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Biomedical subjects

N Venkatesan

Publications and source records attributed to N Venkatesan.

At least 55 records · Page 3Linked to original sources
PubMed1990

Insulin-like growth factor I receptors in adult rat liver: characterization and in vivo regulation.

Although the presence of significant amounts of insulin-like growth factor (IGF)-I receptors in fetal tissues is well documented, adult liver has been reported to contain little or no IGF-I binding activity. In the present investigation, substantial amounts of specific IGF-I receptors were detected in crude membrane fractions and in partially purified receptor preparations of female adult rat liver. Insulin was 100 times less potent than IGF-I in competing for 125I-IGF-I binding. IGF-I binding activity was much less than that of insulin binding in both the microsomal fraction and partially purified receptor preparations. Affinity cross-linking of 125I-IGF-I to purified receptors and microsomal fractions followed by electrophoretic analysis under nonreducing conditions revealed labeling of proteins with relative molecular weight (Mr) of 350,000 and 210,000-220,000, corresponding to the molecular mass of the intact tetramer and alpha-beta dimers, respectively. Under reducing conditions, the labeling of proteins with Mr of 130,000 and 260,000, corresponding to the alpha-subunit of IGF-I receptor and its dimer, respectively, was observed. Treatment of microsomes as well as partially purified receptors with 0.5-1 mM dithiothreitol resulted in decreased IGF-I binding, and this correlated with structural changes in the receptor as detected by affinity labeling and electrophoretic analysis. Hepatic IGF-I binding activity was significantly diminished in female rats exposed to chronic growth hormone excess, suggesting down-regulation of IGF-I receptors by the enhanced circulating levels of IGF-I in these animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1988

Glucose transport is rate limiting for skeletal muscle glucose metabolism in normal and STZ-induced diabetic rats.

To test the hypothesis that glucose transport is rate limiting for skeletal muscle glucose metabolism, intracellular free glucose was measured in in situ abdominal wall muscle and liver that were rapidly freeze-clamped and removed from normal and streptozocin-induced diabetic (STZ-D) Sprague-Dawley rats exposed to different concentrations of serum insulin and glucose achieved by 90-min glucose clamps. Three groups of anesthetized normal (n = 17) and three groups of STZ-D (n = 19) animals were studied. In each group, infusions were adjusted to expose rats to basal, euglycemic-hyperinsulinemic, and hyperglycemic-hyperinsulinemic conditions. Final steady-state serum glucose and insulin concentrations (mean +/- SE) achieved in the six groups ranged from 112 +/- 2 to 467 +/- 26 mg/dl and from 5.8 +/- 1.9 to 3644 +/- 116 microU/ml, respectively. Intracellular free-glucose content of all tissue was calculated from total tissue glucose content minus the contribution of glucose in extracellular water estimated with [14C]inulin. Intracellular free glucose was not detected in muscle of any of the experimental groups. However, intracellular free glucose was detected in liver from all normal and STZ-D rats, and the calculated intracellular concentrations of free glucose correlated with serum glucose concentrations (r = .68, P less than .001). We conclude that intracellular free glucose does not accumulate, regardless of glucose or insulin concentration, in normal skeletal muscle and muscle made insulin resistant (by STZ-D), suggesting that glucose transport remains rate limiting.

Animals↗
PubMed1987

In vivo insulin antagonism but evanescent in vitro tissue effect in rats with growth hormone-secreting tumors.

