Aortic glucose metabolism in the diabetic Chinese hamster

Effect of in vivo exposure to insulin on glucose metabolism in rat aorta

The effect of administration of insulin in vivo on accumulation of [14C]glucose carbon in rat aorta in vitro was studied. Insulin was injected intravenously in a tail vein 5-60 min before the rats were killed and the accumulation of [14C]glucose was determined after incubation for 30-120 min in 5.6 mM [14C]glucose. When determined 30 min after injection of insulin (4 U kg-1) the aortic [14C]glucose incorporation was significantly increased when an incubation period of 120 min was used, while no significant effect was found after incubation for 30 or 60 min. In subsequent experiments an incubation time of 120 min was used. The aortic [14C]glucose accumulation was not increased when determined 5 or 60 min after injection of insulin (4 U kg-1). Injection of insulin (2 U kg-1) 5 or 30 min before the rats were killed had no effect on the aortic [14C]glucose accumulation but it had a pronounced effect on [14C]glucose accumulation in rat diaphragm. Serum insulin determined 30 min after injection of insulin (2 U kg-1) was 731 +/- 58 mU-1 and in saline-treated control rats 37 +/- 3 mU l-1. These results suggest that the glucose metabolism of vascular smooth muscle has a low sensitivity to the immediate effects of insulin.

Effect of fasting and refeeding on glucose metabolism in rat aorta

The effect of refeeding on accumulation of [14C]glucose carbon, oxidation of [14C]glucose and glycogen concentration in rat aortic intima-media was studied in rats fasted for 3 days. Accumulation of glucose carbon and glucose oxidation were determined by incubating rat aorta in vitro for 2 h with 5.6 mM 14C-labelled glucose in the medium. Refeeding with standard pellets for 2-4 h augmented [14C]glucose accumulation in rat aorta but had no significant effect on glucose oxidation. The glycogen concentration in rat aorta tended to increase. After refeeding for 16 h both [14C]glucose incorporation and [14C]glucose oxidation were increased in rat aorta. Refeeding with carbohydrate-rich pellets (92% carbohydrate) for 3 h increased blood glucose more than did protein-rich (92% protein) pellets, whereas the rise in plasma insulin was about the same. The accumulation of [14C]glucose carbon measured during the subsequent in vitro incubation for 2 h was augmented after refeeding with protein-rich pellets and slightly reduced after carbohydrate-rich pellets. Refeeding of diabetic rats for 2 h with standard pellets increased plasma insulin and markedly increased blood glucose but had no effect on aortic [14C]glucose incorporation. Intravenous infusion of glucose in normal rats for 2 h markedly raised blood glucose but did not increase the aortic glucose incorporation. Raising the ambient glucose concentration from 5 to 10 mM during incubation of normal rat aorta in vitro for 2 h slightly decreased the [14C]glucose incorporation determined during a subsequent incubation for 2 h. These results suggest that refeeding with standard pellets augments glucose incorporation and glucose oxidation in rat aorta.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of elevated substrates on substrate oxidation in normal and diabetic aorta

The influence of elevated substrates (glucose, beta-hydroxybutyrate and palmitate) on the gated. The oxidation of 14C-glucose to 14CO2 was depressed in diabetic aorta at a glucose concentration of 5.6 mM in the incubation medium. The glucose oxidation in normal and mM. The oxidation of 14C-beta-hydroxybutyrate (0.5 and 1.5 mM) to 14CO2 was impaired in diabetic aorta, while the oxidation of 14C-palmitate (0.5 and 1.0 mM) to 14CO2 was unaltered. The oxidation of both substrates was concentration dependent. The glucose oxidation in normal rat aorta was reduced by addition of 3.0 and 6.0 mM of beta-hydroxybutyrate to the incubation medium, while 1.5 mM palmitate had no effect on the glucose oxidation. After incubation of normal rat aorta in MEM with glucose (20.0 mM) for 46 h the subsequently determined glucose oxidation was not influenced in comparison to the aorta incubated in 5.6 mM glucose. Incubation with 6.0 mM beta-hydroxybutyrate or 1.5 mM palmitate did not affect the aortic glucose oxidation. In rats which were infused for 24 h with glucose or beta-hydroxybutyrate to obtain diabetic levels of these substrates, there was no change in the aortic glucose oxidation compared to saline-infused controls. The present results suggests that the lowered glucose oxidation in diabetic vascular tissue is not caused by hyperglycemia or high concentrations of ketone bodies or free fatty acids in the diabetic state.

