Hydrogen ion concentration is not the sole determinant of muscle metaboreceptor responses in humans

We examined the effects of exercise conditioning on muscle sympathetic nerve activity (MSNA) during handgrip and post-handgrip circulatory arrest (PHG-CA). Two conditioning stimuli were studied: forearm dominance and bodybuilding. Static handgrip at 30% maximal voluntary contraction followed by PHG-CA led to a rise in MSNA smaller in dominant than in nondominant forearms (99% vs. 222%; P < 0.02) and in body builders than in normal volunteers (28% vs. 244%; P < 0.01). Separate ³¹P NMR experiments showed no effect of dominance on forearm pH but a pH in bodybuilders higher (6.88) than in normal volunteers (6.79; P < 0.02) during PHG-CA. Our second goal was to determine if factors besides attenuated [H⁺] contribute to this conditioning effect. If differences in MSNA during exercise were noted at the same pH, then other mechanisms must contribute to the training effect. We measured MSNA during ischemic fatiguing handgrip. No dominance or bodybuilding effect on pH was noted. However, we noted increases in MSNA smaller in dominant than nondominant forearms (212% vs. 322%; P < 0.02) and in bodybuilders than in normal volunteers (161% vs. 334%; P < 0.01). In summary, MSNA responses were less during exercise of conditioned limbs. Factors aside from a lessening of muscle acidosis contribute to this effect.