Determination of human muscle buffer value by the homogenate technique: methods of measurement: Methods of measurement

The human muscle buffer value (β) is most frequently determined by either fixed acid titration of a homogenate ['in vitro' β (β(vit))] or measurement of the change in lactate concentration (Δ[La]) relative to the change in muscle homogenate pH after high-intensity exercise ['in vivo' β = - Δ[La]/ΔpH (β(viv))]. We sought to compare β(viv), determined after isometric and dynamic exercise to exhaustion (~60 s), with β(vit). Resting (R) and postexercise (E) biopsy samples were taken from vastus lateralis muscles 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). β(vit) was determined by HCl (0.01 M) titration of the homogenate over the pH range 7.1-6.5. Muscle lactate was measured by enzymatic assay. There was no significant difference between β(viv) determined after isometric (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 β(vit) in the corresponding muscle samples (R) were ~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) between the measured decline in muscle homogenate pH after exercise and the reduction in pH predicted from β(vit) and Δ[La], indirectly confirming the lack of any significant difference between β(viv) and β(vit). The components expected to contribute to buffering during each method of measurement are discussed, and we suggest that any discrepancies between values for β(viv) and β(vit), as determined by homogenate technique, cannot simply be attributed to the differential involvement of metabolic buffering or indeed any single mechanism.