The control by insulin of amino acid accumulation in muscle

Muscle Carnosine Concentration with the Co-Ingestion of Carbohydrate with β-alanine in Male Rats

Muscle carnosine is an intracellular buffer. The intake of β-alanine, combined with carbohydrate and protein, enhanced carnosine loading in human muscle. The aim of the present study was to examine if muscle carnosine loading was enhanced by β-alanine intake and co-ingestion of glucose in male rats. Thirty-six male rats were divided into three groups and supplemented for four weeks: β-alanine (βA group, 1.8% β-alanine in drinking water), β-alanine and glucose (βAGL group, 1.8% β-alanine and 5% glucose in drinking water), and control (C group, drinking water). During the supplementation period, rats were exercised (20 m·min^-1, 10 min·day^-1, 4 days·week^-1 for 4 weeks). Muscle carnosine concentration was quantified in soleus (n = 12) and rectus femoris (n = 6) muscles using high-performance liquid chromatography. In soleus muscle, carnosine concentration was 2.24 ± 1.10, 6.12 ± 1.08, and 6.93 ± 2.56 mmol/kg dw for control, βA, and βAGL, respectively. In rectus femoris, carnosine concentration was 2.26 ± 1.31, 7.90 ± 1.66, and 8.59 ± 2.33 mmol/kg dw for control, βA, and βAGL respectively. In each muscle, βA and βAGL resulted in similar carnosine increases compared to the control. In conclusion, β-alanine intake for four weeks, either alone or with glucose co-ingestion, equally increased muscle carnosine content. It appears that the potential insulin response to fluid glucose intake does not affect muscle carnosine loading in male rats.

Caffeine Use in Sports, Pharmacokinetics in Man, and Cellular Mechanisms of Action

Caffeine is the most widely consumed psychoactive 'drug' in the world and probably one of the most commonly used stimulants in sports. This is not surprising, since it is one of the few ergogenic aids with documented efficiency and minimal side effects. Caffeine is rapidly and completely absorbed by the gastrointestinal tract and is readily distributed throughout all tissues of the body. Peak plasma concentrations after normal consumption are usually around 50 microM, and half-lives for elimination range between 2.5-10 h. The parent compound is extensively metabolized in the liver microsomes to more than 25 derivatives, while considerably less than 5% of the ingested dose is excreted unchanged in the urine. There is, however, considerable inter-individual variability in the handling of caffeine by the body, due to both environmental and genetic factors. Evidence from in vitro studies provides a wealth of different cellular actions that could potentially contribute to the observed effects of caffeine in humans in vivo. These include potentiation of muscle contractility via induction of sarcoplasmic reticulum calcium release, inhibition of phosphodiesterase isoenzymes and concomitant cyclic monophosphate accumulation, inhibition of glycogen phosphorylase enzymes in liver and muscle, non-selective adenosine receptor antagonism, stimulation of the cellular membrane sodium/potassium pump, impairment of phosphoinositide metabolism, as well as other, less thoroughly characterized actions. Not all, however, seem to account for the observed effects in vivo, although a variable degree of contribution cannot be readily discounted on the basis of experimental data. The most physiologically relevant mechanism of action is probably the blockade of adenosine receptors, but evidence suggests that, at least under certain conditions, other biochemical mechanisms may also be operational.

Acetylcholine receptors and nerve terminal distribution at the neuromuscular junction of non-obese diabetic mice

Skeletal muscle is one of the main targets of the metabolic alterations in diabetes, in which protein synthesis is markedly reduced followed by increased proteolysis. Ultrastructural and functional changes in the presynaptic compartment of the neuromuscular junction (NMJ) have been demonstrated, but little attention has been paid to the proteins in the postsynaptic muscle fiber membrane. In the present work, we studied the changes in acetylcholine receptors (AChRs) and nerve terminal distribution in the NMJ of non-obese diabetic (NOD) mice. The sternomastoid muscles of adult female NOD mice were double-labeled for AChR and nerve terminal observation by fluorescence and reflected light confocal microscopy. In 62.4% of the diabetic endplates, AChR branches broke apart into receptor islands that stained less than in the normal mice. These patches had regular junctional folds. At most of the endplates studied, the nerve terminals colocalized with AChRs, and sprouts were seen in 10% of the diabetic endplates. The intramuscular nerve branches and axons in the nerve to the sternomastoid muscle showed no degenerative disorders. These results suggest that metabolic alterations in the diabetic muscle fiber can affect the distribution and expression of molecules, such as AChRs, in the postsynaptic membrane of the neuromuscular junction.

