Single-Dose Dexamethasone Induces Whole-Body Insulin Resistance and Alters Both Cardiac Fatty Acid and Carbohydrate Metabolism
Article Figures & Tables
Figures
- FIG. 1.
Effects of DEX on whole-animal insulin resistance. Following injection of vehicle or DEX for 4 h, whole-animal insulin resistance was assessed using a euglycemic-hyperinsulinemic clamp. Insulin (HumulinR; 3 mU · min−1 · kg−1) and d-glucose (50%) were continuously delivered (by a cannula inserted into the left jugular vein) for 3 h, with the glucose infusion started 4 min after commencement of insulin infusion. At regular intervals, blood samples taken from the tail vein were analyzed for glucose using a glucometer. The glucose infusion rate (GIR) was adjusted accordingly to maintain euglycemia. BG, blood glucose; CON, control.
- FIG. 2.
Glycolysis, glucose oxidation, and glycogen in DEX-treated hearts. Four hours after control (CON) rats were treated with DEX, animals were killed and hearts collected. Isolated hearts were perfused in the working mode for 1 h (preload 11.5 mmHg, afterload 80 mmHg) and rates of glycolysis (A) and glucose oxidation (B) determined as described in research design and methods. Mean steady-state rates of glycolysis and glucose oxidation were determined from data obtained during the initial portion of the heart perfusion. C: Glycogen content in rat ventricular muscle. Cardiac glycogen was determined as glucose residues by a glucose kinase method after acid hydrolysis. Values are the means ± SE for five rats in each group. *P < 0.05 vs. control.
- FIG. 3.
PDK mRNA and protein expression in hearts from insulin-resistant rats. PDK2 (A) and PDK4 (B) gene expression were measured using RT-PCR, and total RNA was extracted from 100 mg heart tissue. Expression levels were represented as the ratio of signal intensity for PDK mRNA relative to β-actin mRNA. Western blot analysis for PDK4 (C) was carried out in isolated mitochondria. Results are the means ± SE of three to four animals in each group. *P < 0.05 vs. control. CON, control.
- FIG. 4.
Effects of DEX on luminal LPL and cardiac mRNA expression. Coronary luminal LPL activity was determined in vitro by heparin perfusion (over 10 min) of hearts isolated from animals treated with DEX (A). Hearts were perfused in the retrograde mode with heparin (5 units/ml) and fractions of perfusate collected and analyzed for LPL activity as described previously. The inset represents peak LPL activity at various intervals (1–3 h of a single representative experiment), whereas the graph shows LPL activity after 4 h of DEX (n = 6). Changes in LPL activity in response to heparin perfusion, over time, were analyzed by multivariate ANOVA followed by the Newman-Keul’s test using the Number Cruncher Statistical System. B: Representative photograph showing the effect of DEX (4 h) on LPL immunofluorescence, as visualized by fluorescent microscopy. Heart sections were fixed and then incubated with the polyclonal chicken antibody against bovine LPL, followed by incubations with biotinylated rabbit anti-chicken IgG and streptavidin-conjugated Cy3 fluorescent probe. C: LPL mRNA gene expression as measured using RT-PCR. Results are the means ± SE of three rats in each group. *P < 0.05 vs. control. □, control (CON); ▪, DEX.
- FIG. 5.
Differential effects on coronary LPL in isolated hearts from STZ and DEX animals that are perfused in vitro for 1h. On immediate removal of hearts from STZ (4 days) and DEX (4 h) rats, peak LPL activity was determined after perfusion with heparin (0 min). In a separate experiment, hearts from different groups were first perfused for 60 min with Kreb’s buffer. During the 60-min perfusion, basal LPL activity (in the absence of heparin) was determined in the buffer reservoir over time (inset). Subsequently, LPL was displaced by heparin and activity determined. Results are the mean ± SE of four rats in each group. *P < 0.05 vs. control; #P < 0.05 vs. all other groups. CON, control.
- FIG. 6.
Responses of exogenous insulin on cardiac LPL activity. STZ-induced diabetic rats were injected with a rapid-acting insulin (8 units; IN) 4 days after diabetes induction, the rats killed 180 min later, and heparin-releasable LPL activity determined in perfused hearts (inset). Heparin-releasable LPL activity was also determined before and after termination of the 3-h euglycemic-hyperinsulinemic clamp. Results are the means ± SE of four rats in each group. #P < 0.05 vs. all other groups. CON, control.
- FIG. 7.
Lipid homeostasis following DEX injection. Animals were treated with DEX (1 mg/kg i.p.) and blood samples from the tail vein collected at 60-min intervals for determination of triglyceride (TG) and fatty acid (FA) (A). Results are the means ± SE of four rats in each group. After 4 h, cardiac morphology was evaluated using a transmission electron microscope. B: Representative electron micrograph of hearts from control (CON) and DEX animals. The scale bar represents 500 nm. M, mitochondria; white arrows, lipid-like vacuoles.
Tables
- TABLE 1
General characteristics of the animals
Control DEX Plasma glucose (mmol/l) 8.7 ± 0.2 8.7 ± 0.1 Plasma insulin (ng/ml) 3.2 ± 0.5 3.3 ± 0.7 Heart weight (g) 1.4 ± 0.1 1.2 ± 0.001 Heart rate (bpm) 313 ± 10 312 ± 8 Rate pressure product (bpm × mmHg/1,000) 32 ± 1 34 ± 1 Mean arterial pressure (mmHg) 122 ± 6/93 ± 4 125 ± 6/93 ± 6 Data are means ± SE for six animals in each group. DEX (1 mg/kg) was administered by intraperitoneal injection into control rats and the animals killed 4 h later. Hearts were perfused in the working mode for 1 h at a preload of 11.5 mmHg and an afterload of 80 mmHg. Mean arterial pressure was measured by an in vivo cannula inserted into the carotid artery.
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