When contractures were induced in isolated frog sartorius muscles with 4 mM caffeine, there was an increase in permeability of the muscle cells to 3-methylglucose. This observation suggests that the changes in permeability to sugar that are known to occur in electrically stimulated muscles may not be intimately related to the depolarization phase of the tissue response. Contractures that were elicited by exposing the muscles to a high concentration of K+ were also associated with an increased permeability to sugar. As the concentration of 45 Ca in the medium was raised, more 4Ca entered the muscles during potassium contractures, and the contractures lasted longer, in agreement with the observations of other investigators. There was also a greater change in permeability to sugar when potassium contractures were elicited in the presence of higher concentrations of Ca + + . The possibility that the enhanced permeability to sugar may be related to changes in the intracellular concentration of Ca++ is discussed.

An improved spectrophotometric method for measuring succinate dehydrogenase (EC 1.3.99.1) activity with the use of 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyltetrazolium chloride (INT) is described. The procedure has been evaluated in mitochondrial fractions and homogenates of frog skeletal muscle. For mitochondrial suspensions, extraction of formazan with alcohol was found to be superior to extraction with ethyl acetate. For homogenates, complete extraction of formazan required sequential treatment with alcohol and ethyl acetate; the generally employed procedure of extracting once with ethyl acetate alone led to serious underestimation of the amount of formazan in the tissue. Observations of mitochondrial suspension incubated with various concentrations of INT led to the selection of 0.8 mM INT for optimal results. Higher concentrations, although commonly used, can exert undesirable inhibitory effects on succinate dehydrogenase activity, especially at low concentrations of mitochondria and after longer periods of incubation. The problem of instability of succinate dehydrogenase was solved by the addition of buffer at pH 7.5.

Crystalline insulin or glucagon labeled with I131 was injected intravenously in adult male Sprague-Dawley rats under ether anesthesia. The left kidney was removed at intervals ranging from 10 seconds to 6 minutes following the injection, and radioautographs of histological sections were prepared. Radioactivity was found to be concentrated in the cortex of the kidney. At intervals less than 2 minutes after injection, radioactivity was prominent in the lumina of proximal convoluted tubules. At later intervals radioactivity tended to become more concentrated in the cells of the proximal tubules than in the lumina. The observations support the hypothesis that proteins may be filtered through the glomerular membrane and reabsorbed in the tubules.

Nitrate ions potentiate twitch tension and enhance the increase in permeability to sugar which occurs in electrically stimulated frog sartorius muscles. However, the potentiating effect of nitrate ions on permeability is not dependent upon an increase in twitch tension. The possible relation of changes in permeability to alterations of the concentration of calcium ions in the cell is discussed.

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