. However, resting and peak [Ca²⁺]_c increased significantly in both groups during 5 min of continuous pacing, although diastolic [Ca²⁺]_c in the trained group was less susceptible to this elevation of intracellular Ca²⁺. Run training also significantly reduced the rate of [Ca²⁺]_c decay during relaxation. Myocytes were then exposed to 10 mM caffeine in the absence of external Na⁺ or Ca²⁺ to trigger sarcoplasmic reticular Ca²⁺ release and to suppress cellular Ca²⁺ efflux. This maneuver elicited an elevated steady-state [Ca²⁺]_c. External Na⁺ was then added, and the rate of [Ca²⁺]_c clearance was determined. Run training significantly reduced the rate of Na⁺-dependent clearance of [Ca²⁺]_c during the caffeine-induced contractures. These data demonstrate that the removal of cytosolic Ca²⁺ was depressed with exercise training under these experimental conditions and may be specifically reflective of a training-induced decrease in the rate of cytosolic Ca²⁺ removal via Na⁺/Ca²⁺ exchange and/or in the amount of Ca²⁺ moved across the sarcolemma during a contraction.

sodium/calcium exchange; fura 2; caffeine; sarcolemma; sarcoplasmic reticulum

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