Macronutrient-induced cascade of events leading to parallel changes in hypothalamic serotonin and insulin

Extracellular serotonin (5-HT) and insulin from hypothalamic PVN-VMH region follow parallel changes in response to specific macronutrient ingestion. Possible independent or causal mechanisms have been investigated. A common primary event might be pancreatic insulin secretion for both insulin entry into the brain and 5-HT synthesis through variations in the ratio of tryptophan over competitor amino acids. The steps of this cascade were found to account only partly for the changes in hypothalamic 5-HT and insulin. The central consequences of these metabolic effects may be modulated directly at the hypothalamic level. For instance, we observed a positive relation between the changes in insulin and 5-HT and the satiating potency of each nutrient. In addition, a direct action of dexfenfluramine on insulin has been found at the hypothalamic level showing that an activation of the serotonergic system immediately enhances insulin levels. This central event may be an important step in a cascade of events triggered by macronutrient ingestion leading to common hypothalamic insulin and 5-HT changes involved in feeding regulation.

Insulin reduces leucine oxidation and improves net leucine retention in parenterally fed humans

Protein metabolism during parenteral feeding was measured with and without euglycemic hyperinsulinemic clamping in five healthy human subjects using the primed continuous infusion of [(13)C]leucine (LEU) tracer methodology. All subjects underwent two periods of protein measurement. Subjects were randomized to have the clamp first or second. Two subjects had the clamp first, and they participated in another study in which the measurement period after the clamp was increased by another 2 h. Insulin reduced LEU oxidation (from 26 to 22 microM. kg(-1). h(-1); P < 0.05) and improved net LEU balance (from 3 to 7 microM. kg(-1). h(-1); P < 0. 05). The order of the clamp influenced the effects of insulin. With clamping performed second, insulin reduced LEU flux (from 152 to 134 microM. kg(-1). h(-1); P < 0.01), endogenous LEU rate of appearance (Ra; from 125 to 107 microM. kg(-1). h(-1); P < 0.01), and non-oxidative LEU disappearance (NOLD; from 128 to 113 microM. kg(-1). h(-1); P < 0.01). With clamping performed first, NOLD decreased after insulin was stopped (from 129 to 121 microM. kg(-1). h(-1); P < 0.05), but no change was seen in the flux and LEU Ra, despite the return of plasma insulin and amino acid concentrations to basal levels. The reduction in NOLD was accentuated with time and did not reach plateau even after 6 h and indicated a prolonged carry-over effect for NOLD and Ra. This effect was not seen for leucine oxidation.

The role of insulin and other hormones in the regulation of amino acid and protein metabolism in humans

While realizing the difficulties with the various methods used to study hormonal control of protein metabolism, there appear to be clear effects of both rapid-acting and slower-acting hormones. Moreover, some of these hormones affect protein metabolism in a dose dependent manner. Insulin and IGF-I appear to have differing effects at lower doses, with insulin primarily inhibiting protein degradation and IGF-I stimulating protein synthesis. At higher doses, infusions of insulin and IGF-I both seem to inhibit protein degradation and stimulate protein synthesis. Epinephrine primarily inhibits protein degradation whereas growth hormone primarily increases protein synthesis. Infusion of amino acids themselves can also increase protein synthesis. Thyroid hormone excess increases protein synthesis and protein degradation, with the latter effect predominating. Sex steroids appear to increase protein synthesis. To date, most interventions studying the metabolic effects of these hormones on protein metabolism have involved varying the concentration of one hormone at a time. In the complex milieu of many pathologic states (e.g. sepsis, renal failure or even the transition from fasting to feeding) multiple hormones change simultaneously. How interactions among these factors determine the overall response of body and muscle protein remains to be defined.

