Relationship between ionic surroundings and insulin actions on glucose transport and Na,K-pump in muscles

1. It is well known that insulin has various effects on glucose transport and the Na,K-pump in muscles. It is also known to have some effects on the membrane potential--in general, insulin induces a hyperpolarization of the membrane in muscles. Furthermore, it is suggested that the actions of insulin are modified by changes in ionic surroundings. 2. In this review article, the actions of ionic surroundings and insulin on glucose transport in muscles are discussed; in particular, the effects of changes in extracellular and/or intracellular concentrations of Na, K, Ca and H ions will be mentioned. 3. The actions of ionic surroundings and insulin on the Na,K-pump in muscles are discussed; in particular, the effects of changes in extracellular an/or intracellular concentrations of Na, K, Ca and H ions will be examined. 4. The relationship between the actions of ionic surroundings and insulin are discussed. 5. In particular, the effects of changes in ionic surroundings on the insulin-induced hyperpolarization of the membrane are discussed by relating it to the Na,K-pump function. The relationship between the insulin-induced change in membrane potential and glucose transport will be also mentioned.     

Insulin-induced changes in membrane potential and 3-O-methylglucose uptake at various external K concentrations in frog skeletal muscle

Insulin induced a hyperpolarization of the membrane and stimulated the 3-O-methylglucose (3-O-MG) uptake in frog skeletal muscle. In the present study, the relationship between the insulin-induced changes in the membrane potential and the 3-O-MG uptake was investigated. The stimulatory action of insulin on the 3-O-MG uptake was mediated by two different mechanisms. One of them was dependent on the change in the membrane potential and the other was independent of the change in the membrane potential. Both values of the insulin-induced changes in the membrane potential and the 3-O-MG uptake were diminished by increasing the external K concentration. One of the causes for the diminution of the 3-O-MG uptake with a rise of the external K concentration would be the decrease in the magnitude of the insulin-induced hyperpolarization.     

Diverse effects of insulin-induced hyperpolarization on 3-O-methyl-D-glucose (3-O-MG) transport in frog skeletal muscles

It has been suggested that the insulin-induced hyperpolarization might be a mediator of the stimulatory action of insulin on glucose transport. The purpose of the present study was to investigate the relationship between the insulin-induced hyperpolarization and the stimulatory action of insulin on glucose transport in skeletal muscle. Satorius muscles dissected from bullfrogs (Rana catesbeiana) were used. Insulin induced a hyperpolarization of the membrane and an increase in the 3-O-Methyl-D-glucose (3-O-MG) uptake and extrusion. In the presence of valinomycin, insulin had no significant effect on the membrane potential. Insulin still had the stimulatory action on both the 3-O-MG uptake and extrusion even in the presence of valinomycin, under whose condition insulin had no significant effect on the membrane potential. The magnitude of the stimulatory action of insulin on the 3-O-MG uptake in the presence of valinomycin was smaller than that in the absence of valinomycin. The magnitude of the stimulatory action of insulin on the 3-O-MG extrusion was, on the contrary, larger than that in the absence of valinomycin. The abolishment of the insulin-induced hyperpolarization decreased the 3-O-MG uptake and increased the 3-O-MG extrusion. The observation in the present study concludes that insulin has two different actions on glucose transport. One of them is developed through the insulin-induced hyperpolarization, which increases the 3-O-MG uptake and decreases the 3-O-MG extrusion. The other action is irrelevant of the insulin-induced hyperpolarization and stimulates both the 3-O-MG uptake and extrusion.     

Na+-sensitive component of 3-O-methylglucose uptake in frog skeletal muscle

Summary A Na⁺-sensitive uptake of 3-O-methylglucose (3-O-MG), a nonmetabolized sugar, was characterized in frog skeletal muscle. A removal of Na⁺ from the bathing solution reduced 3-O-MG uptake, depending on the amount of Na⁺ removed. At a 3-O-MG concentration of 2mm, the Na⁺-sensitive component of uptake in Ringer's solution was estimated to be about 26% of the total uptake. The magnitude of Na⁺-sensitive component sigmoidally increased with an increase of 3-O-MG in bathing solution, whereas in Na⁺-free Ringer's solution the uptake was proportional to the concentration. The half saturation of the Na⁺-sensitive component was at a 3-O-MG concentration of about 13mm, and the Hill coefficient was 1.4 to 1.6. Phlorizin (5mm), a potent inhibitor specific for Na⁺-coupled glucose transport, reduced the uptake in a solution containing Na⁺ to the level in Na⁺-free Ringer's solution. Glucose of concentrations higher than 20mm suppressed 3-O-MG uptake to a level slightly lower than that in Na⁺-free Ringer's solution. These observations indicate that there are Na⁺-coupled sugar transport systems in frog skeletal muscle which are shared by both glucose and 3-O-MG.     