Rats bearing mammosomatotropic tumors have raised insulin but lowered glucose concentrations. To determine if growth hormone (GH) secreted by these tumors causes insulin antagonism, pancreatic suppression tests utilizing infusions (per kg per min) of glucose (8 mg), insulin (200 ng) and somatostatin (1.4 micrograms) for 130 min were performed. Although the steady state plasma glucose and insulin levels (mean of 90, 100, 110, 120 and 130 min samples) were similar in 8 control and 13 tumor-bearing rats, the decrease from the already depressed basal glucose concentration (mmoles/l +/- SE) in the tumor animals was less than in the controls (0.90 +/- 0.30 vs. 2.56 +/- 0.040, p less than 0.005). Since the interpretation of these results was not entirely clear, glucose and insulin-glucose tolerance tests were performed. The glucose disappearance rates (%/min +/- SE) in the glucose tolerance test were lower in 17 tumor rats (2.00 +/- 0.13) compared to 17 control animals (2.51 +/- 0.22). This difference just missed statistical significance (t = 2.00, value of 2.04 necessary for p = 0.05). The decrease occurred in the presence of increased insulin (nmoles/l X 16 min) levels (4.29 +/- 0.38 vs. 2.58 +/- 0.29, p less than 0.005) suggesting insulin antagonism. The glucose disappearance rates (%/min +/- SE) in the insulin-glucose tolerance test were less in 12 tumor-bearing rats compared to 11 control animals (2.80 +/- 0.29 vs. 4.12 +/- 0.35, p less than 0.02). Thus, these GH-secreting tumors cause insulin antagonism in vivo. Freshly isolated hepatocytes from these tumor-bearing animals manifest decreased insulin binding and action (Diabetologia 25:60, 1983). In the present study, however, insulin binding and action (net glucose-C14 incorporation into glycogen) were normal after the hepatocytes were cultured for two days. This suggests that the changes induced by GH in vivo that lead to insulin antagonism are short-lived.

Animals↗
PubMed1987

Possible role for the glucose-fatty acid cycle in dexamethasone-induced insulin antagonism in rats.

This study was undertaken to elucidate the mechanism(s) involved in glucocorticoid-induced insulin antagonism. Male Sprague-Dawley rats (200 to 210 g) were injected with 1 mg/kg dexamethasone-phosphate (Dex) or the vehicle every other day for 10 days. Two days after the last injection, fasted anesthetized animals were infused (per kg body weight per min) with 8 mg glucose, 5 mU porcine insulin and 1.4 micrograms somatostatin with blood sampling before, and at 10 min intervals between 90 and 130 min after the pancreatic suppression test was begun. At the end of the test, abdominal muscle was quickly freeze-clamped and the substrate and products of the rate-determining reactions of glycolysis and glycogenesis were measured. Dex-treated rats had higher basal (0 min) and steady-state levels (90-130 min) of both glucose and insulin signifying insulin antagonism. The pattern of muscle tissue metabolites revealed no free intracellular glucose in either group and concentrations of all other metabolites in the Dex-treated rats were less than those in the control animals (except for a small increase in glycogen). These results suggest a site of insulin antagonism between (and including) insulin binding and glucose transport. Further studies in the Dex-treated rats revealed normal: a) insulin binding to freshly isolated hepatocytes; b) basal and insulin-stimulated xylose transport in soleus muscle; c) basal and insulin-stimulated glucose uptake in hemidiaphragms. These normal in-vitro results suggested that a circulating factor may be responsible. Repeat pancreatic suppression tests in the Dex-treated rats revealed blunted suppression of serum FFA concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Adipose Tissue↗
PubMed1985

Insulin secretion and action in the hyperthyroid rat.

To gain insight into the mechanism of the altered carbohydrate metabolism in thyrotoxicosis, intravenous glucose tolerance tests (IVGTT) and pancreatic suppression tests (PST) were performed in hyperthyroid rats (0.1 mg/kg T4 X 5 days) to assess insulin secretion and action in vivo. Thyroid hormone injections significantly increased T4 levels (182.8 nM +/- 11.6 (SEM) versus 50.2 +/- 6.4; P less than 0.001) and baseline glucose concentrations (9.3 mM +/- 0.2 versus 7.1 +/- 0.2; P less than 0.001). Body weights, basal insulin concentrations, glucose concentrations during IVGTT, glucose disappearance rates and steady state plasma glucose levels (SSPG) were normal. Insulin concentrations during the glucose tolerance test and during the PST were significantly decreased. The metabolic clearance rate of insulin (ml/min/kg +/- SEM) was significantly (P less than 0.01) increased (54.4 +/- 3.5 versus 41.6 +/- 2.3) in the hyperthyroid rats. If the different baseline glucose values were subtracted from the glucose concentrations achieved during the 2 tests, both the glucose disappearance rate and the fall in SSPG levels were significantly enhanced in the T4-injected animals. Thus, in the hyperthyroid rat, insulin secretion is decreased, the clearance of insulin is increased and insulin sensitivity is either normal or possibly enhanced.

