Activation of the sarcoplasmic reticulum Ca 2+-ATPase induced by exercise

Abstract

Prolonged exercise has been shown to cause disruption of intracellular calcium homeostasis in skeletal muscle, which is normally maintained by the sarcoplasmic reticulum (SR) Ca 2+-ATPase. We have investigated the response of this enzyme to increased intracellular calcium levels by investigating the functional and physical characteristics of the SR Ca 2+-ATPase and membrane lipids following 2 h of treadmill running and throughout a period of post-exercise recovery. The Ca 2+-ATPase of SR membranes purified from exercised rats shows increases in enzymatic activity correlating with post-exercise recovery time. Corresponding increases in active Ca 2+-ATPase pump units are observed, as measured by the concentration of phosphorylated enzyme intermediate formed from ATP. However, catalytic turnover rates of the Ca 2+-ATPase are unchanged. Using spin-label electron paramagnetic resonance to assess both membrane fluidity and associations between individual Ca 2+-ATPase polypeptide chains, we find no exercise-induced alterations in membrane dynamics which could explain the observed increases in Ca 2+-ATPase activity. Nor do we find evidence for altered membrane purification as a result of exercise. We suggest that the cell responds to the challenge of increased cytosolic calcium levels by increasing the proportion of functional SR Ca 2+-ATPase proteins in the membrane for the rapid restoration of calcium homeostasis.