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Biomedical subjects

A J Garber

Publications and source records attributed to A J Garber.

At least 55 records · Page 3Linked to original sources
PubMed1980

Skeletal muscle amino acid metabolism in chronic uremia.

The kinetics and factors regulating acid release were studied in epitrochlaris preparations of control and chronically uremic rats. These data were correlated with abnormal glucose and glucose precursor kinetics of azotemic patients and the impact of hemodialysis on these precursor-product interrelationships. In chronically uremic rat skeletal muscle, a subsensitivity to catecholamines was evident. A 1000-fold greater concentration of epinephrine was required in uremic muscle to inhibit alanine release compared to control preparations. The dose-response curve for cAMP formation stimulated by epinephrine was shifted 1 log unit to the right in uremic muscle and adenylyl cyclase stimulation by epinephrine was shifted 1 log unit. Using simultaneous primed-injection-continuous infusions [U-14C]-alanine and [2-3H]glucose, rates of metabolite turnovers were calculated from the plateau portions of the specific activity curves. In 14 azotemic patients, alanine and glucose turnover rates were found to be increased 276 and 154%, respectively, over that of the control group of 17 subjects. Glucose production was found to be 3-fold greater. Hemodialysis decreased alanine turnover by 35% and glucose production by 19%. However, glucose turnover was not affected by dialysis. Although the percentage of glucose produced from alanine approached normal levels, the percent of alanine used for gluconeogenesis increased 60%. These studies demonstrate that abnormal carbohydrate metabolism in uremia is a result of multiple factors. Diminished response of adenylyl cyclase and decreased levels of cAMP contributes to, but does not entirely account for, increased amino acid release. The increased alanine synthesis in skeletal muscle supports glucose overproduction found in chronic azotemia. Hemodialysis ameliorates but does not fully normalize glucose and alanine metabolism and may actually introduce additional abnormalities.

Adult↗
PubMed1980

The impact of streptozotocin-induced diabetes mellitus on cyclic nucleotide regulation of skeletal muscle amino acid metabolism in the rat.

The impact of diabetes on cyclic nucleotide-associated mechanisms regulating skeletal muscle protein and amino acid metabolism was assessed using epitrochlaris preparations from streptozotocin-induced diabetic rats. 1 nM epinephrine inhibited alanine and glutamine release from control preparations, but no inhibition was observed from diabetic preparations with <0.1 mM. 10 nM epinephrine stimulated lactate production from control muscle but stimulation in diabetic preparations was observed only at 0.1 mM. Serotonin inhibited amino acid release and stimulated lactate production equally in control and diabetic muscle. 0.1 mM epinephrine increased cyclic (c)AMP levels by 360% in control muscles, but these levels were increased only 83% in diabetic muscle. Basal-, fluoride-, and serotonin-stimulated adenylyl cyclase activities were equal in membrane preparations of diabetic and control muscle, but epinephrine-stimulated adenylyl cyclase was reduced by 60% in diabetic muscle. Carbamylcholine stimulation of alanine and glutamine release was blunted in diabetic preparations. Carbamylcholine increased cGMP levels in control but not in diabetic muscle. In diabetic muscle, guanylyl cyclase activity was 65% of control and the stimulation of cyclase activity by sodium azide was less in diabetic than control preparations. Added cGMP stimulated alanine and glutamine release from control, but not from diabetic muscle. These data suggest a loss of adrenergic and cholinergic responsiveness in diabetic muscle. Because amino acid release also showed a decreased responsiveness to added cAMP and cGMP, the presence of other derangements in the mechanism(s) of cyclic nucleotide regulation of muscle amino acid metabolism also seems likely.

Adenylyl Cyclases↗
PubMed1980

The effect of hemodialysis on levels of cyclic nucleotide-associated hormones in patients with chronic renal failure.

