Nitrate ions: Potentiation of increased permeability to sugar associated with muscle contraction.

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.

Selective loss of a plasma membrane protein associated with contraction of skeletal muscle.

Muscle proteins were labeled by incubating isolated frog sartorius muscles with [3H]- or [14C]phenylalanine. Sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis of plasma membrane fractions revealed a major protein band with an apparent molecular weight of approx. 96,000. Radioactivity in this band showed a clearly delineated decrease, relative to other bands, when previously labeled muscles were induced to contract either by electrical stimulation or by increasing the influx of Ca2+ from the incubation medium. It is postulated that a Ca2+-activated neutral protease may account for this decrease in labeled membrane protein.

Use of tritiated 3-O-methyl-D-glucose for studies of membrane transport. Caveat.

Tritiated 3-O-methyl-D-glucose has many useful attributes as a model substance for studies of the transport of glucose across cell membranes. However, preparations of high specific radioactivity can decompose within a few months, producing radioactive impurities that can cause a several-fold increase in the apparent rate of sugar transport. In our investigation radioactive contaminants entered frog skeletal muscle cells by free diffusion rather than by facilitated transport. Much of the contaminating radioactive material could be removed by evaporating the solvent and redissolving the sugar. Tritiated sugar samples that had a specific activity below 0.1 Ci/mmol remained stable and suitable for transport measurements after several years of storage at -20 degrees C. In order to evaluate the suitability of a given tritiated preparation of sugar for transport measurements, it is recommended that its behavior be compared with that of a stable reference standard of low specific activity.

Isolation of plasma membrane vesicles, derived from transverse tubules, by selective homogenization of subcellular fractions of frog skeletal muscle in isotonic media.

A new technique for isolating fragmented plasma membranes from skeletal muscle has been developed that is based on gentle mechanical disruption of selected homogenate fractions. (Na+ + K+)-stimulated, Mg2+-dependent ATPase was used as an enzymatic marker for the plasma membrane, Ca2+-stimulated, Mg2+-dependent ATPase as a marker for sarcoplasmic reticulum, and succinate dehydrogenase for mitochondria. Cell segments in an amber low-speed (800 x g) pellet of a frog muscle homogenate were disrupted by repeated gentle shearing with a Polytron homogenizer. Sarcoplasmic reticulum was released into the low-speed supernatant, whereas most of the plasma membrane marker remained in a white, fluffy layer of the sediment, which contained sarcolemma and myofibrils. Additional gentle shearing of the white low-speed sediment extracted plasma membranes in a form that required centrifugation at 100,000 x g for pelleting. This pellet, the fragmented plasma membrane fraction, had a relatively high specific activity of (Na+ + K+)-stimulated ATPase compared with the other fractions, but it had essentially no Ca2+-stimulated ATPase activity and only a small percentage of the succinate dehydrogenase activity of the homogenate. Experimental evidence suggests that the fragmented plasma membrane fraction is derived from delicate transverse tubules rather than from the thicker, basement membrane-coated sarcolemmal sheath of muscle cells. Electron microscopy showed small vesicles lined bu a single thin membrane. Hydroxyproline, a characteristic constituent of collagen and basememt membrane, could not be detected in this fraction.

Enhanced permeability to sugar associated with muscle contraction. Studies of the role of Ca++.

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 45Ca in the medium was raised, more 45Ca 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.

Assay of succinate dehydrogenase activity by the tetrazolium method: evaluation of an improved technique in skeletal muscle fractions.

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.

Action of insulin on the transport of 3-O-methylglucose in frog sartorius muscles.

The initial rate of entry of 3-O-methyl-D-glucose into isolated frog sartorius muscles was measured at various concentrations of substrate, at 0, 9, 19, or 29 degrees C, after prior incubation at 19 degrees C with a maximally stimulating concentration of insulin. Control muscles were treated similarly, except for the omission of insulin. A saturable transport system provided for most of the entry of 3-O-methylglucose into muscle cells, but a small amount of penetration occurred by a nonsaturable route. The major effect of insulin was to produce a large increase in activity of the suturable system. The Vmax of entry increased, but there was no significant change in the apparent Km. The ratio of insulin-stimulated to basal Vmax was 10 when transport was measured at 29 degrees C but was 22 at 0 degrees C. These findings support the hypothesis that, although a large part of the effect of insulin on sugar transport can be accounted for by an increase in the number of functional transporters in the plasma membrane, there is a separate hormonal effect that permits a relatively greater activity of transporters at lower temperatures, compared with control rates. An additional effect of insulin was to produce a small but definite increase in the entry of sugar by the nonsaturable transport system.

Degradation of skeletal muscle plasma membrane proteins by calpain.

Observations described here provide the first demonstration that calpain (Ca2+-dependent cysteine protease) can degrade proteins of skeletal muscle plasma membranes. Frog muscle plasma membrane vesicles were incubated with calpain preparations and alterations of protein composition were revealed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Calpain II (activated by millimolar concentrations of Ca2+) was isolated from frog skeletal muscle, but the activity of calpain I (activated by micromolar concentrations of Ca2+) was lost during attempts at fractionation. Calpain I obtained from skeletal muscle and erythrocytes of rats was tested instead, and exerted effects similar to those of frog muscle calpain on the membrane proteins. All of the calpain preparations caused striking losses of a major membrane protein of molecular mass of approximately 97 kDa, designated band c, and diminution of a thinner band of approximately 200 kDa. There were concomitant increases in 83- and 77-kDa polypeptides. These effects were absolutely dependent on the presence of free Ca2+, and were completely blocked by calpastatin, a specific inhibitor of calpain action. Frog muscle calpain differed only in being relatively more active at 0 degree C than were the calpains from rat tissues. Experimental observations suggest that calpain acts at the cytoplasmic surface of the plasma membrane.