<i>In vitro</i> action of insulin on minced avian and mammalian muscle

Krebs & Eggleston reported the interesting observation of an in vitro insulin effect upon oxidative metabolism. The experimental conditions which they employed to demonstrate this phenomenon consisted in the use of minced pigeon muscle as against the tissue slice technique which has been uniformly unsuccessful for the demonstration of insulin effects on respiration. A M/10 phosphate (pH 6.8) buffer containing the minced tissue in at most a one to ten dilution, the addition of citrate, and finally of boiled muscle juice resulted in a system which gave maximum results. With this system, increases of respiration of as much as 90 % were noted in the presence of insulin. These effects were obtained with an amorphous insulin hydrochloride preparation (B.D.H.), but were absent when crystalline insulin was employed. This failure with crystalline insulin was attributed to its zinc content, an interpretation based on the in- hibiting effects of zinc on the respiration of pigeon mince. The stimulation of respiration was roughly proportional to the insulin concentration between 0 05 and 5 0 mg./100 ml. Instead of citrate, other members of the postulated citric acid cycle could be substituted with similar effects. The only tissue employed in these experiments was the breast muscle of the pigeon, which was minced in a Latapie mincer.