Plasma lactic dehydrogenase activities in men during bed rest with exercise training.

Peak oxygen uptake (peak VO2) and basal total activity of lactic dehydrogenase (LDH-T) and its five isoenzymes were measured in seven men, 19-21 years old, before, during, and after three 2-week bed rest (BR) periods, each separated by 3-week recovery periods. For 1 h X d-1 during BR, they performed isometric (250 kcal X -1) leg exercise, or no exercise. LDH-T was reduced (p less than 0.05) with all three regimens by day 10 of BR, but the decrease occurred at different rates. The earliest significant (p less than 0.05) reduction in LDH-T with the no-exercise regimen was associated with the greatest decrease in peak VO2 of 12.3%; isotonic exercise was next with a peak VO2 decrease of 9.2%; finally, isometric exercise had the least decrease in peak VO2 at 4.8%. Irrespective of the exercise regimen during BR, the increases in isoenzyme activities occurred mainly in LDH-1 and LDH-2 (heart subunits). Isometric (aerobic) muscular strength training appears to maintain skeletal muscle integrity and, perhaps, oxygen uptake capacity better during bed rest than isotonic exercise training. Reduced hydrostatic pressure during bed rest, however, ultimately overrides effects of both moderate isometric and isotonic exercise training (metabolism) in the resulting decrease in LDH-T.