ELSEVIER Biochimica el BiophysicaActa 1185 (1994) 327-335 ~ ~ ~ Effects of the creatine analogue g-guanidinopropiomc acid on skeletal muscles of mice deficient in muscle creatine kinase Jan van Deursen a Paul Jap a, Arend Heerschap b, Henk ter Laak c, Wim Ruitenbeek d, 136 Wieringa a'* a Depart~wnts of Cell Biology and Histology, b Diagnostic Radiolo~, ~ Neuroto~, and d Paediatrics, Facul O" o f Medical Sciences. Uniz'ersio' of Nijmegen, P.O. Box 9101, 6500 lib Nijmegen, ITw Netherlands (Received 4 October 1993) Abstract To evaluate the effects of phosphocreatine (PCr) and creatine (Cr) depletion on skeletal muscles of mice deficient in muscle crcatine kinase (M-CK), we have fed mutant mice a diet containing the creatine analogue ~-guanidinopropionic acid (g3GPA). After 8-10 weeks of feeding, accumulation of the creatine analogue in M-CK-deficient muscles was comparable to that observed in muscles of wild-type mice. Strikingly, and unlike wild types, mutants did not accumulate pbosphorylated ~GPA, indicating that MM-CK is the only muscle CK isoform which can phosphorylate 8GPA. in M-CK-defk-ient muscles there was respective depletion of PCr, Cr and ATP levels to 31, 4l and 83% of normal. ~ avcra~',. ,-~'~s-sectional area of type 213 fibres in gastrocnemius muscles was very much reduced and was similar to type I and type 2A fibres which maintained their normal size. The maximal isometric twitch force developed by gastrocnemius-plantaris-soleus (GPS) muscle complexes of tgGPA-treated mutants was reduced by about 30%, but these muscles showed an increased fatigue resistance during I and 5 Hz contraction. Mitoehondrial enzyme activities in the upper hind limb musculature of null mutants were 20-35% increased by the g3GPA diet. Altogether, these results provide evidence that certain functions of the creatine kinase/phosphocreatine (CK/PCr) system are not eliminated solely by the loss of M-CK. Key words: CK/PCr system; Creatine kinase deficiency; Phos,ahoc:c., :,,=: 8-Guanidinopropionic acid; Muscle; Skeletal muscle bioenergetics; (M-CK-deficient mice) 1. Introduction Skeletal muscles contain high levels of phospho- creatine (PCr) and creatine kinase (CK), which are presumed to function in energy buffering and energy transport [1-3]. Thus far, two kinds of animal model have been used to evaluate the physiological impor- tance of the CK/PCr system in energy metabolism of skeletal muscle. One model involves the feeding of synthetic analogues of Cr, such as /3-gnanidinopro- pionie acid, which competitively inhibit uptake of crea- fine (Cr) in skeletal muscle cells [4,5]. Rats put on a * Corresponding author, 0005-2728/oA/$07.00 © 1994 Elsevier Science B.V. All rights reserved SgDI 0005-2728(94)00016-X diet containing !%/3GPA (w/w) for 6-8 weeks accu- mulate high concentrations of phosphorylated g~GPA (/3GPAP) in their skeletal muscles, and display PCr and ATP levels which are respectively 12- and 2-fold -educed compared to normal. A reduction in the flux of high-energy phosphates between PCr and ATP par- allels the decline of PCr, as shown by steady-state nuclear magnetic resonance (NMR) magnetization transfer experiments [6,7]. The phosphorylated g3GPA cannot effectively substitute for PCr, because it is a poor substrate for CK [8]. g3GPA feeding results in a number of adap*ivc changes in skeletal muscle. Glyco- gen levels arc elevatt~, while glycolytic enzymes and glycogen consumption during muscle exercise are re- duced [7,9]. The aerobic energy-generating capacity of type 2 fibres is enhanced [9]. Mitochondria in perinu- clear and subsarcolemmal areas of type 1 fibres fre- quently contain paracrystailine inclusions and often are