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

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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.