The opposing effects of insulin and hyperglycemia in modulating amino acid metabolism during a glucose tolerance test in lean and obese subjects

An intravenous glucose tolerance test (IVGTT) was administered in both lean (n = 9) and obese (n = 14) volunteers to ascertain the importance of the dynamic interactions between insulin and glucose on plasma concentrations of two amino acids known to be primarily used by skeletal muscle, namely leucine and isoleucine, and one amino acid, phenylalanine, which is primarily metabolized by the liver. After a 30-minute basal period, each subject received a bolus injection of glucose (0.3 g/kg IV) followed 20 minutes later by a bolus injection of tolbutamide (300 mg). Blood samples were drawn frequently for 180 minutes after the glucose infusion to determine plasma concentrations of glucose, insulin, leucine, isoleucine, and phenylalanine. Plasma glucose and insulin concentrations during the basal period were increased by 10% and 100%, respectively, in obese compared with lean individuals (P < .05), whereas phenylalanine, isoleucine, and leucine levels were similar between groups. During the IVGTT, plasma glucose level initially increased by twofold and slowly returned to basal level thereafter, whereas insulin level responded to glucose and tolbutamide infusions in a typical biphasic manner. Plasma leucine and isoleucine levels did not change from basal levels during the first 60 minutes of the IVGTT as hyperglycemic hyperinsulinemia prevailed in both groups. However, when plasma glucose had returned to near-basal levels, plasma leucine and isoleucine levels began to decrease, reaching a plateau of approximately 20% and 35% below basal, and plasma insulin level remained elevated in the lean and obese individuals, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Amphibolic role of the Krebs cycle in the insulin-stimulated protein synthesis

It has been a generally held view that insulin does not significantly affect the incorporation of amino acids into liver protein. This interpretation was based on data obtained from studies using the branched chain amino acids, which are poorly metabolized by the hepatic tissue. The effect of insulin on 14CO2 formation and protein incorporation of several 1-14C-labeled or U-14C-labeled amino acids was studied in isolated rat hepatocytes and diaphragm pieces. It was shown that insulin enhanced 14CO2 formation and protein incorporation primarily of those carbons of amino acids which are metabolized through the mitochondrial Krebs cycle. Using aminooxyacetic acid (0.5 mM), a potent inhibitor of the transamination reaction, it was shown that there exists an "insulin-sensitive" pool of glutamate which is preferentially utilized for protein synthesis in the presence of insulin. The insulin effect on protein incorporation of 14C-labeled glutamate generated in the Krebs cycle was abolished in the presence of aminooxyacetic acid. We interpret these results to signify that mitochondrial transamination of alpha-ketoglutarate to glutamate is essential for insulin stimulation of 14C incorporation into hepatocyte protein.

Neutral amino acid transport into rat skeletal muscle: competition, adaptive regulation, and effects of insulin

Amino acid (AA) transport systems A and L, which transfer preferentially small neutral AA (SNAA) and large neutral AA (LNAA), respectively, were studied in the isolated soleus muscle with the specific models, 2-(methylamino)isobutyrate (MeAIB) and 2-aminobicyclo[2,2,1]heptane-2-carboxylate (BCH). Affinity for MeAIB was greater than for BCH (Km = 3.2 +/- 0.2 and 8.7 +/- 0.2 mm, respectively). Rate of transport of MeAIB (Vmax = 104 +/- 3 pmol/microL/min) was slower than for BCH (970 +/- 12 pmol/microL/min), but accumulation was far more concentrative; transport of BCH, but not MeAIB, rapidly reached a steady-state level. MeAIB transport was reduced in the presence of SNAA; BCH transport was reduced to a lesser extent only by LNAA. Mixtures of AA at concentrations resembling those in plasmas of rats fed either a 6% or 50% casein diet reduced transport of MeAIB, whereas BCH transport was low only with the latter mixture. Only MeAIB transport was stimulated by insulin. Preincubation of muscles for 5 hours in a AA-free medium stimulated subsequent MeAIB uptake by about twofold to fourfold; this effect was suppressed by inhibitors of protein synthesis. Selective differences were thus observed in transport by skeletal muscle of model AA for the A and L systems: increased transport resulting from various stimuli was limited to the model for the A system, and transport of either model was depressed with mixtures containing physiological levels of AA. Changes in dietary protein or AA intake may thus alter transport of certain neutral AA into skeletal muscle via changes in plasma AA pools.

