Interstitial pH, K+, lactate, and phosphate determined with MSNA during exercise in humans

Abstract

The purpose of the present study was to use the microdialysis technique to simultaneously measure the interstitial concentrations of several putative stimulators of the exercise pressor reflex during 5 min of intermittent static quadriceps exercise in humans (n = 7). Exercise resulted in approximately a threefold (P < 0.05) increase in muscle sympathetic nerve activity (MSNA) and 13 ± 3 beats/min (P < 0.05) and 20 ± 2 mmHg (P < 0.05) increases in heart rate and blood pressure, respectively. During recovery, all reflex responses quickly returned to baseline. Interstitial lactate levels were increased (P < 0.05) from rest (1.1 ± 0.1 mM) to exercise (1.6 ± 0.2 mM) and were further increased (P < 0.05) during recovery (2.0 ± 0.2 mM). Dialysate phosphate concentrations were 0.55 ± 0.04, 0.71 ± 0.05, and 0.48 ± 0.03 mM during rest, exercise, and recovery, respectively, and were significantly elevated during exercise. At the onset of exercise, dialysate K+ levels rose rapidly above resting values (4.2 ± 0.1 meq/l) and continued to increase during the exercise bout. After 5 min of contractions, dialysate K+ levels had peaked with an increase (P < 0.05) of 0.6 ± 0.1 meq/l and subsequently decreased during recovery, not being different from rest after 3 min. In contrast, H+ concentrations rapidly decreased (P < 0.05) from resting levels (69.4 ± 3.7 nM) during quadriceps exercise and continued to decrease with a mean decline (P < 0.05) of 16.7 ± 3.8 nM being achieved after 5 min. During recovery, H+ concentrations rapidly increased and were not significantly different from baseline after 1 min. This study represents the first time that skeletal muscle interstitial pH, K+, lactate, and phosphate have been measured in conjunction with MSNA, heart rate, and blood pressure during intermittent static quadriceps exercise in humans. These data suggest that interstitial K+ and phosphate, but not lactate and H+, may contribute to the stimulation of the exercise pressor reflex.