Effect of fasting on the metabolism in rat aorta

The oxidation of 14C-labelled glucose, beta-hydroxybutyrate and palmitate to CO2 and the incorporation of 14C-labelled amino acid into alkali-soluble protein were studied in aorta from fed and fasted male Sprague-Dawley rats, weighting about 200 g. Substrate oxidation and amino acid incorporation were measured during incubation of rat aorta in vitro for 2-3 h. After fasting for 6 h there was a slight but significant increase in the plasma concentration of beta-hydroxybutyrate. Blood glucose was lowered after 12 h while an increase in the plasma concentration of free fatty acids was found after 24 h. A decrease in the oxidation of glucose in rat aorta was found after fasting for 12 h and with prolonged fasting a further decrease in the aortic glucose oxidation was found. After fasting for 4 days the oxidation of beta-hydroxybutyrate and the incorporation of 14C-leucine and 14C-phenylalanine into protein were reduced in rat aorta while the oxidation of palmitate was not altered. The effects of fasting on substrate oxidation and amino acid incorporation in rat aorta, found in this study are similar to the known effects of diabetes on vascular metabolism.

Response of the aorta of the obese Zucker rat to injury

The response of the thoracic aorta to balloon-catheter-induced endothelial denudation was studied in two animal models of diabetes: the obese Zucker rat and the streptozotocin-treated Wistar rat. The obese Zucker rat, an animal with a metabolic profile similar to that of noninsulin-dependent human diabetics, was characterized by excessive body weight, hyperinsulinemia, hyperlipidemia, and a mild increase in plasma glucose levels. In Zucker rats sacrificed 3 weeks after endothelial denudation, the cross-sectional areas of the fibrocellular intimal lesions were found to be approximately twice as large as those of the lean control. In all other respects, the morphology of the lesions in the obese rats, as assessed by both light and transmission electron microscopy, was similar to that of the lean control rats. In streptozotocin-treated Wistar rats, neither the cross-sectional areas nor the morphology of the intimal lesions differed from those in control rats. These results indicate that, in the obese Zucker rat, the response to aortic intimal injury is altered.

Influence of exogenous substrates on adenosinetriphosphate (ATP) concentration in normal and diabetic rat aorta in vitro

In diabetic rat aorta glucose metabolism is impaired and changes in the activities of several enzymes are found. The aim of this study was to investigate whether the metabolic alterations in diabetic vascular tissue influence energy production. Aortas of normal and diabetic male Sprague-Dawley rats were incubated in vitro for up to 120 min with various substrates added to the incubation medium. Diabetes was induced by streptozotocin and the rats were used after a diabetes duration of two weeks. When normal and diabetic aorta were incubated in the presence of 5.6 mmol/l glucose no significant difference in ATP-concentration was found after 60 min while after 120 min the ATP-concentration was lowered in diabetic aorta. Addition to the incubation medium of a substrate mixture containing amino acids in the same concentrations as in rat plasma, 3 mmol/l DL-beta-hydroxybutyrate and 1.5 mmol/l palmitate increased the ATP-concentration, measured after 120 min, in diabetic aorta but not in normal aorta. No significant difference in ATP-concentration was found between normal and diabetic aorta incubated in a medium containing all substrates. When diabetic aorta was incubated with each substrate separately beta-hydroxybutyrate but not glucose, palmitate or amino acids, increased the ATP-concentration to about the same level as the complete substrate mixture. The results suggest that the ability to utilize glucose for ATP production is impaired in diabetic vascular tissue and that other substrates such as ketone bodies are of importance for energy production in diabetic vessels.

Effects of in vitro and in vivo exposure to insulin upon glucose carbon accumulation in rat aorta: different patterns of response for intima-media and adventitia