Regeneration of skeletal muscle in streptozotocin-induced diabetic rats

The present study analyzes the regeneration of skeletal muscle in diabetic rats. Intravenous injection of streptozotocin (STZ) was used to induce diabetes. Six weeks later the extensor digitorum longus (EDL) muscles from diabetic rats were either transplanted into diabetic or normal hosts to initiate regeneration. Normal EDL muscle transplants in normal and diabetic hosts were also performed for comparison. One, 2, 4, and 12 weeks after transplantation, the EDL regenerates were morphologically analyzed. Regeneration and formation of neuromuscular junctions were observed in all transplants, including diabetic regenerates in diabetic hosts. The overall mass and myofiber size of the diabetic EDL regenerate in the diabetic host was significantly reduced in spite of complete regeneration. Recovery of the diabetic muscle mass and the myofiber size was observed after transplantation into normal hosts. A reduction in mass and myofiber size was observed in normal EDL muscles transplanted into diabetic hosts. It is concluded that poor recovery of diabetic muscle is related to metabolic and structural alterations in the diabetic host, rather than to innate capacity of the muscle to per se undergo regeneration and reinnervation. The observed enhancement in recovery of diabetic muscle after transplantation in a normal host and deterioration of normal muscle after transplantation in a diabetic host shows that the host environment determines the success of muscle regeneration.

Amino acids augment insulin's suppression of whole body proteolysis

Leucine (LEU) kinetics were assessed using a primed-continuous infusion of L-[1-14C]LEU in normal overnight-fasted male volunteers during a basal period and an experimental period where insulin (INS) was infused at either 0.6, 1.2, 2.5, 5.0, 10, or 20 mU.kg-1.min-1 with euglycemia maintained. Two protocols were used: 1) subjects were allowed to develop hypoaminoacidemia or 2) plasma essential amino acids (AA) were maintained near basal levels by frequently monitoring plasma LEU in conjunction with variable infusions of an AA solution (LEU infused = 0.41, 0.72, 0.93, 1.03, 1.31, and 1.35 mumol.kg-1.min-1 at escalating INS doses, respectively). Basal rates of LEU appearance (Ra), nonoxidative disappearance (NORd) and oxidative disappearance (OXRd) were similar in both protocols (means = 1.74, 1.40, and 0.36 mumol.kg-1.min-1, respectively). INS infusions without AA resulted in a progressive decrement in LEU Ra (14 to 45%), NORd (16-41%), and OXRd (3-56%) compared with basal values. The infusion of AA resulted in an additional reduction in endogenous Ra (P less than 0.01; approximately 100% suppression achieved at plasma INS greater than 1,000 microU/ml) and a blunting of NORd reduction (P less than 0.05) at each dose of INS. Observed differences in INS's suppression of LEU Ra between the two protocols suggests the existence of a component of whole body proteolysis that is highly dependent on circulating plasma AA. Therefore, hypoaminoacidemia associated with INS treatment would appear to blunt the responsiveness of INS's suppression of protein breakdown and in the presence of near basal plasma AA, proteolytic suppression by INS is enhanced.

