Muscle Studies by³¹P MRS

Skeletal muscle ATP synthesis and cellular H+ handling measured by localized 31P-MRS during exercise and recovery

“…The transition between these two metabolic regimes corresponds approximately to what is defined in whole body physiology as ‘lactate threshold’ or ‘anaerobic threshold’111213. The relationship between PCr and pH is rather an indirect one.…”

“…as described in refs 4 and 11, using a Hill coefficient of n  = 2, and making common assumptions of exchange constants ( K m  = 30 μM, K CK  = 1.66 × 10 9  l/mol) and basal concentrations ([PCr] rest  = 33 mM, [ATP] = 8.2 mM) contained therein and in ref. 35.…”

“…where β is the total buffer capacity of the cytosol11, and γ is a negative stoichiometric coefficient arising from the pH-dependent difference in charge3738. Assuming total ATP demand to be constant throughout the (constant-power) exercise, it can be calculated from the first few seconds of exercise, where contributions from Q and L are still negligible (see Fig.…”

“…exercise at relatively low intensity, below the lactate threshold, in which PCr breakdown buffers the temporary mismatch between the step increase in ATP demand and the slower (usually exponential) increase in oxidative ATP synthesis, with negligible contribution by glycolytic ATP synthesis; black PCr and pH lines depict behavior during ‘mixed’ exercise, i.e. exercise at higher intensity, above the lactate threshold, in which glycolytic ATP synthesis is also appreciable11. The rates of ATP generation by PCr breakdown, glycolysis ( L ) and oxidative phosphorylation ( Q ) must equal ATP demand ( U ), which is shown here as constant for constant-intensity exercise below ‘critical power’11: the notional pattern of L and Q illustrated here shows an intermediate case where glycolytic ATP synthesis is not negligible, but is transient, so that by the end of exercise ATP is supplied only by oxidative ATP synthesis.…”

“…exercise at higher intensity, above the lactate threshold, in which glycolytic ATP synthesis is also appreciable11. The rates of ATP generation by PCr breakdown, glycolysis ( L ) and oxidative phosphorylation ( Q ) must equal ATP demand ( U ), which is shown here as constant for constant-intensity exercise below ‘critical power’11: the notional pattern of L and Q illustrated here shows an intermediate case where glycolytic ATP synthesis is not negligible, but is transient, so that by the end of exercise ATP is supplied only by oxidative ATP synthesis. During recovery from exercise at any intensity, PCr is replenished at the expense purely of oxidative ATP synthesis, the kinetics being exponential at low exercise intensity, following more complicated kinetics after exercise where pH has significantly fallen.…”