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Articles in PresS, published online ahead of print October 11, 2002
J Appl Physiol, 10.1152/jap.00565.2002
Submitted on June 27, 2002
Accepted on October 2, 2002
1 Department of Kinesiology, University of Wisconsin, Madison, WI, USA
* To whom correspondence should be addressed. E-mail: gmdiffee{at}facstaff.wisc.edu.
Myocardial function is known to be enhanced by endurance exercise training but the cellular mechanisms underlying this improved function remain unclear. We previously showed that endurance exercise training increases the sensitivity of rat cardiac myocytes to activation by Ca2+. The Ca2+ sensitivity of tension in cardiac myocytes has been previously shown to be highly dependent on sarcomere length. In this study we tested the hypothesis that endurance exercise training increases this length dependence of maximal and submaximal tension in rat cardiac myocytes. Female Sprague-Dawley rats were divided into sedentary control(C) and exercise trained (T) groups. The T rats underwent 11 weeks of progressive treadmill exercise (1 h/day, 5 days/wk, 26 m/min, 20% grade). Heart weight was increased by 14% in T compared to C rats and plantaris muscle citrate synthase activity showed a 39% increase in T compared to C animals, evidence of a significant endurance training effect. At the end of the training, myocytes were isolated from T and C hearts, chemically skinned (i.e. the sarcolemma was removed), and attached to a force transducer. Steady state tension was determined in solutions of various [Ca2+] (pCa) and tension-pCa curves were generated at 2 different sarcomere lengths for each myocyte (1.9µm and 2.3µm). We measured maximal tension and the pCa50 (the [Ca2+] giving 50% of maximal tension) for each sarcomere length. We found an increased sarcomere length dependence of both maximal tension and pCa50 in T compared to C myocytes. The 
pCa50 between the long and short sarcomere length was 0.084 ± 0.023 (mean ± SD) in myocytes from C hearts compared to a pCa50 of 0.132 ± 0.014 in myocytes from T hearts (n= 50 myocytes per group). The max. tension was 5.11 ± 1.42 kN/m2 in C myocytes and 9.01 ± 1.28 in T myocytes. We conclude that exercise training increases the length-dependence of maximal and submaximal tension in cardiac myocytes and this change may underlie, at least in part, training induced enhancement of myocardial function.
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