Effect of insulin on excretion and retention of infused glutamine amide-N in chickens (Gallus domesticus)

1. The purpose of this study was to examine the effects of insulin on urinary excretion and retention of the intravenously infused glutamine amide-15N in chickens. 2. Insulin pretreatment reduced urinary total 15N excretion (P less than 0.05) and enhanced 15N retention in chicken body (P less than 0.05), but it did not affect non protein-15N retained in the liver and blood. 3. Insulin decreased the incorporation of the infused glutamine amide-15N into urinary uric acid as well as the excretion of other N-derived urinary uric acid (P less than 0.05), which resulted in a significant decrease in total urinary uric acid (P less than 0.05). 4. There was no effect of insulin on urinary appearance of the infused glutamine amide-15N in the form of ammonia.

Stress induces an increased hexose uptake in cultured cells

Temperature-sensitive mutants have revealed a region of the herpes simplex virus 1 genome that affects both the uptake of hexose and the synthesis of heat shock proteins. Other inducers of heat-shock proteins, namely heat shock itself and arsenite, likewise induce an increased uptake of hexose. The increased uptake, like that induced by insulin, is insensitive to the presence of actinomycin D or cycloheximide. It is concluded that an increased hexose uptake, reflecting an activation or relocation of existing hexose transport protein, is a general biochemical response of stressed cells.

Insulin-mediated reduction of whole body protein breakdown. Dose-response effects on leucine metabolism in postabsorptive men

In vivo effects of insulin on plasma leucine and alanine kinetics were determined in healthy postabsorptive young men (n = 5) employing 360-min primed, constant infusions of L-[1-13C]leucine and L-[15N]alanine during separate single rate euglycemic insulin infusions. Serum insulin concentrations of 16.4 +/- 0.8, 29.1 +/- 2.7, 75.3 +/- 5.0, and 2,407 +/- 56 microU/ml were achieved. Changes in plasma 3-methyl-histidine (3-MeHis) were obtained as an independent qualitative indicator of insulin-mediated reduction in proteolysis. Hepatic glucose output was evaluated at the lowest insulin level using D-[6,6-2H2]glucose. The data demonstrate a dose-response effect of insulin to reduce leucine flux, from basal values of 77 +/- 1 to 70 +/- 2, 64 +/- 3, 57 +/- 3, and 52 +/- 4 mumol(kg X h)-1 at the 16, 29, 75, and 2,407 microU/ml insulin levels, respectively (P less than 0.01). A parallel, progressive reduction in 3-MeHis from 5.8 +/- 0.3 to 4.3 +/- 0.3 microM was revealed. Leucine oxidation estimated from the 13C-enrichment of expired CO2 and plasma leucine (12 +/- 1 mumol[kg X h]-1) and from the 13C-enrichment of CO2 and plasma alpha-ketoisocaproate (19 +/- 2 mumol[kg X h]-1) increased at the 16 microU/ml insulin level to 16 +/- 1 and 24 +/- 2 mumol(kg X h)-1, respectively (P less than 0.05 for each), but did not increase at higher insulin levels. Alanine flux (206 +/- 13 mumol(kg X h)-1) did not increase during the clamp, but alanine de novo synthesis increased in all studies from basal rates of 150 +/- 13 to 168 +/- 23, 185 +/- 21, 213 +/- 29, and 187 +/- 15 mumol(kg X h)-1 at 16, 29, 75, and 2,407 microU/ml insulin levels, respectively (P less than 0.05). These data indicate the presence of insulin-dependent suppression of leucine entry into the plasma compartment in man secondary to a reduction in proteolysis and the stimulation of alanine synthesis during euglycemic hyperinsulinemia.

Effects of insulin and glucagon on amino acid transport into the liver and opercular muscle of the eel in vitro

Effects of insulin and glucagon on amino acid transport in vitro into liver slices and the opercular muscle of the eel were studied using a labeled nonmetabolizable amino acid analog, alpha-[1-14C]aminoisobutyric acid (AIB). Addition of insulin (0.1 IU/ml) to the medium increased the radioactivity in the deproteinized fractions of both the liver slices and opercular muscle, indicating an accelerated movement of this amino acid analog into these tissues. Glucagon (5 micrograms/ml) also enhanced the entry of [14C]AIB into the liver slices; however, treatment of opercular muscle with glucagon did not alter the radioactivity in the deproteinized fraction. These findings clearly indicate that the entry of amino acids into eel tissues independent of protein synthesis can be altered by insulin and glucagon.

