Mitochondria and sarcoplasmic reticulum (SR) fractions were isolated from exercised trained (E T) and sedentary control dog hearts. Measurements of mitochondrial respiratory functions indicated no changes in energy producing (ATP synthesis) capacity in mitochondria from E T compared to control dog hearts. However, the ability of isolated mitochondria from E T hearts to retain accumulated calcium was markedly decreased compared to controls. Inhibition of mitochondrial rates of calcium uptake with the inhibitor, ruthenium red, revealed fewer binding and/or transport sites in mitochondrial membranes from exercised trained heart preparations. ATP dependent binding (-oxalate) and uptake (+ oxalate) of calcium by SR preparations from E T hearts were unchanged compared to controls. In contrast, significant differences in the rates of release of bound calcium were found in SR isolated from E T hearts. Total myocardial protein, nucleic acids, and connective tissue levels were unchanged in E T hearts compared to controls. The results suggest subtle changes are occurring in the energy utilizing mechanism(s) involving calcium transport of the myocardial cell during exercise training. These changes may be related to alterations in the performance of the exercised trained heart.
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