In order to determine the molecular basis for the loss of catecholamine responsiveness in skeletal muscle in chronic azotemia, plasma and serum levels of catecholamines and other hormones whose mechanisms of action are associated in part with cyclic nucleotide mediation were assessed in 37 patients with chronic azotemia. Samples were obtained prior to and immediately following conventional hemodialysis. Plasma epinephrine and norepinephrine levels in patients predialysis were increased 50% and 25% respectively compared to control subjects. Levels of insulin, prolactin, aldosterone and renin were also in creased in azotemic patients prior to dialysis. Conventional hemodialysis reduced serum levels of growth hormone, but had no effect of the elevated levels of all other hormones found in patients predialysis. In particular, plasma epinephrine and norepinephrine levels were unaffected by hemodialysis. Despite these findings, hemodialysis did reduce to normal levels the elevated plasma levels of cyclic AMP and cyclic GMP observed in uremic subjects predialysis. These data are consistent with increased adrenergic outflow in patients with chronic azotemia, and suggest a mechanism of homologous desensitization of the catecholamine receptor adenylyl cyclase unit in chronic azotemia.

Aldosterone↗
PubMed1979

Impact of hemodialysis on the abnormal glucose and alanine kinetics of chronic azotemia.

Rates of alanine and glucose turnover and precursor-product interrelationships were determined in patients on chronic hemodialysis and in matched controls using simultaneous primed injection-continuous infusions of [U-14C] alanine and [2-3h] glucose. In eight chronically dialyzed patients studied before their first dialysis of the week, glucose turnover was 866 +/- 120 micromole/min (mean +/- SE); after their last dialysis of the week, glucose turnover was 880 +/- 63 micromole/min. These rates were 35% (p less than 0.05) and 37% (p less than 0.01) greater than rates observed in ten normal volunteers (642 +/- 28.3 micromole/min). Fasting glucose and insulin levels in dialyzing patients were unchanged from normal. Alanine turnover was increased predialysis (318 +/- 55.2 micromoles/min; p less than 0.01) and postdialysis (248 +/- 32.4 micromole/min; p less than 0.01) as compared to normal (168 +/- 14.3 micromole/min). In patients pre- and postdialysis, gluconeogenesis from alanine was increased to 34.6 +/- 10.9 micromole/min (p less than 0.05) and 39.0 +/- 6.33 micromole/min (p less than 0.05) compared to 20.9 +/- 1.63 micromole/min in normal subjects. We conclude that neither acute nor chronic hemodialysis corrects the increased glucose and alanine production and utilization and gluconeogenesis observed in chronic renal failure.

Adult↗
PubMed1978

Cholinergic stimulation of skeletal muscle alanine and glutamine formation and release. Evidence for mediation by a nicotinic cholinergic receptor and guanosine 3':5'-monophosphate.

The mechanism of cholinergic stimulation of alanine and glutamine formation and release from skeletal muscle was studied using rat epitrochlaris preparations. The increased alanine and glutamine release produced by carbamylcholine (10(-6) M) was reproduced by tetramethylammonium (10(-6) M) but not by pilocarpine (10(-6) M) and was blocked by hexamethonium (10(-4) M) but not by atropine (10(-7) M). This increased alanine and glutamine release was not associated with altered muscle cAMP levels. However, carbamylcholine (10(-6) M) and tetramethylammonium (10(-6) M) did not increase levels of cGMP, 134% and 101%, respectively, and these increments in cGMP were blocked by hexamethonium but not by atropine. Carbamylcholine produced a concentration-dependent increase in cGMP levels. Methylisobutylxanthine and theophylline augmented the increased amino acid release and increased cGMP levels produced by carbamylcholine. Neither xanthine derivative alone altered alanine and glutamine release or cyclic nucleotide levels. Added cGMP increased amino acid release and the uptake of [U-14C]alanine and alpha-amino[14C]isobutyric acid. Carbamylcholine did not alter muscle phosphorylase a activity, glycogen levels, or basal adenylate cyclase activity. These data indicate that cholinergic stimulation of muscle alanine and glutamine formation and release involves a nicotinic cholinergic receptor and may be mediated by increased levels of cGMP, which in turn may result from a cholinergic stimulation of muscle guanylyl cyclase.

1-Methyl-3-isobutylxanthine↗
PubMed1978

Bioactivity of instant glucose. Failure of absorption through oral mucosa.