Regulation of membrane transport sites for amino acids in myogenic cells. A differentiation dependent phenomenon

Myoblasts from 12-day chick embryos in cell culture transport the nonmetabolizable amino acid alpha-aminoisobutyric acid (AIB) two to three-fold more rapidly than multinucleated myotubes which form from them. This decrease in transport is due to a relative decrease in the number of transport sites per unit area of cell surface suggesting a compositional change in the plasma membrane during myogenesis. In studies reported here, AIB transport was monitored throughout myogenesis and correlated with other aspects of differentiation. During myogenesis the number of amino acid transport sites remains constant per myotube nucleus. As myogenesis proceeds, there is a marked increase in cellular protein and cell surface without a commensurate increase in amino acid transport sites. The net consequence of the surface area change is fewer amino acid transport sites per unit area of myotube membrane surface. The decrease in membrane transport sites for AIB per unit area of membrane is not a result of length of time in culture per se, medium depletion, or cell density, but is a result of differentiation, since blocking myoblast fusion by deprivation of calcium delays the decrease in AIB transport sites per unit cell surface area while reversal of the calcium deprivation block is accompanied by a rapid decrease in the number of AIB transport sites per unit cell surface area. Thus, the decrease in AIB transport sites is an aspect of differentiation which accompanies the marked elaboration of surface membrane during myogenesis.

Preliminary report: the effects of acute acidosis on alanine and glucose metabolism across the liver, gut, kidney, and muscle in the dog

Acute metabolic acidosis (AMA) in the dog results in a 48% increase in arterial alanine concentration and an 11% fall in blood glucose. Extraction/production of glucose and alanine were studied by A/V sampling and electromagnetic blood flow probe measurements. Alanine and total amino-N release by muscle increased in acidosis Hepatic glucose release was reduced. More marked changes in metabolism occur in AMA in the dog than previously considered.

Tissue resistance to insulin stimulation of amino acid uptake in acutely uremic rats

Peripheral tissue resistance to insulin-mediated glucose uptake occurs in uremia. A similar resistance also may exist to insulin-mediated amino acid uptake. The present study examined the effect of insulin on the uptake or distribution ratios (DR) of alpha-aminoisobutyric acid (AIBA) attained in the tissues of rats made acutely uremic by bilateral nephrectomy. Both uremic and control rats concentrated equally AIBA in the gastrocnemius muscle in the absence of insulin (DR = 2.82 +/- 0.39 in controls, 2.52 +/- 0.40 in uremics). Compared to control rats, the diaphragm and liver of uremic rats, in the absence of insulin, had higher concentrations of AIBA. Insulin had the expected effect of increasing the uptake in the gastrocnemius, diaphragm, and liver of control rats but had no effect on uptake in the gastrocnemius muscle of uremic rats (DR = 5.82 +/- 0.78 in controls, 3.07 +/- 0.60 in uremics). The effect of insulin on the distribution ratio of AIBA in diaphragm and liver or uremic rats was less than it was in control rats. Uremia is associated with inhibition of insulin-mediated amino acid uptake as well as peripheral resistance to glucose utilization.

Amino acid transport by a muscle cell line

An established rat muscle cell culture, L-6, was employed for the evaluation of muscle amino acid transport. The initial influx of 16 radiolabeled amino acids into these cells was determined, and an overall uptake profile was constructed. L-6 cells accumulate amino acids progressively with time by saturable, temperature-dependent processes. As found in other preparations, several amino acids have sodium-dependent uptake systems. There is a wide range in the rate of transport of the different amino acids. The overall profile of uptake is fairly reproducible and may be characteristic of muscle tissue; it does not resemble that previously found in other cell types. The profile of amino acid uptake into these cells compares favorably with the pattern of amino acid transfer from the bloodstream to muscle found in intact rats. It is concluded that muscle cell transport is basically similar to that of other cell types. L-6 muscle cells have an amino acid uptake profile which resembles that of intact muscle; they should thus prove useful in the further delineation of muscle amino acid transport mechanisms.

