Role of Citrate in Stimulation of Oxygen Consumption by Insulin in Frog Muscle

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Abstract
When isolated intact frog muscles were placed in Ringer''s solution containing 10-2 [image] citrate they became spontaneously active. Twitching was seen with the unaided eye and a large number of action potentials could be recorded. Lactate did not produce these effects. Both citrate and lactate increased the rate of O2 consumption by the muscles. At any given molar concentration the acceleration produced by lactate was greater than that produced by citrate. In the presence of both insulin and lactate the rate of O2 consumption by the muscles was higher than in the presence of lactate alone. If the rate in Ringer''s solution be taken as 100 then the rate in the presence of lactate was approximately 150, the rate in the presence of both insulin and lactate 270. This degree of stimulation was produced by an insulin concentration in the medium surrounding the muscles as low as 3.0 x 10-2 units/ ml. No detectable effect was produced by 10-3 units insulin/ml. Insulin which was heated with alkali had no effect on the rate of O2 consumption by the muscles. Such heating destroys the ability of insulin to lower the blood sugar level when injected into mice. The concentrations of insulin required to obtain the effect on the isolated muscles were identical with the concentration necessary to produce typical systemic effects in intact frogs. The effect of insulin in the presence of citrate is probably due, in this type of experiment at least, to the production of lactate as a consequence of the muscular activity induced by the citrate.

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