PLASMA-VOLUME AND ION REGULATION DURING EXERCISE AFTER LOW-CARBOHYDRATE AND HIGH-CARBOHYDRATE DIETS

This study compared plasma volume (PV) and ion regulation during prolo nged exercise in control vs. glycogen-depleted (GD) conditions, with e mphasis on the initial minutes of exercise. In two trials separated by 1-2 wk, four adult males cycled at 75% of peak: oxygen consumption (V O2) until exhaustion (50 +/- 7 min for GD) or until the GD exhaustion time in the control trial. Blood was sampled from catheters placed in the brachial artery and retrograde in the femoral vein (fv). Arterial PV decreased rapidly and by 15 min PV was 83% (control) and 88% (GD) o f initial. The decrease in PV was accompanied by a net osmotic flux of water from plasma and inactive tissues to contracting muscles. The si gnificantly greater decrease in PV in control compared with GD was ass ociated with a higher muscle lactate content (Lac(-); 36 vs. 17 mu mol /g dry wt, respectively). Increases in plasma [Cl-] and [Na+] were les s than predicted from decreased PV, indicating net loss of these ions from the plasma compartment. Increases in arterial and fv [K+] were 50 % greater than could be accounted for by decreased PV, corresponding w ith increased arterial and fv plasma K+ contents. The rapid net releas e of K+ and Lac(-) from contracting muscle during the first few minute s of exercise in both trials was abolished (control) or reversed (GD) within 15 min of beginning exercise. In summary, 1) the greatest rate of water and ion flux occurred during the initial 2 min of exercise; 2 ) the reduced magnitude of PV and ion disturbances during exercise in the GD trial, compared with control, was associated with a reduced acc umulation of lactate in contracting muscle; and 3) a steady state was achieved in 10-15 min, at which time there was no evidence of net lact ate or K+ release from contracting muscle to the circulation. It can b e concluded that water flux between vascular and tissue compartments d uring exercise occurred independent of ion flux.