Compensatory hypertrophy of skeletal muscle fibers in streptozotocin-diabetic rats

Previous studies have demonstrated an apparent differential response of the fiber types in mixed skeletal muscles of rats to streptozotocin diabetes. The purpose of the present study was to examine the ability of the different fiber types to hypertrophy in muscles from diabetic rats, which should further clarify the apparent differential trophic influence of insulin on the fibers. One group of rats was injected with streptozotocin to induce diabetes. The gastrocnemius muscle was then removed from one hindlimb of rats of both the diabetic and a second, normal group, resulting in compensatory growth of ipsilateral plantaris muscle. Rats were sacrificed 60 days following the surgery. Experimental muscles in normal and diabetic rats enlarged 79% and 61% over control muscles, respectively. In normal hypertrophied muscles there was an 8% increase in relative cross-sectional area composed of slow-twitch fibers, whereas in diabetic rats the slow-twitch component increased 17%. The results indicate that slow-twitch fibers in diabetic rats were capable of responding to the chronic power overloaded condition, but that the fast-twitch fibers had a reduced capacity to undergo compensatory growth. These findings support our previous observations suggesting that insulin may exert a differential trophic effect upon the muscle fiber types.

Effect of aminophylline on tryptophan and other aromatic amino acids in plasma, brain and other tissues and on brain 5-hydroxytryptamine metabolism

1 Aminophylline and other methylxanthines increase brain tryptophan and hence 5-hydroxytryptamine turnover. The mechanism of this effect of aminophylline was investigated. 2 At lower doses (greater than 100 mg/kg i.p.) the brain tryptophan increase could be explained by the lipolytic action of the drug, i.e. increased plasma unesterified fatty acid freeing plasma tryptophan from protein binding so that it became available to the brain. 3 Plasma unesterified fatty acid did not increase when aminophylline (109 mg/kg i.p.) was given to nicotinamide-treated rats but as both plasma total and free tryptophan rose, a tryptophan increase in the brain still occurred. 4 The rise in brain tryptophan concentration following the injection of a higher dose of the drug (150 mg/kg i.p.) could no longer be explained by a rise of plasma free tryptophan as the ratio of brain tryptophan to plasma free tryptophan rose considerably. Plasma total tryptophan fell and the plasma insulin concentration rose. 5 The increase of brain tryptophan concentration after injection of 150 mg/kg aminophylline appeared specific for this amino acid as brain tyrosine and phenyllanine did not increase. However as their plasma concentrations fell the brain/plasma ratio for all three amino acids rose. 6 The higher dose of aminophylline increased the muscle concentration of tryptophan but that of tyrosine fell and that of phenylalanine remained unaltered. The liver concentrations were not affected. 7 The aminophylline-induced increase of the ratio of brain tryptophan of plasma free tryptophan no longer occurred when the drug was given to animals injected with the beta-adrenoreceptor blocking agent propranolol or the diabetogenic agent streptozotocin. 8 The changes in brain tryptophan upon aminophylline injection may be explained by (a) increased availability of plasma tryptophan to the brain due to increased lipolysis and (b) increased effectiveness of uptake of tryptophan by the brain due to increased insulin secretion.

Possible role of insulin in the transport of tyrosine and tryptophan from blood to brain

The administration of insulin or the ingestion of glucose increases the concentration of tryptophan and tyrosine in the brain. This increase is associated with a parallel decrease in the concentration of free tryptophan and tyrosine in serum. The results suggest that insulin enhances the transport of tyrosine and tryptophan from blood to brain. The results, moreover, suggest that exogenous or endogenous insulin enhances the transport of tyrosine and tryptophan from blood to brain.

