The effect of insulin on oxidations in isolated muscle tissue

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Abstract
The optimal conditions for the respiration of minced pigeon breast muscle were investigated. The best suitable saline medium was found to be 0.1 M phosphate buffer, pH 6 [center dot] 8-7 [center dot] 4. Ringer''s soln. and similar "balanced" solns. inhibited oxidations owing to the presence of Ca++. If the ratio medium/tissue was above 10 the respiratory activity of the muscle was diminished. Minced liver or kidney respired (for 1 or 2 hr.) at about the same rate as sliced tissue if suspended in 5 vols. of phosphate buffer. The respiration of minced tissue, unlike that of sliced tissue, was inhibited by physiological concs. of Ca. The respiration of minced muscle fell off after 30-60 min. The falling off was retarded by the addition of (a) boiled muscle extract, (b) citrate or substances derived from it ([alpha]-ketoglutarate, succinate, fumarate, oxaloacetate), (c) insulin. The conditions under which these factors were effective were investigated. Insulin had the greatest effect if added together with citrate and muscle extract. This indicated that insulin became a limiting factor in oxidations only when heat-stable coenzymes (muscle extract) and a source for the carbon skeleton required for the "citric acid cycle" were present in excess. Observations were discussed which suggested that insulin acted as a catalyst in the "citric acid cycle." Zinc insulinate had practically no effect on the respiration of minced muscle. Traces of Zn (10-6[image]) inhibited respiration. Zinc insulinate was not the active form of insulin in metabolism.

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