Animals↗
PubMed1983

Hepatic insulin effects independent of changes in Ca++ metabolism.

The possibility of Ca++ acting as second messenger for insulin in rat liver was investigated using the net stimulation of 14C-glucose incorporation into glycogen by isolated hepatocytes as an index of insulin action. An insulin effect could be partially sustained in the virtual absence of Ca++ and Mg++ and a maximal insulin effect could be observed in the presence of either Ca++ or Mg++, suggesting that extracellular Ca++ is not required for insulin action. Inhibiting the activity of calmodulin, an intracellular mediator of Ca++ action, with trifluoperazine had little effect on insulin action. The efflux of 45Ca from prelabeled hepatocytes was not altered by the presence of insulin arguing against insulin-induced changes in Ca++ fluxes. Collectively, these results do not support the role of Ca++ as second messenger for insulin action in liver.

Animals↗
PubMed1982

Persistence of a curvilinear Scatchard plot for insulin binding despite correcting for degradation.

We evaluated whether correcting for insulin degradation products (both cell-associated and those recovered in the media) in the analysis of insulin binding to rat hepatocytes linearized the Scatchard plot. In the presence of bacitracin, the average amount of intact insulin in the cell pellet increased from 68% to 90% and in the media from 21% to 69% when assayed by G-50 Sephadex separation. However, using only the amount of intact insulin in Scatchard analysis still resulted in a curvilinear form whether bacitracin was present or not. Persistence of curvilinear Scatchard plots is probably due to the fact that binding of insulin to hepatocytes in our system is over twice as high as in the cells used in the two studies which reported linearization of the Scatchard plot when these corrections were made. Because of the relatively low binding in those studies, corrections for insulin degradative products resulted in an amount of bound insulin that fell on the initial linear portion of the typical curvilinear Scatchard plot. Bacitracin significantly inhibited insulin degradation and correcting for the amount of insulin degradation in Scatchard analysis does not yield a linear plot.

Animals↗
PubMed1982

Release of lymphotoxin by control and chemical carcinogen-treated lymphocyte cultures derived from black healthy subjects and cancer patients.

In an age-adjusted comparison with white men, black men have a significantly higher increase in esophageal and other types of cancer associated with environmental causes. The basis of this increase in cancer rates in blacks over the last two decades is unknown. Since cancer patients generally show an impairment in cell-mediated immune (CMI) functions, we measured certain CMI reactions in cultured lymphocytes derived from black healthy subjects and cancer patients. We also determined the levels of aryl hydrocarbon hydroxylase (AHH) induced in these lymphocytes. AHH catalyzes the activation of polycyclic aromatic hydrocarbons (PAH) to intermediates which might alter CMI functions.Lymphocytes from 33 black patients with squamous cell carcinoma and 22 healthy volunteers were mitogen-activated with phytohemagglutinin and concurrently treated with the environmental carcinogen, 3-methylcholanthrene (MCA). Measurements were made of the effects of MCA (0.5 to 3.0 μM on blastogenesis, T-cell growth, lymphotoxin (LT) release, and AHH induction in these lymphocyte cultures. MCA treatment depressed blastogenesis but had no depressive effect on T-cell growth in cultures. Blastogenesis and T-cell growth were mitogen dose-dependent, while LT release was independent of mitogen concentration. There was a significantly lower LT release by lymphocytes from lung cancer patients, compared to those from healthy and head/neck cancer subjects. The reduced levels of LT release in lung cancer patients might reflect an impairment in this CMI function. Studies on the role of lymphocyte subpopulations in CMI functions are in progress.

Adult↗
PubMed1978

Cyclophosphamide-induced oncogenic transformation, chromosomal breakage, and sister chromatid exchange following microsomal activation.

Cyclophosphamide, an extensively used cancer chemotherapeutic agent, requires metabolic activation through a mixed-function oxygenase system. The capacity of this agent to produce oncogenic transformation and chromosomal damage, including increases in sister chromatid exchanges, was investigated in cell culture with or without an exogenous liver metabolic activation system. No oncogenic transformation or chromosomal aberrations were produced by cyclophosphamide in the absence of metabolic activation, whereas significant transformation, chromosomal breaks, and increases in sister chromatid exchanges were observed when the activation system was incorporated into the assays. The oncogenic transformation and chromosomal changes were completely eliminated by removing glucose 6-phosphate and nicotinamide adenine dinucleotide phosphate from the metabolic generating system. These studies emphasize the necessity to incorporate some activation procedure into short-term assays used for evaluating the mutagenic and/or oncogenic potential of various chemicals.