The efficacy of instant glucose as a potential treatment for hypoglycemia was studied in normal volunteers, with therapeutic doses administered in the buccal cavity. 2-Tritiated glucose (50 mu Ci) was homogenized into each dose before use. Mean blood glucose and serum insulin concentrations were unaltered by instant glucose. Glucose absorption was less than 0.05 mg at any time, and total glucose absorbed was less than 0.1 mg. For comparison purposes, volunteers swallowed a dose of instant glucose. Approximately 88% of the dose was absorbed during a 30-minute interval. Blood glucose and insulin levels increased. Instant glucose appears to be of therapeutic value only if swallowed by fully conscious, hypoglycemic patients. It should not benefit unconscious patients because of its poor absorption through the buccal mucosa.

Adult↗
PubMed1978

Abnormal carbohydrate metabolism in chronic renal failure. The potential role of accelerated glucose production, increased gluconeogenesis, and impaired glucose disposal.

To delineate the potential role of disordered glucose and glucose-precursor kinetics in the abnormal carbohydrate metabolism of chronic renal failure, alanine and glucose production and utilization and gluconeogenesis from alanine were studied in patients with chronic compensated renal insufficiency and in normal volunteers. With simultaneous primed injection-continuous infusions of radiolabeled alanine and glucose, rates of metabolite turnover and precursor-product interrelationships were calculated from the plateau portion of the appropriate specific activity curves. All subjects were studied in the postabsorption state. In 13 patients with chronic renal failure (creatinine = 10.7+/-1.2 mg/100 ml; mean+/-SEM), glucose turnover was found to be 1,035+/-99.3 mumol/min. This rate was increased 56% (P = 0.003) over that observed in control subjects (664+/-33.5 mumol/min). Alanine turnover was 474+/-96.0 mumol/min in azotemic patients. This rate was 191% greater (P = 0.007) than the rate determined in control subjects (163+/-19.4 mumol/min). Gluconeogenesis from alanine and the percent of glucose production contributed by gluconeogenesis from alanine were increased in patients with chronic renal failure (192% and 169%, respectively) as compared to controls (P < 0.05 for each). Alanine utilization for gluconeogenesis was increased from 40.2+/-3.86 mumol/min in control subjects to 143+/-39.0 mumol/min in azotemic patients (P < 0.05). The percent of alanine utilization accounted for by gluconeogenesis was not altered in chronic renal insufficiency. In nondiabetic azotemic subjects, mean fasting glucose and immunoreactive insulin levels were increased 24.3% (P = 0.005) and 130% (P = 0.046), respectively.These results in patients with chronic renal failure demonstrate (a) increased glucose production and utilization, (b) increased gluconeogenesis from alanine, (c) increased alanine production and utilization, and (d) a relative impairment to glucose disposal. We conclude that chronic azotemia is characterized by increased rates of glucose and glucose precursor flux and by a relative impairment to glucose disposal. These findings may suggest an underlying hepatic and peripheral insensitivity to the metabolic action of insulin in patients with chronic renal insufficiency.

Adult↗
PubMed1978

Skeletal muscle protein and amino acid metabolism in experimental chronic uremia in the rat: accelerated alanine and glutamine formation and release.

The kinetics and factors regulating alanine and glutamine formation and release were investigated in skeletal muscle preparations from control and experimentally uremic rats. These preparations maintained phosphocreatine and ATP levels in vitro which closely approximated levels found in vivo. Alanine and glutamine release from uremic muscle were increased 45.8 and 36.0%, respectively, but tissue levels were unaltered. The increased release of alanine by uremic muscle was not accounted for by decreased rates of medium alanine reutilization via oxidation to CO(2) or incorporation into muscle protein. The maximal capacity of added amino acids such as aspartate, cysteine, leucine, and valine to stimulate net alanine and glutamine formation was the same in uremic and control muscle. Epitrochlearis preparations were partially labeled in vivo with [guanido-(14)C]-arginine. On incubation, preparations from uremic animals showed a 54.6% increase in the rate of loss of (14)C-label in acid precipitable protein. Correspondingly, these same uremic preparations showed a 62.7% increase in (14)C-label appearance in the acid-soluble fraction of muscle and in the incubation media. Insulin decreased alanine and glutamine release to an extent threefold greater in uremic than in control preparations, and increased muscle glucose uptake approximately threefold in all preparations. Although basal rates of [4,5-(3)H]leucine incorporation into protein were decreased 25% in uremic muscles as compared with control muscles, insulin stimulated [(3)H]leucine incorporation nearly equally in both preparations. These data demonstrate increased alanine and glutamine production and release from skeletal muscle of chronically uremic rats. This increase appears to derive in part from an enhancement of net protein degradation which could be caused by an acceleration in the breakdown of one or more groups of muscle proteins, or by an inhibition of protein synthesis, or by both processes. The increased alanine and glutamine formation and release in uremia appears not to result from an insensitivity to insulin action. The implications of these findings for an understanding of the abnormal carbohydrate metabolism of uremia are discussed.

