Endocrine control of amino acid transfer; distribution of an unmetabolizable amino acid

Actionable Metabolic Pathways in Heart Failure and Cancer-Lessons From Cancer Cell Metabolism

Recent advances in cancer cell metabolism provide unprecedented opportunities for a new understanding of heart metabolism and may offer new approaches for the treatment of heart failure. Key questions driving the cancer field to understand how tumor cells reprogram metabolism and to benefit tumorigenesis are also applicable to the heart. Recent experimental and conceptual advances in cancer cell metabolism provide the cardiovascular field with the unique opportunity to target metabolism. This review compares cancer cell metabolism and cardiac metabolism with an emphasis on strategies of cellular adaptation, and how to exploit metabolic changes for therapeutic benefit.

Influence of Anabolic Steroids on Uptake of Alpha-Aminoisobutyric Acid by Levator Ani Muscle

Two representative anabolicsteroids caused an increase in the uptake of alpha-aminoisobutyric acid-1-C(14) in the levator ani muscle of rats. The distribution ratio between that muscle and the plasma was increased fourfold by the administration of a synthetic anabolic steroid and twofold by administration of testosterone propionate. The determination of this increase may serve as an indicator for the myotrophic effect of anabolic steroids.

Reactivity of homocysteine-thiolactone and alpha-methylhomocysteine-thiolactone with e-(aq) and OH-radical: a pulse radiolysis study

The efficient radiation protecting agents homocysteine-thiolactone x HCl (HCTL x HCl) and its alpha-alkylated derivative (alpha-methyl-homocysteine, alpha-MHCTL x HCl) have been investigated in respect to the identification of the primarily formed species after absorption of ionizing radiation using pulse radiolysis technique.The reaction of e-(aq) with the unprotonated form of HCTL (k = 2.1 x 10(9) dm3 mol(-1) s(-1)) is leading to the formation of a radical anion having two absorption bands: at 275 nm (epsilon = 2500 dm3 mol(-1) cm(-1)) and 510 nm (epsilon = 930 dm3 mol(-1) cm(-1)), which decay with 2k = 2.3 x 10(9) dm3 mol(-1) s(-1). The protonated form of HCTL reacts with e-(aq) with k = 4.0 x 10(10) dm3 mol(-1) s(-1). The OH-radicals react with HCTL with k = 1.95 x 10(9) dm 3 mol(-1) x s(-1) resulting in a transient spectrum with lambda(max) = 265 nm (epsilon =2000 dm3 mol(-1) cm(-1)). The transients disappear with 2k = 2.1 x 10(9) dm3 mol(-1) s(-1). The reactivity of e-(aq) with alpha-MHCTL was determined for both forms: for the protonated, k = 1.25 x 10(10) dm3 mol(-1) s(-1) and for the unprotonated, k = 2.6 x 10(9) dm3 mol(-1) s(-1). The transient absorption spectrum at pH = 8.4 shows two absorption bands: lambda = 275 nm (epsilon = 3500 dm3 x mol(-1) x cm(-1)) and 490 nm (epsilon = 1160 dm3 x mol(-1) x cm(-1)). The transients disappear with 2k = 2.2 x 10(9) dm3 x mol(-1) x s(-1). The reaction of OH with alpha-MHCTL x HCl,k = 8.4 x 10(9) dm3 x mol(-1) x s(-1) (pH = 8.6) is resulting in an absorption spectrum with lambda(max) < 260 nm and an absorption band at 350 nm (epsilon = 510 dm3 x mol(-1) x cm(-1)). Up to 50 micros after pulse the transients decay with 2k = 5.5 x 10(9) dm3 x mol(-1) x s(-1) and thereafter by a k = 8.4 x 10(9) dm3 x mol(-1) x s(-1) (pH = 8.6) is resulting in an absorption spectrum with lambda(max) < 260 nm and an absorption band at 350 nm (epsilon = 510 dm3 x mol(-1) x cm(-1)). Up to 50 micros after pulse the transients decay with 2k = 5.5 x 10(9) dm3 x mol(-1) x s(-1) and thereafter by a first order reaction. In addition, the formation of some products was also studied. The yield of ammonia resulting from alpha-MHCTL x HCl strongly depends on pH, e.g. at pH = 5.1 Gi (NH3) = 0.95, whereas at pH = 9.15 it increases to Gi = 3.1. Hydrogen sulphide is formed in airfree solutions, Gi (H2S) = 0.29, whereas in the presence of N2O it is reduced to Gi (H2S) = 0.10. Some probable reaction mechanisms are presented.