ADH Action on Whole-Cell Currents by Cytosolic Ca2+-dependent Pathways in Aldosterone-treated A6 Cells

We studied the characteristics of the basal and antidiuretic hormone (arginine vasotocin, AVT)-activated whole cell currents of an aldosterone-treated distal nephron cell line (A6) at two different cytosolic Ca²⁺ concentrations ([Ca²⁺] _c , 2 and 30 nm). A6 cells were cultured on a permeable support filter for 10 ∼ 14 days in media with supplemental aldosterone (1 μm). At 30 nm [Ca²⁺] _c , basal conductances mainly consisted of Cl⁻ conductances, which were sensitive to 5-nitro-2-(3-phenylpropylamino)-benzoate. Reduction of [Ca²⁺] _c to 2 nm abolished the basal Cl⁻ conductance. AVT evoked Cl⁻ conductances at 2 as well as 30 nm [Ca²⁺] _c . In addition to Cl⁻ conductances, AVT induced benzamil-insensitive nonselective cation (NSC) conductances. This action on NSC conductances was observed at 30 nm [Ca²⁺] _c but not at 2 nm [Ca²⁺] _c . Thus, cytosolic Ca²⁺ regulates NSC and Cl⁻ conductances in a distal nephron cell line (A6) in response to AVT. Keeping [Ca²⁺] _c at an adequate level seems likely to be an important requirement for AVT regulation of ion conductances in aldosterone-treated A6 cells. Received: 6 May 1996/Revised: 28 June 1996     

Seasonality and vertical structure of light-attracted insect communities in a dipterocarp forest in Sarawak

Nocturnal flying insects were collected monthly for 13 months using ultra violet light-traps set at various vertical levels in a weakly-seasonal, tropical lowland dipterocarp forest in Sarawak, Malaysia. Abundance, faunal composition, size distribution and guild structure of these samples were analyzed with respect to temperal and vertical distributions. The nocturnal flying insect community in the canopy level was highly dominated by fig wasps (84%) in individual number, and by scarabaeid beetles (28%) in weight. A principal component analysis on monthly catches detected non-random, seasonal trends of insect abundance. The first two principal trends were an alternation of wetter (September to January) and less wet seasons (February to August) and an alternation between the least wet (January to March) and the other seasons. Many insect groups were less abundant in the least wet season than the other seasons, whilst inverse patterns were found in Scarabaeidae and Tenebrionidae. Significantly positive and negative correlations between monthly catch and rainfall were detected only in ovule-feeders and in phloem-feeders, respectively. Delayed, significant negative correlations between monthly catch and 1–3 month preceding rainfall were more frequently detected in phytophages, phloem-feeders, seed-feeders, wood-borers and scavengers. The peak in abundance along vertical levels were found at the canopy level (35 m) for phloem-, ovule-, seed-, root-, fungal-feeders and nectar collectors, at an upper subcanopy level (25 m) for scavengers and aquatic predators, and at a middle subcanopy level (17 m) for ants. Catches at the emergent level (45 m) did not exceed those at the canopy level.     

Detection of the Nicotiana rustica chloroplast genome coding for the large subunit of Fraction I protein in a somatic hybrid in which only the N. tabacum chloroplast genome appeared to have been expressed

Nine plants were produced from anthers of a somatic hybrid which had been obtained by fusion of Nicotiana tabacum L. and N. rustica L. protoplants. As determined by electrofocusing, the Fraction I protein of the original somatic hybrid had largesubunit polypeptides exclusively of the N. tabacum type. Two of the plants regenerated from anthers contained Fraction-I-protein large subunits exclusively of the N. rustica type. Since each plant was regenerated from a single cell, the somatic hybrid must have had cells containing both the N. tabacum and N. rustica chloroplast genome although the latter was not expressed. Possibilities to account for this non-expression of a chloroplast genome in the somatic hybrid are discussed.     

Simultaneous determination of phenylbutazone and its metabolites in plasma and urine by high-performance liquid chromatography

A high-performance liquid chromatographic method was developed for the simultaneous determination of phenylbutazone and its metabolites, oxyphenbutazone and γ-hydroxyphenylbutazone, in plasma and urine. Samples were acidified with hydrochloric acid and extracted with benzene—cyclohexane (1:1, v/v). The extract was redissolved in methanol and chromatographed on a μBondapak C₁₅ column using a mobile phase of methanol—0.01 M sodium acetate buffer (pH 4.0) in a linear gradient (50 to 100% methanol at 5%/min; flow-rate 2.0 ml/min) in a high-performance liquid chromatograph equipped with an ultra-violet absorbance detector (254 nm). The detection limit for phenylbutazone, oxyphenbutazone and for γ-hydroxyphenylbutazone was 0.05 μg/ml.A precise and sensitive assay for the determination of phenylbutazone and its metabolites was established.     

Intermolecular Forces Involved in the Gelation and Gel Stability of Sesame 13S Globulin

The effect of various reagents on the formation and stability of heat-induced gels of sesame 13S globulins were investigated. Electrostatic interaction, the hydrophobic bond and the disulfide bond were important for forming the network structure of gels, and the hydrogen bond also had an influence on the formation of the gel. Hydrophobic bonds mainly contributed to the stability of the gel. Subunit analyses of the proteins solubilized from the gels showed the presence of a free acidic subunit (AS) and basic subunit (BS), a polymer of AS, a dimer of BS and the dimer of a fragment from AS or BS. From the results, sulfhydryl-disulfide exchange reactions during gelation are suggested.