We have studied the effect of insulin upon the accumulation of glucose carbon into the intima-media and the adventitia of rat aorta following in vitro exposure by adding insulin to the incubation medium, and in vivo exposure by producing relative hyperinsulinemia during life and subsequently incubating excised tissue in a medium containing no insulin. Hyperinsulinemia in vivo was either (1) endogenous--following 1.5-2 hr of refeeding after a 48-hr fast--or (2) exogenous--following an i.v. injection of insulin. For adventitia, a significant stimulation was found after both in vitro (+ 53%, p < 0.001) and in vivo exposure to insulin (+ 96%, p < 0.01 with endogenous insulin; + 75%, p < 0.05 with a dosage of 2 U/kg of exogenous insulin). For intima-media, the stimulation was weak and insignificant after in vitro exposure (+ 11%, p > 0.30), but became important and significant after in vivo exposure to insulin (+ 100%, p < 0.001 with endogenous insulin; + 50%, p < 0.05 with a dosage of 0.5 U/kg and + 49%, p < 0.05 with a dosage of 2 U/kg of exogenous insulin). For the in vivo exposure experiments, we found a significant linear correlation (r = 0.611, p < 0.005) between plasma insulin concentrations and glucose carbon accumulations into intima-medias of control and refed rats. These results establish the insulin sensitivity of the adventitia and show a sensitivity of intima-media only to in vivo insulin exposure. These different behaviors of intima-media in vivo and in vitro may have a hemodynamic basis.

Time course of the effect of insulin on bovine mesenteric artery

The time course for the effect of insulin on the bovine mesenteric artery, a medium sized artery with a high content of smooth muscle, was studied. The arteries were preincubated for varying times (0--180 min.) with and without insulin. The magnitude of the insulin effect, subsequently assayed on the accumulation of glucose carbon and the distribution of 3-0-methylglucose, was found to increase with the time of prior exposure to insulin, approaching a maximum after about 120 min. Insulin exposure also augmented its effect on the incorporation of leucine-14C into protein in a similar way. Preincubation without insulin had no appreciable influence on the magnitude of the insulin effect, indicating that the sensitivity to the hormone did not change with time. The threshold concentration for the effect of insulin on glucose-14C accumulation was 0.01 U/ml without preincubation and 0.001 U/ml after prior exposure to the hormone for 180 min. An action on protein synthesis may be involved in the increase in the magnitude of the insulin effect caused by prior exposure to insulin since no increase was found after inhibition of protein synthesis by puromycin.

Influence of diabetes on metabolism of vascular smooth muscle

Glucose utilization and the conversion of glucose to lactate, CO2, glycogen and lipids are decreased in the aorta from diabetic rats and rabbits. In addition the incorporation of amino acid into protein is reduced in diabetic rat aorta. The metabolic changes produced by diabetes are counteracted by insulin treatment, but there is a time lag of about 2 days before the effect of insulin treatment appears. The membrane transport of glucose in smooth muscle is carried out by a specific transport system of the facilitated diffusion type. A rate limiting influence of membrane transport on glucose metabolism is found in bovine mesenteric arteries and rabbit colon smooth muscle. In these preparations the influence of glucose concentrations on glucose metabolism is most pronounced in the range 0-11.1 mmol exhibiting saturation at higher glucose concentrations. Insulin in a high concentration (0.1 U/ml) has acute (less than or equal to 3 h) metabolic effects in vitro on smooth muscle which are qualitatively similar to those in skeletal muscle, but are weaker and appear later. The threshold concentration for the acute metabolic effects of insulin on smooth muscle in vitro is 10-100 times above the physiological levels, indicating a low acute sensitivity to insulin.

The relationship of abnormal circulating insulin levels to atherosclerosis

The evidence linking insulin with atherosclerosis can be divided into two parts. First, there is evidence that a proportion of subjects who have atherosclerosis or who are at risk of developing atherosclerosis hav elevated circulating insulin levels. The high insulin levels may be associated with another metabolic abnormality such as obesity, hypertriglyceridaemia, uraemia or consumption of oral contraceptives, may be inappropriate to the blood sugar levels as in mild diabetes, or may be of exogenous origin as in insulin-treated diabetics. The tissues of these subjects are exposed to high concentrations of insulin, and it seems reasonable to suggest that elevated insulin levels may have pathological effects on these tissues. Secondly, there is increasing evidence that the arterial wall is an insulin sensitive tissue. Exposure of arterial tissue to insulin results in proliferation of smooth muscle cells, inhibition of lipolysis, and synthesis of cholesterol, phospholipid and triglyceride. Chronic exposure to high concentrations of insulin results in the development of lipid filled lesions similar to those of early atherosclerosis. Thus, insulin has the ability to promote changes in the artery which, in the long term, may progress to atherosclerosis. The two lines of evidence together suggest that high levels of circulating insulin may have a role in the development of atherosclerosis.