Stimulation of amino acid transport in isolated diaphragm by growth hormone added in vitro

The influence of the pituitary on the transport of alpha-aminoisobutyric acid-1-C(14) into the cells of isolated rat diaphragm was investigated. Hypophysectomy results in a lower-than-normal rate of alpha-aminoisobutyric acid-1-C(14) penetration into muscle. Adding either simian or bovine growth hormone preparations to the incubation medium in concentrations of 2.5 to 25 microg/ml of medium resulted in a doubling of the alpha-aminoisobutyric acid-l-C(14) penetration rate. The stimulatory effect was minimized when the hormone concentration was reduced to 0.25 microg/ml of medium.

"Facilitated" amino acid transport is upregulated in brain tumors

The goal of this study was to determine the magnitude of "facilitated" amino acid transport across tumor and brain capillaries and to evaluate whether amino acid transporter expression is "upregulated" in tumor vessels compared to capillaries in contralateral brain tissue. Aminocyclopentane carboxylic acid (ACPC), a non-metabolized [14C]-labeled amino acid, and a reference molecule for passive vascular permeability, [67Ga]-gallium-diethylenetriaminepentaacetic acid (Ga-DTPA), were used in these studies. Two experimental rat gliomas were studied (C6 and RG2). Brain tissue was rapidly processed for double label quantitative autoradiography 10 minutes after intravenous injection of ACPC and Ga-DTPA. Parametric images of blood-to-brain transport (K1ACPC and K1Ga-DTPA, microL/min/g) produced from the autoradiograms and the histology were obtained from the same tissue section. These three images were registered in an image array processor; regions of interest in tumor and contralateral brain were defined on morphologic criteria (histology) and were transferred to the autoradiographic images to obtain mean values. The facilitated component of ACPC transport (deltaK1ACPC) was calculated from the K1ACPC and K1Ga-DTPA data, and paired comparisons between tumor and contralateral brain were performed. ACPC flux, K1ACPC, across normal brain capillaries (22.6 +/- 8.1 microL/g/min) was >200-fold greater than that of Ga-DTPA (0.09 +/- 0.04 microL/g/min), and this difference was largely (approximately 90%) due to facilitated ACPC transport. Substantially higher K1ACPC values compared to corresponding K1DTPA values were also measured in C6 and RG2 gliomas. The deltaK1ACPC values for C6 glioma were more than twice that of contralateral brain cortex. K1ACPC and deltaK1ACPC values for RG2 gliomas was not significantly higher than that of contralateral cortex, although a approximately 2-fold difference in facilitated transport is obtained after normalization for differences in capillary surface area between RG2 tumors and contralateral cortex. K1ACPC, deltaK1ACPC, and K DTPA were directly related to tumor cell density, were higher in regions of "impending" necrosis, and the tumor/contralateral brain ACPC radio-activity ratios (0 to 10 minutes) were very similar to that obtained with 0 to 60 minutes experiments. These results indicate that facilitated transport of ACPC is upregulated across C6 and RG2 glioma capillaries, and that tumors can induce upregulation of amino acid transporter expression in their supporting vasculature. They also suggest that early imaging (e.g., 0 to 20 minutes) with radiolabeled amino acids in a clinical setting may be optimal for defining brain tumors.

Imaging experimental brain tumors with 1-aminocyclopentane carboxylic acid and alpha-aminoisobutyric acid: comparison to fluorodeoxyglucose and diethylenetriaminepentaacetic acid in morphologically defined tumor regions

