REGULATION OF OXIDATIVE AND GLYCOGENOLYTIC ATP SYNTHESIS IN EXERCISING RAT SKELETAL-MUSCLE STUDIED BY P-31 MAGNETIC-RESONANCE SPECTROSCOPY

P-31 magnetic resonance spectroscopy measurements of pH and the concen trations of orthophosphate and phosphocreatine were used to estimate r ates of glycogenolytic and oxidative ATP synthesis in rat leg muscle d uring 6 min sciatic nerve stimulation at different rates (1-4 Hz). To study the regulation of glycogenolysis during exercise, the apparent ' glycogenolytic capacity' (L(MAX)) was calculated from glycogenolytic A TP synthesis rate and orthophosphate concentration as a measure of the Ca2+-dependent activation of glycogen phosphorylase. This was found t o be proportional to the total ATP synthesis rate (F), and to decline with time; expressed relative to total ATP turnover rate as L(MAX)/F, its initial value was 2.9 +/- 0.6, declining with half-time 1.4 +/- 0. 4 min. The apparent 'mitochondrial capacity' (Q(MAX)), calculated from oxidative ATP synthesis rate and [ADP], was independent of ATP turnov er rate, but increased with half-time 0.8 +/- 0.1 min to 29 +/- 2 mmol kg(-1) min(-1): thus [ADP] was the predominant but not the only influ ence on oxidative ATP synthesis. Numerical simulation shows that time- dependent changes in L(MAX)/F exert a strong influence on pH and on th e concentrations of phosphocreatine and ADP. (C) 1997 by John Wiley & Sons, Ltd.