Metabolic variation among rat lumbosacral alpha-motoneurons

Despite the wealth of literature concerned with muscle fiber biochemical, ultrastructural and physiological characteristics, little information is available regarding the metabolic enzyme activities of alpha-motoneurons. The present study examines the metabolism of alpha-motoneurons located in the lateral cell column of the rat lumbosacral enlargment with a quantitative histochemical technique. Variation in the activities of alpha-glucan phosphorylase, NADH-diaphorase, succinic dehydrogenase and acid phosphatase were detectable with the photographic densitometry and atomic absorption spectrophotometry technique. No difference in the glycolytic enzyme activity (mitochondrial alpha-glycerophosphate dehydrogenase) was observed. Analysis of lactate dehydrogenase isoenzymes demonstrated the existence of H type isoenzyme in alpha-motoneurons, consistent with other observations indicating predominance of aerobic metabolism within these neurons. The activities of the former enzymes in alpha-motoneurons formed a complete spectrum of activities, distributed unimodally. Smaller motoneurons exhibited the greatest NADH-D and acid phosphatase activities; phosphorylase activity was greatest in larger motoneurons. Significant variation in the enzyme activity of similar-sized motoneurons suggests that the metabolism of the motoneuron is regulated by factors other than cell size. Relationships between motoneuron metabolic enzyme activity and motor unit type are under current investigation.