NA+-K+ ATPASE CONCENTRATION IN DIFFERENT ADULT-RAT SKELETAL-MUSCLES IS RELATED TO OXIDATIVE POTENTIAL

To investigate the relationship among fibre type, oxidative potential, and Na+-K+ ATPase concentration in skeletal muscle, adult male Wistar rats weighing 259 +/- 8 g (X(BAR) +/- SE) were sacrificed and the soleus (SOL), extensor digitorum longus (EDL), red vastus lateralis (RV), and white vastus lateralis (WV) removed. These muscles were chosen as being representative of the two major fibre type populations: slow twitch (SOL) and fast twitch (EDL, RV, WV) and exhibiting either a high (SOL, EDL, RV) or low (WV) oxidative potential. Na+-K+ ATPase concentration (pmol.g(-1) wet weight), measured by the [H-3]ouabain binding technique, differed (p < 0.01) only between the WV (238 +/- 7.9) and the SOL (359 +/- 9.6), EDL (365 +/- 10), and RV (403 +/- 12). Similarly, muscle oxidative potential as measured by the maximal activity of citrate synthase was different (p < 0.01) only between the WV and the other three muscles. Citrate synthase activity (mu mol.min(-1).g(-1) wet weight) was 4.0 +/- 0.7, 12.3 +/- 0.9, 9.1 +/- 0.7, and 11.3 +/- 1.0 in the WV, SOL, EDL, and RV, respectively. These results indicate that Na+-K+ ATPase concentration is not related to the speed of contraction but to the oxidative potential of the muscle. Since chronic activity is a primary determinant of oxidative potential, it would be expected that increases in Na+-K+ ATPase would accompany increases in muscle utilization.