The goal of this study was to evaluate the differences and define the advantages of imaging experimental brain tumors in rats with two nonmetabolized amino acids, 1-aminocyclopentane carboxylic (ACPC) acid and alpha-aminoisobutyric (AIB) acid compared with imaging with fluorodeoxyglucose (FDG) or the gallium-diethylenetriaminepentaacetic acid chelate (Ga-DTPA). 1-aminocyclopentane carboxylic acid, AIB, and FDG autoradiograms were obtained 60 minutes after intravenous injection to simulate positron emission tomography (PET) imaging, whereas the Ga-DTPA autoradiograms were obtained 5 or 10 minutes after injection to simulate gadolinium (Gd)-DTPA-enhanced magnetic resonance (MR) images. Three experimental tumors were studied (C6, RG2, and Walker 256) to provide a range of tumor types. Triple-label quantitative autoradiography was performed, and parametric images of the apparent distribution volume (Va, mL/g) for ACPC or AIB, relative glucose metabolism (R, micromol/100 g/min), vascular permeability to Ga-DTPA (K1, microL/min/g), and histology were obtained from the same tissue section. The four images were registered in an image array processor, and regions of interest in tumor and contralateral brain were defined on morphologic criteria (histology) and were transferred to the autoradiographic images. A comparative analysis of all measured values was performed. The location and morphologic characteristics of the tumor had an effect on the images and measurements of Va, R, and K1. Meningeal extensions of all three tumors consistently had the highest amino acid uptake (Va) and vascular permeability (K1) values, and subcortical portions of the tumors usually had the lowest values. Va and R (FDG) values generally were higher in tumor regions with high-cell density and lower in regions with low-cell density. Tumor areas identified as "impending" necrosis on morphologic criteria consistently had high R values, but little or no change in Va or K1. Tumor necrosis was seen consistently only in the larger Walker 256 tumors; low values of R and Va for AIB (less for ACPC) were measured in the necrotic-appearing regions, whereas K1 was not different from the mean tumor value. The highest correlations were observed between vascular permeability (K1 for Ga-DTPA) and Va for AIB in all three tumors; little or no correlation between vascular permeability and R was observed. The advantages of ACPC and AIB imaging were most convincingly demonstrated in C6 gliomas and in Walker 256 tumors. 1-aminocyclopentane was substantially better than FDG or Ga-DTPA for identifying tumor infiltration of adjacent brain tissue beyond the macroscopic border of the tumor; ACPC also may be useful for identifying low-grade tumors with an intact blood-brain barrier. Contrast-enhancing regions of the tumors were visualized more clearly with AIB than with FDG or Ga-DTPA; viable and necrotic-appearing tumor regions could be distinguished more readily with AIB than with FDG. [11C]-labeled ACPC and AIB are likely to have similar advantages for imaging human brain tumors with PET.

Radiation protection by alpha-methyl-homocysteine thiolactone in vitro

The radiation protective effect of thiol compounds is unequivocal and their use is only limited by their toxic effects. We used the principle of alpha alkylation, which renders amino acids unmetabolizable, to reduce the toxicity of homocysteine. This product, alpha-methyl-homocysteine thio-lactone, was tested for toxicity and radiation protective effect along with known protectors L-cysteine, cysteamine and WR 1065 in cell culture using V79-4 Chinese hamster lung cells. The three-day growth curve assays, useful to measure overall effects on cell growth, revealed lowest toxicity for alpha-methyl-homocysteine thiolactone (GL-2). Clonogenic survival tests, used to evaluate the retention of reproductive integrity, were carried out and revealed that GL-2 had no adverse effects in this test system. Radiation protection tests showed that GL-2 exhibited protective activity against radiation induced lethality above that seen with cysteine and cysteamine, but below WR 1065. However, GL-2 showed little or no negative effects toward the cell itself, in direct contrast to WR 1065. Our findings show a potentially important tool and principle to reduce toxicity of radiation protectors with analogous structures.

Alpha-methyl-proline restores normal levels of bone collagen Type I synthesis in ovariectomized rats

Collagen type I is the major protein of bone matrix and significantly reduced in osteoporosis. We tested the effect of alpha - methyl - proline on collagen synthesis in the model of the ovariectomized rat. Collagen synthesis was studied at the transcriptional level using Northern and dot blotting and at the protein level using hydroxyproline determination and a specific dye binding collagen assay. Alpha - methyl- proline treatment significantly increased collagen synthesis as compared to untreated ovariectomized and estradiol treated ovariectomized rats and restored collagen synthesis to levels of sham operated rats. Proline analogues were described to stimulate procollagen synthesis at the transcriptional level, however, if incorporated, lead to negative collagen production due to rapid intracellular degradation of the deficient collagen. Our synthesized analogue is not being incorporated, thus not interfering with collagen conformation and can therefore induce collagen production.