Adenosine Diphosphate↗
PubMed1978

The regulation of skeletal muscle alanine and glutamine formation and release in experimental chronic uremia in the rat: subsensitivity of adenylate cyclase and amino acid release to epinephrine and serotonin.

The mechanism of the increased alanine and glutamine formation and release from skeletal muscle in experimental uremia was investigated using epitrochlearis preparations from control and chronically uremic rats. In uremic muscle, insensitivity to epinephrine or serotonin suppression of alanine and glutamine release was observed. With control muscles, 1 nm or greater, epinephrine inhibited alanine and glutamine release, whereas with uremic muscles, epinephrine concentrations <1 muM did not alter amino acid release. Decreased alanine and glutamine release with 1 nM serotonin was observed in control muscles, but no inhibition was observed with concentrations <1 muM in uremic muscle. Muscle amino acid levels were the same in control and uremic muscles in the presence or absence of epinephrine or serotonin. The reutilization of released alanine by protein synthesis or oxidation to CO(2) was not differentially affected by epinephrine in uremic muscles as compared with control muscle. Dibutyryl-cAMP inhibited amino acid release equally in uremic and control muscles. Epinephrine or serotonin increased cAMP levels two- to four-fold or more in control than in uremic muscle. Basal- and fluoride-stimulated adenylate cyclase activities were equal in uremic and control muscle homogenates and in membrane fractions, but 10 muM epinephrine-stimulated adenylate cyclase was reduced 30-60% with uremia. At any concentration of epinephrine (0.001-100 muM), the stimulation of membrane adenylate cyclase activity was one- to twofold greater with control membranes than with uremic muscle membranes. With either control or uremic muscle, peak adenylate cyclase activity was observed at 1 muM epinephrine. These data indicate that skeletal muscle in chronic uremia acquires an insensitivity to the metabolic action of epinephrine or serotonin. This insensitivity may be attributable in part to the diminished increments in muscle cAMP levels produced by adrenergic and serotonergic agonists. The decreased cAMP levels may derive in turn from a decreased activity or subsensitization of the agonist-stimulated adenylate cyclase in uremic muscle.

Adenylyl Cyclases↗
PubMed1976

Alanine and glutamine synthesis and release from skeletal muscle. I. Glycolysis and amino acid release.

The synthesis and release of alanine and glutamine were investigated with an intact rat epitrochlaris muscle preparation. This preparation will maintain on incubation for up to 6 hours, tissue levels of phosphocreatine, ATP, ADP, lactate, and pyruvate closely approximating those values observed in gastrocnemius muscles freeze-clamped in vivo. The epitrochlaris preparation releases amino acids in the same relative proportions and amounts as a perfused rat hindquarter preparation and human skeletal muscle. Since amino acids were released during incubation without observable changes in tissue amino acids levels, rates of alanine and glutamine release closely approximate net amino acid synthesis. Large increases in either glucose uptake or glycolysis in muscle were not accompanied by changes in either alanine or glutamine synthesis. Insulin increased muscle glucose uptake 4-fold, but was without effect on alanine and glutamine release. Inhibition of glycolysis by iodacetate did not decrease the rate of alanine synthesis. The rates of alanine and glutamine synthesis and release from muscle decreased significantly during prolonged incubation despite a constant rate of glucose uptake and pyruvate production. Alanine synthesis and release were decreased by aminooxyacetic acid, an inhibitor of alanine aminotransferase. This inhibition was accompanied by a compensatory increase in the release of other amino acids, such as aspartate, an amino acid which was not otherwise released in appreciable quantities by muscle. The release of alanine, pyruvate, glutamate, and glutamine were observed to be interrelated events, reflecting a probable near-equilibrium state of alanine aminotransferase in skeletal muscle. It is concluded that glucose metabolism and amino acid release are functionally independent processes in skeletal muscle. Alanine release reflects the de novo synthesis of the amino acid and does not arise from the selective proteolysis of an alanine-rich storage protein. It appears that the rate of alanine and glutamine synthesis in skeletal muscle is dependent upon the transformation and metabolism of amino acid precursors.

