Effects of chronic run training on Na+-dependent Ca2+ efflux from rat leftventricular myocytes

The effects of endurance run training on Na+-dependent Ca2+ regulation in r at left ventricular myocytes were examined. Myocytes were isolated from sed entary and trained rats and loaded with fura 2. Contractile dynamics and fl uorescence ratio transients were recorded during electrical pacing at 0.5 H z, 2 mM extracellular Ca2+ concentration, and 29 degrees C. Resting and pea k cytosolic Ca2+ concentration ([Ca2+](c)) did not change with exercise tra ining. However er, resting and peak [Ca2+](c) increased significantly in bo th groups during 5 min of continuous pacing, although diastolic [Ca2+](c) i n 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 abse nce of external Na+ or Ca2+ to trigger sarcoplasmic reticular Ca2+ release and to suppress cellular Ca2+ efflux. This maneuver elicited an elevated st eady-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 contractur es. These data demonstrate that the removal of cytosolic Ca2+ was depressed with exercise training under these experimental conditions and may be spec ifically 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 acro ss the sarcolemma during a contraction.