Growth hormone regulates amino acid transport in human and rat liver

Human growth hormone (GH) has been shown to improve nitrogen balance in surgical patients and to decrease urea production. This has been thought to be due primarily to an increase in protein synthesis in skeletal muscle. Little attention has focused on the liver as a possible site where GH may modulate amino acid uptake and thereby divert nitrogen away from urea-genesis. The authors hypothesized that GH regulates amino acid transport in hepatocytes at the plasma membrane level. They studied hepatic amino acid transport in 20 healthy surgical patients that received saline, low-dose GH (0.1 mg/kg/day), or high-dose (0.2 mg/kg/day) GH for 3 days before operation. At operation, a 5- to 10-g wedge biopsy of the liver was obtained, and hepatocyte plasma membrane vesicles were prepared by Percoll density gradient centrifugation. Vesicle transport of [3H]-MeAIB, a highly selective system A substrate, and [3H]-glutamine, a selective system N substrate, was measured, employing a rapid mixing/filtration technique. Hepatocyte plasma membrane vesicles were also prepared from 14 rats treated with saline or one of three different GH treatment regimens: (A) 12 hours after chronic GH treatment (6 mg/kg every 12 hours x 4 doses); (B) 4 hours after acute (1 dose) GH treatment; and (C) 4 hours after chronic GH treatment. In human liver vesicles, low-dose GH resulted in a 13% decrease in system A activity (p = not significant), whereas high-dose GH caused a marked 79% decrease (6.7 +/- 1.7 pmol/mg protein/10 seconds in control patients versus 1.4 +/- 0.7 in GH, p less than 0.05). System N was unaffected. Kinetic analysis of MeAIB transport by vesicles from high-dose GH patients showed the reduction in transport to be due to a 63% decrease in the Vmax (maximal transport velocity) with no alteration in the transport Km (carrier affinity). Vesicles from rats treated chronically with GH using a protocol similar to that used for human subjects exhibited decreased system A transport activity (10.4 +/- 0.4 pmol/mg pro/10 seconds in controls versus 7.5 +/- 0.2 in GH, p less than 0.05) secondary to a 59% reduction in the transport Vmax. Chronic growth hormone treatment decreases the activity of system A in both human and rat hepatocytes. This may be one mechanism by which GH diminishes hepatic urea-genesis and spares amino acids for peripheral protein synthesis.

Plasma free amino acid values in normal children and adolescents

We measured plasma free amino acids in 52 children (mean age 8 years) and 80 adolescents (mean age 16 years); conditions of diet and time of day were similar in the two groups. The protocols allowed us to compare their interindividual variation with values previously reported by us for adults. In children, the values for all but seven amino acids were normally distributed; in adolescents there were only six exceptions. Effects of age were apparent: values for only two amino acids were higher in children than adolescents. Values were significantly lower for ten amino acids in children v adolescents and for 11 amino acids in younger children (1 to 6 years) v older children (7 to 12 years). An effect of sex was apparent for five amino acids in adolescents; such differences were not apparent in children. All differences were quasicontinuous and occurred within the global distributions that define values for plasma amino acids in normal children and adolescents. Two artifacts (choice of anticoagulant and delay in deproteinization) affected values for taurine and cystine, respectively.

Insulin-induced formation of ruffling membranes of KB cells and its correlation with enhancement of amino acid transport

Insulin induced the formation of ruffling membranes in cultured KB cells (a cell strain derived from human epidermoid carcinoma) within 1-2 min after its addition. The ruffled regions were stained strongly with antibody to actin but not that to tubulin. Pretreatment of KB cells with agents disrupting microfilaments (cytochalasins), but not with those disrupting microtubules (colcemid, nocodazole, and colchicine) completely inhibited the formation of ruffling membranes. Pretreatment of KB cells with dibutyryl cyclic AMP, but not with dibutyryl cyclic GMP, also inhibited the formation of ruffling membranes. Addition of insulin enhanced Na+-dependent uptake of a system A amino acid (alpha-amino isobutyric acid; AIB) by the cells within 5 min after the addition, and decreased the cyclic AMP content of the cells. Treatments that inhibited insulin-induced formation of ruffling membranes of KB cells also inhibited insulin-induced enhancement of their AIB uptake. From these observations, the mechanism of insulin-induced formation of ruffling membranes and its close correlation with AIB transport are discussed.

