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

Abstract

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, Pi) feed back to activate glycogenolytic enzymes, thus providing for resynthesis of ATP. However, both in vivo 31P 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 31P 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-coupiing was reassessed using a quantitatively accurate 1H 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-glycogen- olysis-coupling. © 2003 Wiley-Liss, Inc.