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Interrelations of ATP synthesis and proton handling in ischaemically exercising human forearm muscle studied by ³¹P magnetic resonance spectroscopy

Graham J. Kemp, Magali Roussel *, David Bendahan *, Yann Le Fur * and Patrick J. Cozzone *

1. In ischaemic exercise ATP is supplied only by glycogenolysis and net splitting of phosphocreatine (PCr). Furthermore, 'proton balance' involves only glycolytic lactate/H⁺ generation and net H⁺ 'consumption' by PCr splitting. This work examines the interplay between these, metabolic regulation and the creatine kinase equilibrium.
2. Nine male subjects (age 25-45 years) performed finger flexion (7 % maximal voluntary contraction at 0.67 Hz) under cuff ischaemia. ³¹P magnetic resonance spectra were acquired from finger flexor muscle in a 4.7 T magnet using a 5 cm surface coil.
3. Initial PCr depletion rate estimates total ATP turnover rate; glycolytic ATP synthesis was obtained from this and changes in [PCr], and then used to obtain flux through 'distal' glycolysis (phosphofructokinase and beyond) to lactate; 'proximal' flux (through phosphorylase) was obtained from this and changes in [phosphomonoester]. Total H⁺ load (lactate load less H⁺ consumption) was used to estimate cytosolic buffer capacity ().
4. Glycolytic ATP synthesis increased from near zero while PCr splitting declined. Net H⁺ load was approximately linear with pH, suggesting = 20 mmol l^-1 (pH unit)^-1 at rest, increasing as pH falls.
5. Relationships between glycolytic rate and changes in [PCr] (i.e. the time-integrated mismatch between ATP use and production), and thus also [P_i] (substrate for phosphorylase), suggest that increase in glycolysis is due partly to 'open-loop' Ca²⁺-dependent conversion of phosphorylase b to a, and partly to the 'closed loop' increase in P_i consequent on net PCr splitting.
6. The 'settings' of these mechanisms have a strong influence on changes in pH and metabolite concentrations.

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