Histochemical properties of skeletal muscle fibers in streptozotocin-diabetic rats

The response of rat gastrocnemius muscle fibers to chronic streptozotocindiabetes was studied. Transverse sections of this muscle from normal and diabetic rats were histochemically assayed for reduced diphosphopyridine nucleotide-diaphorase, myofibrillar adenosine triphosphatase, mitochondrial alpha-glycerophosphate dehydrogenase, beta-hydroxybutyrate dehydrogenase, and alkaline phosphatase activities. Cross-sectional areas of the fiber types were measured, and fiber capillarization and populations estimated. Chemically-induced diabetes appeared to have little effect on the metabolic or morphological properties of slow-twitch fibers. However, a general dedifferentiation occurred in the 2 fast-twitch fiber populations. There was a loss of oxidative potential in the fast-twitch-oxidative-glycolytic fibers, and a significant decrease in size in the fast-twitch-glycolytic fibers. No change in the proportions of slow- and fast-twitch fibers in the muscles of diabetic rats occurred. It is concluded that hypoinsulinism has differential effects on the 3 fiber types in heterogeneous rat skeletal muscle, and that slow-twitch fibers are least affected by the diabetic condition.

Effect of insulin on muscle glutamate uptake. Whole blood versus plasma glutamate analysis

For decades, investigators concerned with protein metabolism in man have performed detailed amino acid analyses of human plasma obtained under a wide range of experimental situations. A large body of information has been used to calculated rates of protein synthesis and proteolysis. During the course of an investigation of the effect of intrabrachial artery infusion of insulin (70 muU/min per kg body weight) on glutamate uptake by human forearm muscle, it was discovered that plasma arterio-deep venous glutamate difference analysis failed to document any increase in the uptake of this amino acid, suggesting that insulin had little influence on glutamate uptake by muscle. However, whole blood glutamate analyses, performed on the same blood samples, revealed that (a) the resting muscle uptake of glutamate is smaller than previously reported and (b) insulin is capable of markedly increasing glutamate uptake by muscle from whole blood. Since the hematocrit was obtained on all samples, detailed analyses of the various compartments in which glutamate could be found were performed. It was determined that circulating blood cells have a dynamic role in glutamate transport. These data underscore the need for both whole blood and plasma amino acid analysis in investigations concerned with protein synthesis and/or amino acid flux, for analysis of plasma samples alone could be misleading as illustrated in the present study.

The effect of insulin on free amino acid pools and protein synthesis in rat skeletal muscle in vitro

1. The effects of insulin in vitro on tissue pools and incorporation into protein of glycine and leucine in the extensor digitorum longus muscle of the rat are reported. 2. It was found that insulin decreased the lag period before the establishment of a linear rate of incorporation of radioactive glycine into protein. 3. The hormone increased the size of the free intracellular glycine pool. No such effect was found for leucine. The accumulation of radioactive glycine in the intracellular fluid compartment was increased. The content of radioactive leucine in the intracellular compartment was decreased. 4. Insulin decreased the specific radioactivity of both glycine and leucine in the extracellular fluid. 5. The hormone also decreased protein catabolism. 6. The effect on protein synthesis was not caused by an increase in the specific radioactivity of the extracellular pool but was possibly related to increased amino acid concentrations in this pool, which could in turn have affected the aggregation of ribosomes.

Kinetic analysis of insulin action on amino acid uptake by isolated chick embryo heart cells

1. Isolated chick embryo heart cells were used to investigate the mode of action of insulin on the transport of three naturally occurring amino acids: l-proline, l-serine and glycine. Initial velocities of uptake were measured over a period of 5min with an 80-fold range of amino acid concentration. Corrections for amino acid diffusion, incorporation into protein and conversion into carbon dioxide were introduced. 2. The uptake processes approximated Michaelis-Menten kinetics within definite ranges of amino acid concentrations. A single transport system for proline and at least two transport systems for serine and glycine were detected. 3. The kinetic effects of insulin on transport systems for the amino acids tested were consistent with an acceleration of the maximal velocity of the process, without substantial changes in substrate concentration for half-maximal transport velocity. 4. These hormonal effects were not essentially altered by the corrections for amino acid incorporation into protein and conversion into carbon dioxide.