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

1. In ischaemic exercise ATP is supplied only by glycogenolysis and net spl itting of phosphocreatine (PCr). Furthermore, 'proton balance' involves onl y glycolytic lactate/H+ generation and net H+ 'consumption' by PCr splittin g. 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 % maxim al voluntary contraction at 0.67 Hz) under cuff ischaemia. P-31 magnetic re sonance spectra were acquired from finger flexor muscle in a 4.7 T magnet u sing 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 t o obtain flux through 'distal' glycolysis (phosphofructokinase and beyond) to lactate; 'proximal' flux (through phosphorylase) vas obtained from this and changes in [phosphomonoester]. Total H+ load (lactate load less H+ cons umption) was used to estimate cytosolic buffer capacity (beta). 4. Glycolytic ATP synthesis increased from near zero while PCr splitting de clined. Net H+ load was approximately linear with pH, suggesting beta = 20 mmol 1(-1) (pH unit)(-1) at rest, increasing as pH falls. 5. Relationships between glycolytic rate and changes in [PCr] (i.e. the tim e-integrated mismatch between ATP use and production), and thus also [P-i] (substrate for phosphorylase), suggest that increase in glycolysis is due p artly 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 splittin g. 6. The 'settings' of these mechanisms have a strong influence on changes in pH and metabolite concentrations.