Differential expression of the fast skeletal muscle proteome following chronic low-frequency stimulation

Physiological and biochemical responses of skeletal muscle fibres to enhanced neuromuscular activity under conditions of maximum activation can be studied experimentally by chronic low-frequency stimulation of fast muscles. Stimulation-induced changes in the expression pattern of the rabbit fast skeletal muscle proteome were evaluated by two-dimensional gel electrophoresis and compared to the altered isoform expression profile of established transformation markers such as the Ca2+-ATPase, calsequestrin and the myosin heavy chain. Sixteen muscle proteins exhibited a marked change in their expression level. This included albumin with a 4-fold increase in abundance. In contrast, glycolytic enzymes, such as enolase and aldolase, showed a decreased expression. Concomitant changes were observed with marker elements of the contractile apparatus. While the fast isoforms of troponin T and myosin light chain 2 were drastically down-regulated, their slow counterparts exhibited increased expression. Interestingly, mitochondrial creatine kinase expression increased while the cytosolic isoform of this key muscle enzyme decreased. The expression of the small heat shock protein HSP-B5/alphaB-crystallin and the oxygen carrier protein myoglobin were both increased 2-fold following stimulation. The observed changes indicate that the conversion into fatigue-resistant red fibres depends on: (i) the optimum utilization of free fatty acids via albumin transportation, (ii) a rearrangement of the creatine kinase isozyme pattern for enhanced mitochondrial activity, (iii) an increased availability of oxygen for aerobic metabolism via myoglobin transport, (iv) the conversion of the contractile apparatus to isoforms with slower twitch characteristics and (v) the up-regulation of chaperone-like proteins for stabilising myofibrillar components during the fast-to-slow transition process.