Synthesis and quality assurance of [11C]alpha-aminoisobutyric acid (AIB), a potential radiotracer for imaging and amino acid transport studies in normal and malignant tissues

Carbon-11 labeled alpha-aminoisobutyric acid (AIB), a synthetic amino acid, was prepared by the modified Bucherer-Strecker amino acid synthesis from acetone, ammonium carbonate and [11C]KCN in the presence of carrier KCN. This method results in the labeling of AIB in the carboxyl group. The label is stable in this position because AIB is not a metabolized after cellular uptake. AIB is rapidly accumulated in viable cells including malignant cells. Since it is a non-metabolized amino acid, AIB offers the possibility of studying amino acid transport in vivo without interference by radiolabeled metabolic products. Radiochemical yields of [11C]AIB of 35-60% have been obtained in 70-80 min with radiopurities greater than 99%. Carrier added syntheses gave 15-25 mCi of [11C]AIB with specific activities of 0.3 Ci/mmol. Our quality control program which insures that [11C]AIB is suitable for imaging studies in patients with cancer includes HPLC analyses of product identity and purity, apyrogenecity and isotonicity assays, and a sensitive test for cyanide.

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.

Amino acid transport in isolated rat hepatocytes

Improvements in the collagenase perfusion techniques have made isolated rat hepatocytes a popular model in which to study hepatic function. Our knowledge of hepatic amino acid transport has been advanced as a result of this methodology. Translocation across the hepatocyte plasma membrane can, in some instances, represent the rate-limiting step in the overall metabolism of certain amino acids. Furthermore, regulation of amino acid uptake by hepatocytes appears to play a role in diabetes, and perhaps in malignant transformation. Comparisons between normal adult hepatocytes and several hepatoma cell lines show basic differences in amino acid transport. There are at least eight distinct systems in normal hepatocytes for transport of the hormones. Systems A and N exhibit enhanced uptake rates after the cells have been maintained in the absence of extracellular amino acids, a phenomenon termed adaptive control. Further studies using isolated hepatocytes will increase our basic understanding of membrane transport processes and their regulation.

The effects of dexamethasone on metabolic activity of hepatocytes in primary monolayer culture

The effects of dexamethasone on multiple metabolic functions of adult rat hepatocytes in monolayer culture were studied. Adult rat liver parenchymal cells were isolated by collagenase perfusion and cultured as a primary monolayer in HI/WO/BA, a serum free, completely defined, synthetic culture medium. Cells inoculated into the culture medium formed a monolayer within 24 hr. Electron microscopy showed that the cells in primary culture had a fine structure identical to liver parenchymal cells in vivo, including the observation of desmosomes and bile canaliculi in intercellular space. There was significant gluconeogenesis by the cells 24 hr postinoculation but it had decreased markedly by 48 hr. There was a marked induction of tyrosine aminotransferase (TAT) by dexamethasone, which was maintained for up to 72 hr postinoculation of cells. The transport of alpha-aminoisobutyric acid into the cells in monolayer culture was stimulated by dexamethasone and was dependent on the concentration of dexamethasone. Albumin synthesis and secretion by the cells was measured by a quantitative electroimmunoassay. Albumin production was shown to increase linearly over an incubation period of 24 to 48 hr postinoculation. Dexamethasone depressed the albumin synthesis. The effects of dexamethasone are slow, and at times require more than 6 hr to show variation from the control, indicating that dexamethasone is not a single controlling hormone. Possibly it functions in a cooperative and coordinating role in the regulation of cell metabolism.

Hormonal regulation of amino acid transport and cAMP production in monolayer cultures of rat hepatocytes