Animals↗
PubMed1977

Regulation of DNA chain elongation in SV3T3 cells in relation to growth rate.

Regulation of DNA synthesis was investigated in SV40 transformed 3T3 cells exhibiting variable growth rates and residence times in S phase when cultured in the presence of different serum concentrations. Pulse-labeled DNA was chased into large molecular weight material in vivo much more slowly in slowly growing cells than in cells growing at the normal rate. Consistent with this, the joining of short (less than 10 S) chains to form long (greater than 10 S) chains by whole cell lysate system in vitro was greatly impaired in slowly growing cells compared to controls. Thus the lengthening of S phase in SV3T3 cells growing slowly in low serum is reflected in a reduced rate of DNA chain elongation. The presence of cycloheximide during chase in vivo reduced the rate of conversion of pulse-labeled molecules into large molecular weight DNA in both slowly growing and normally growing cells.

Cell Cycle↗
PubMed1976

Dimethylnitrosamine-demethylase: absence of increased enzyme catabolism and multiplicity of effector sites in repression. Hemoprotein involvement.

Evidence is presented that the previously observed decrease of the Vmax of hepatic microsomal demethylation of dimethylnitrosamine (DMN), following pretreatment of rats with 3-methylcholanthrene (MC), is not due to increase in the rate of breakdown but to decrease of de novo synthesis. Determinations of Vmax at time intervals in the transition from the high steady-state level induced by a carbohydrate-devoid casein diet, down to the low steady-state level of carbohydrate-containing basal diet, yielded two consecutive slopes; descent from the basal diet level to the lower steady-state level following pretreatment with MC yielded one slope. Plotting these slopes against the initial Vmax values gave a typical exponential curve (or straight line if the logs of slopes are used) indicating that the rate of enzyme decay in the MC-treated animals is not greater than that expected from normal enzyme catabolism. A multiplicity of effector sites appears to be involved in the repressor action of different structural types; for example, repression by MC (46.6%) and by phenobarbital (23.9%) in combination are approximately additive (62.0%), rather than competitive, indicating that the two agents act at different sites. A P-450 type cytochrome is involved in the demethylation of DMN. DMN-demethylase is inhibited by carbon monoxide, but the susceptibility to CO is far greater than that observed previously with 3,4-benzopyrene hydroxylation; inhibition of DMN-demethylase as a function of CO concentration follows typical enzyme kinetics. However, while both phenobarbital and MC powerfully repress the DMN-demethylase, we have confirmed that they are strong inducers of the synthesis of P-450 and P-448, respectively, as estimated from the difference spectra.

Animals↗
PubMed1975

Effects of phenobarbital on protein synthesis and polysome levels in rat liver.

Administration of phenobarbital to rats increases the rate of synthesis of certain microsomal drug-metabolizing enzymes in a selective manner and promotes proliferation of smooth endoplasmic reticulum in the liver. Phenobarbital increased a number of factors by which protein synthesis could be enhanced in the liver. It produced a 30% increase in the amount of ribosomes and mRNA per cell. The proportion of ribosomes associated with polysomes was increased by 5-10% over normal liver. There was a 10-30% increase in the rate of ploypeptide elongation and a small increase or no change in polysome size, indicating that the rate of polypeptide initiation was increased proportionately. The product of these effects accounts for the 1.5-fold increase in the rate of total protein synthesis previously reported. The average polysome size, and the size of free polysomes in particular, was maintained when actinomycin D was administered to phenobarbital-pretreated rats, suggesting that the rate of mRNA degradation was decreased selectively. Phenobarbital did not, however, affect the distribution of ribosomes between the free and membrane-bound states or the activity of ribonucleases associated with isolated free and bound polysomes. Thus, we conclude that phenobarbital stimulates protein synthesis by expanding the mRNA pool, at least partially through effects on mRNA degradation, and by augmenting the rate of mRNA translation.

Animals↗