Lactate dehydrogenase expression at the onset of altered loading in rat soleus muscle

Abstract

dehydrogenase expression at the onset of altered loading in rat soleus muscle. J Appl Physiol 97: 1424 -1430, 2004; 10.1152/japplphysiol.00222.2004.-Both functional overload and hindlimb disuse induce significant energy-dependent remodeling of skeletal muscle. Lactate dehydrogenase (LDH), an important enzyme involved in anaerobic glycolysis, catalyzes the interconversion of lactate and pyruvate critical for meeting rapid high-energy demands. The purpose of this study was to determine rat soleus LDH-A and -B isoform expression, mRNA abundance, and enzymatic activity at the onset of increased or decreased loading in the rat soleus muscle. The soleus muscles from male Sprague-Dawley rats were functionally overloaded for up to 3 days by a modified synergist ablation or subjected to disuse by hindlimb suspension for 3 days. LDH mRNA concentration was determined by Northern blotting, LDH protein isoenzyme composition was determined by zymogram analysis, and LDH enzymatic activity was determined spectrophotometrically. LDH-A mRNA abundance increased by 372%, and LDH-B mRNA abundance decreased by 43 and 31% after 24 h and 3 days of functional overload, respectively, compared with that in control rats. LDH protein expression demonstrated a shift by decreasing LDH-B isoforms and increasing LDH-A isoforms. LDH-B activity decreased 80% after 3 days of functional overload. Additionally, LDH-A activity increased by 234% following 3 days of hindlimb suspension. However, neither LDH-A or LDH-B mRNA abundance was affected following 3 days of hindlimb suspension. In summary, the onset of altered loading induced a differential expression of LDH-A and -B in the rat soleus muscle, favoring rapid energy production. Long-term altered loading is associated with myofiber conversion; however, the rapid changes in LDH at the onset of altered loading may be involved in other physiological processes.atrophy; hypertrophy; disuse; hindlimb suspension; glycolytic enzymes THE LOADING DEMANDS PLACED on skeletal muscle can dramatically affect fiber cross-sectional area, phenotype, and metabolic profile (14,30,40). These adaptations are specific to the type of loading stimulus applied. Contractile and metabolic properties of skeletal muscle are linked during myofiber conversion, as evidenced by the fact that metabolic enzyme activity and gene expression are related to the new functional demands placed on that muscle. An overall shift in muscle phenotype will occur after an extended period of altered loading; however, a brief period of altered loading is also a powerful stimulus for changes to skeletal muscle that can regulate several processes associated with muscle remodeling. Functionally overloading the rat soleus muscle for 3 days alters gene expression for immune function, inflammation, and cell cycle regulation (9). Brief periods of muscle unloading initiate signaling that decreases muscle mass that includes a decrease in protein synthesis (26) and satellite cell proliferation (11).Increasing or decreasing muscle loading for extended period...

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