The effects of insulin, glucagon of Dexamethasone (DEX) and of glucagon with insulin or DEX were examined on the uptake of 2-amino [1-14C]isobutyric acid (AIB) and N-Methyl-2-amino [1-14C]isobutyric acid (NMe AIB) in monolayer cultures of rat hepatocytes. Insulin and glucagon stimulated the uptake of both the amino acids and DEX inhibited it, showing that all three of these hormones regulate the A system (the sodium-dependent system that permits the transport of NMe AIB) for amino acid transport in these cultures. Experiments investigating the transport of aminocyclopentane-1-carboxylic acid, 1- [carboxyl-14C] in the presence of excess AIB or in the absence of sodium showed that insulin had no effect on the activity of the L system (the sodium-independent system that prefers leucine). Experiments on the uptake of AIB in the presence of excess NMe AIB showed insulin had no effect on the transport activity of the ASC system (the sodium-dependent system that does not transport NMe AIB). Insulin concentrations ranging from 0.1 nM to 100 nM did not antagonize the stimulatory effect of optimum or suboptimum concentrations of glucagon on the uptake of either AIB or NMe AIB. Similarly, glucagon did not antagonize the stimulatory effect of optimum or suboptimum concentrations of insulin on the uptake of both the amino acids. The combined effect of insulin and glucagon was additive on the rate as well as the cumulative uptake of both AIB and NMe AIB. DEX alone inhibited the transport of both AIB and NMe AIB by about 25%, while glucagon caused a 2--3-fold increase; however, the addition of glucagon to cultures containing DEX caused a 7--8-fold increase in the uptake of both AIB and NMe AIB when compared to cultures containing DEX alone. The effect of insulin on the levels of cAMP was also investigated. Insulin had no effect on the cAMP levels in cultures treated or untreated with optimum or suboptimum concentrations of glucagon.

Food and protein intake, glucagon, and distribution of alpha-aminoisobutyrate in the rat

Distribution of alpha-aminoisobutyric acid (AIB) in the rat was modified by food, dietary protein, and glucagon. In rats last fed 24 h before AIB injection, AIB clearance from plasma and uptake into liver were greater in rats fed a high-protein diet (60% casein) than in rats fed the control diet (18% casein); AIB clearance from plasma and uptake into muscle were lowered by a low-protein diet (6% casein). Feeding rats lowered clearance of AIB from plasma in low- and high-protein groups. Distribution ratios (AIB concentration in tissue water/AIB in plasma) were low in all tissues but liver during the first 7 h after feeding high protein when compared to the control values; ratios were low in muscle, heart, and kidney after feeding low protein. Maximum ratios occurred at different times for different tissues; the time was delayed by the high-protein diet in all tissues but liver. Glucagon increased all ratios in rats fed the control or low-protein diets, with the smallest changes occurring in liver and muscle from low-protein rats.

The effect of endotoxin on plasma alpha-aminoisobutyric acid

The i.v. injection of bacterial endotoxin into dogs was found to cause a rapid increase in plasma levels of infused alpha-aminoisobutyric acid. The findings suggest that nonmetabolic factors (tissue uptake, fluid shifts) influence amino acid distribution during endotoxemia.

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.

The effect of fluoride on the cellular uptake and pool of amino acids

Fluoride decreased the initial uptake of 2-aminoisobutyric acid (AIB) in LS cells only in high (supralethal) concentrations and after several hours of exposure. The steady state level of AIB seemed somewhat more sensitive as it was halved after 23 hours exposure to 6 mM-NaF. The cellular levels of some natural amino acids were also reduced by fluoride. However, cells resistant to fluoride showed equally lowered levels of amino acids, indicating that the observed effect of fluoride on cellular amino acid levels may be of little importance.

Glucocorticoids and the plasma membrane

Glucocorticoids affect the composition and function of the plasma membrane in a variety of cell types. Cultured rat hepatoma (HTC) cells in tissue culture provide an excellent model system for analysis of such effects. In these cells, dexamethasone rapidly and dramatically inhibits the influx of amino acids sharing the A or alanine-preferring transport system. Inhibition is half-maximal within 2 h, and maximal after 6 h incubation with the hormone. The inhibition is rapidly reversed by insulin, and more slowly by removing the steroid. Microtubules and microfilaments are not apparently involved in this hormonal effect, but continuous protein synthesis is required for the glucocorticoid inhibition of transport. Dexamethasone also decreases the number of microvilli on the surface of HTC cells, increases their adhesiveness to a substratum, and dramatically decreases the production of plasminogen activator, but it does not affect the growth rate or plating efficiency of the cells. Variant cell lines stably resistant to dexamethasone inhibition of plasminogen activator production have been isolated using an agar-fibrin overlay technique to detect protease production by individual colonies of HTC cells. The hormonal resistance to inhibition of protease production is associated witha maintenance of inducibility of other glucocorticoid-regulated functions and therefore is not apparently secondary to abnormal or absent glucocorticoid receptor, but due to a lesion in a later step in hormone action specific for plasminogen activator. Combined genetic and biochemical analysis of such dexamethasone-resistant variants should facilitate study of the hormonal regulation of specific membrane phenotypes and of the role of proteases in this regulation.