Objective—To investigate the effects of moderate
short-term training on K+ regulation in plasma and erythrocytes
during exercise and on skeletal muscle
Na+,K+-ATPase concentration in young adult and
Animals—Four 4- to 6-year-old and four 10- to 16-yearold
Dutch Warmblood horses.
Procedure—The horses underwent a 6-minute exercise
trial before and after 12 days of training. Skeletal
muscle Na+,K+-ATPase concentration was analyzed in
gluteus medius and semitendinosus muscle specimens
before and after the 12-day training period.
Blood samples were collected before and immediately
after the trials and at 3, 5, 7, and 10 minutes after
cessation of exercise for assessment of several
hematologic variables and analysis of plasma and
whole-blood K+ concentrations.
Results—After training, Na+,K+-ATPase concentration
in the gluteus medius, but not semitendinosus, muscle
of middle-aged horses increased (32%), compared
with pretraining values; this did not affect the
degree of hyperkalemia that developed during exercise.
The development of hyperkalemia during exercise
in young adult horses was blunted (albeit not significantly)
without any change in the concentration of
Na+,K+-ATPase in either of the muscles. After training,
the erythrocyte K+ concentration increased (7% to
10%) significantly in both groups of horses but did not
change during the exercise trials.
Conclusions and Clinical Relevance—In horses, the
activation of skeletal muscle Na+,K+-ATPase during
exercise is likely to decrease with age. Training
appears to result in an increase in Na+,K+-ATPase
activity in skeletal muscle with subsequent upregulation
of Na+,K+-ATPase concentration if the existing
Na+,K+-ATPase capacity cannot meet requirements.
(Am J Vet Res 2005;66:1252–1258)