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

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 presser reflex during 5 min of intermittent sta tic quadriceps exercise in humans (n = 7). Exercise resulted in approximate ly a threefold (P < 0.05) increase in muscle sympathetic nerve activity (MS NA) and 13 +/- 3 beats/min (P < 0.05) and 20 +/- 2 mmHg (P < 0.05) increase s in heart rate and blood pressure;respectively. During recovery, all refle x 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). D ialysate 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 contin ued to increase during the exercise bout. After 5 min of contractions, dial ysate 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) fr om resting levels (69.4 +/- 3.7 nM) during quadriceps exercise and continue d to decrease with a mean decline (P < 0.05) of 16.7 +/- 3.8 nM being achie ved after 5 min. During recovery, H+ concentrations rapidly increased and w ere not significantly different from baseline after 1 min. This study repre sents the first time that skeletal muscle interstitial pH, K+, lactate, and phosphate have been measured in conjunction with MSNA, heart rate, and blo od pressure during intermittent static quadriceps exercise in humans. These data suggest that interstitial K+ and phosphate, but not lactate and H+, m ay contribute to the stimulation of the exercise presser reflex.