DETERMINATION OF HUMAN SKELETAL-MUSCLE BUFFER VALUE BY HOMOGENATE TECHNIQUE - METHODS OF MEASUREMENT

The human muscle buffer value (beta) is most frequently determined by either fixed acid titration of a homogenate [''in vitro'' beta (beta(v it))] or measurement of the change in lactate concentration (DELTA[La] ) relative to the change in muscle homogenate pH after high-intensity exercise [''in vivo'' beta = -DELTA[La]/DELTApH (beta(viv))]. We sough t to compare beta(viv), determined after isometric and dynamic exercis e to exhaustion (approximately 60 s), with beta(vit). Resting (R) and postexercise (E) biopsy samples were taken from vastus lateralis muscl es of 43 human volunteers. Freeze-dried muscle was homogenized (30 mg/ ml) in NaF (0.01 M) for the measurement of muscle pH (R and E). Beta(v it) was determined by HCI (0.01 M) titration of the homogenate over th e pH range 7.1-6.5. Muscle lactate was measured by enzymatic assay. Th ere was no significant difference between beta(viv) determined after i sometric (n = 35) or dynamic (n = 8) exercise to fatigue (170 vs. 168 mmol H+.kg dry muscle mass-1.pH-1, respectively; P > 0.05). Values for beta(vit) in the corresponding muscle samples (R) were approximately 7-8% lower (156 +/- 25 vs. 157 +/- 18 mmol H+.kg dry muscle mass-1.pH- 1, respectively). There was no significant difference (P = 0.278) betw een the measured decline in muscle homogenate pH after exercise and th e reduction in pH predicted from beta(vit) and DELTA[La], indirectly c onfirming the lack of any significant difference between beta(viv) and beta(vit). The components expected to contribute to buffering during each method of measurement are discussed, and we suggest that any disc repancies between values for beta(viv) and beta(vit), as determined by homogenate technique, cannot simply be attributed to the differential involvement of metabolic buffering or indeed any single mechanism.