Simulation of The Epidermal Growth Factor Signal Transduction Pathway

Abstract

In ischaemic exercise ATP is produced by glycogenolysis and net phosphocreatine (PCr) splitting, and lactatem' generation is opposed by net H+ 'consumption' as a consequence of PCr splitting [I-31. Here we examine proton buffering [41, control of glycogenolysis [ I , 21, and the interplay between these and the constraints imposed by the creatine kinase equilibrium [2] in magnetic resonance spectroscopy studies of human muscle in vivo. Spectra (8-scan at 1.5 s i.p. delay) were acquired in a 4.7 T magnet using a 5 cm surface coil placed over finger flexor muscles in 9 males (25-45 y) while cuff ischaemia was maintained during and for 3 min preceding dynamic finger flexion exercise (7% maximum voluntary contraction force at 0.7 Hz). During exercise pH and [PCr] fell and [ADPI rose, as expected (end-exercise values: 6.50k0.05, 1322 mM and 48r6 pM respectively: meankSEM). PCr depletion rate ( D ) at the start of exercise was used to calculate total ATP turnover rate (F = 26+4 mM/min). During exercise, glycogenolytic ATP synthesis rate (L , estimated as F-D) [ I , 31 increased while D declined, reaching 2323 mM/min by 3 min. The total H' load to the cytosol, calculated as A[lactate] less net Hf consumption by PCr splitting, was linear with pH, suggesting overall non-Pi buffer capacity = 27r5 slykes [ I ] . The quantity LMAx (estimated L at saturating [Pi], which is proposed as an approximate measure of Ca''-dependent generation of the 'active' a form of glycogen phosphorylase [2]) started off low, increasing to an approximately constant plateau of 45k2 mM/min. This is consistent with the hypothesis that glycogenolysis increases partly as a result of the 'open-loop' Ca*'-dependent conversion of phosphorylase b to a , and the 'closed-loop' increase in Pi consequent on PCr splitting. The proportionality between L and F in studies at different powers [ I ] implies that similar open-loop mechanisms play a major part in the activation of other glycolytic enzymes, notably phosphofructokinase [ 11. I . Conley KE, Blei ML etal. [I9971 Am. J. Physiol. 273: C306-C315. 2. Kemp GJ (19971 Magma 5 : 231-241 3. Wackerhage H e t a l . [1996] Magma4: 151-5. 4. Kemp GJ et al. [ 19931 NMR in Biomed. 6: 73-83.