Muscle glycogenolysis is not activated by changes in cytosolic P-metabolites: a 31P and 1H MRS demonstration.

Skeletal muscle contraction and glycogenolysis are closely coupled. The standard explanation for this coupling, as taught in modern biochemistry textbooks, is that the metabolic products of contraction (ADP, AMP, P(i)) feed back to activate glycogenolytic enzymes, thus providing for resynthesis of ATP. However, both in vivo (31)P MRS analyses and chemical analyses of muscle extracts have provided results that are contrary to this theory, at least in its simplest form. The MRS studies suffer from ambiguous assumptions. More importantly, in (31)P MRS studies the dependent and independent variables are often confounded because the glycogenolytic rate is calculated from the same data which are used to calculate the other metabolic variables. The analysis of biopsies has been necessarily quite limited, and suffers from a different set of experimental artifacts. Thus, the problem of contraction-glycogenolysis-coupling was reassessed using a quantitatively accurate (1)H MRS method. It is confirmed that glycogenolysis and contractions are closely coupled during repetitive exercise, while glycogenolysis and P-metabolite concentrations are not. A simple metabolic feedback system cannot explain contraction-glycogenolysis-coupling.

The carnosine C-2 proton's chemical shift reports intracellular pH in oxidative and glycolytic muscle fibers.

The appearance of new peaks in the 7.7-8.6 and 6.8-7.4 ppm regions of the postexercise (1)H spectrum of frog muscle is reported. These new peaks result from the splitting of single pre-exercise carnosine C-2 and C-4 peaks into two peaks, representing the intracellular pH (pH(I)) of oxidative and glycolytic fibers. The following data support this conclusion: 1) comparison of means and regression analysis indicates equivalence of the pH(I) measurements by (1)H and (31)P NMR; 2) the pre- and poststimulation concentrations of carnosine are equal; 3) in ischemic rat hindlimb muscles, the presence of a single, more acidic peak in the plantaris; a single, less acidic peak in the soleus; and two peaks (more and less acidic) in the gastrocnemius correspond to published values for the fiber-type composition of these muscles; and 4) in muscles treated with iodoacetate prior to and during stimulation, a second peak never appears. These data indicate that it is feasible to measure separately the pH(I) of oxidative and glycolytic fibers using (1)H NMR spectroscopy.