HARD TRAINING FOR 5 MO INCREASES NA-K+ PUMP CONCENTRATION IN SKELETAL-MUSCLE OF CROSS-COUNTRY SKIERS()

To study how training affects the Na+-K+ pump concentration, 11 male a nd 9 female elite junior cross-country skiers trained 12-15 h/wk at 60 -70% (moderate-intensity group) or 80-90% (high-intensity group) of th eir maximal O-2 uptake for 5 mo. Muscle biopsies taken from the vastus lateralis muscle before and after the training period were analyzed f or Na+-K+ pump concentration by the [H-3]ouabain-binding technique. Be fore training, the concentration was 343 +/- 11 nmol/kg wet muscle mas s (mean +/- SE) for the men and 281 +/- 14 nmol/kg for the women (18% less than for the men, P = 0.003). The Na+-K+ pump concentration rose by 49 +/- 11 nmol/kg (16%, P < 0.001) for all subjects pooled during t he training period, and there was no difference between the two traini ng groups (P = 0.3) or the sexes (P = 0.5) in this increase. The Na+-K + pump concentration correlated with the maximal O-2 uptake (r = 0.6, P = 0.003), with the performance during a 20-min treadmill run (r = 0. 6, P = 0.003), and to the rank of the subjects' performance as cross-c ountry skiers (Spearman's rank correlation coefficient = 0.76, P < 0.0 01). These data could mean that for elite cross-country skiers the per formance is related to the Na+-K+ pump concentration. However, other s tudies have shown an equally high pump concentration for far less fit subjects, suggesting that the pump concentration may not be a limiting factor.