Adenine Nucleotides↗
PubMed1976

Alanine and glutamine synthesis and release from skeletal muscle. II. The precursor role of amino acids in alanine and glutamine synthesis.

The synthesis and release of alanine and glutamine have been studied in the intact rat epitrochlaris skeletal muscle preparation. Aspartate, cysteine, leucine, valine, methionine, isoleucine, serine, theronine, and glycine increased significantly the formation and release of alanine from muscle. Cysteine, leucine, valine, methionine, isoleucine, tyrosine, lysine, and phenylalanine increased the rate of glutamine synthesis. Only ornithine, arginine, and tryptophan were without effect on the synthesis of either alanine or glutamine. Half-maximal stimulation of alanine and glutamine formation by added amino acids was observed with concentrations ranging between 0.5 and 1.0 mM. Increases in alanine and glutamine formation were not accompanied by changes in pyruvate production or glucose uptake. The progressive decline in alanine and glutamine synthesis noted on prolonged incubation was prevented by the addition of amino acids to the incubation medium. Stimulation of alanine synthesis by added amino acids was unaffected by inhibition of glycolysis with iodoacetate. Inhibition of alanine aminotransferase with aminooxyacetate significantly decreased alanine formation. Pyruvate and ammonium chloride did not increase further the rate of either alanine or glutamine formation above that produced by added amino acids. These data indicate that most amino acids are precursors for alanine and glutamine synthesis in skeletal muscle. A general mechanism is presented for the de novo formation of alanine from amino acids in skeletal muscle, and the importance of proteolysis for the supply of amino acid precursors for alanine and glutamine synthesis is discussed.

Alanine↗
PubMed1976

Alanine and glutamine synthesis and release from skeletal muscle. III. Dietary and hormonal regulation.

Alanine and glutamine formation and release were studied using the intact epitrochlaris preparation of rat skeletal muscle. Alanine release from skeletal muscle was increased by fasting (65%), cortisone (145%), thyroxine (200%), and diabetes (185%). Glutamine release was decreased by cortisone (37%) and diabetes (23%) but not significantly affected by fasting or thyroxine. Tissue levels of alanine were unchanged but tissue glutamine levels were markedly reduced (30 to 60%) in all treatment groups. Insulin added in vitro did not affect amino acid release even with preparations obtained from diabetic animals. Inhibition of glycolysis with 0.2 mM iodoacetate had no effect on the rate of alanine and glutamine formation in any treatment group. Pyruvate generation was increased by all treatments even in the presence of the inhibitor. Total skeletal muscle alanine, aspartate, and branched chain aminotransferase, glutamate dehydrogenase, and malic enzyme activities were not significantly altered in any treatment groups. The addition of 10 mM aspartate, cysteine, branched chain amino acids, and serine significantly increased alanine formation, whereas the maximal rate of glutamine formation in the presence of stimulating amino acids was reduced in each treatment groups--the most marked effects were noted with cortisone and diabetic preparations. Although accelerated muscle proteolysis is an important factor regulating alanine formation in skeletal muscle, the redirection of carbon flow from glutamine toward alanine formation observed in fasting, cortisone, thyroxine-treated, and diabetic rats, indicates that factors other than proteolysis also participate in the control of amino acid release from muscle.

Alanine↗