Aims/hypothesis GW501516, an agonist of peroxisome proliferator-activated receptor-δ (PPAR-δ), increases lipid combustion and exerts antidiabetic action in animals, effects which are attributed mainly to direct effects on skeletal muscle. We explored such actions further in isolated rat skeletal muscle. Materials and methods Specimens of rat skeletal muscle were pretreated with GW501516 (0.01-30 μmol/l) for 0.5, 4 or 24 h and rates of fuel metabolism were then measured. In addition, effects on mitochondrial function were determined in isolated rat liver mitochondria.

In order to investigate the effects of exposure to possible environmental pollutants such as Cd, Pb and Hg on haematological and serum biochemistry values, New Zealand White female rabbits were treated orally with distilled water solutions of CdSO 4 ·H 2 O, Pb(NO 3 ) 2 and HgCl 2 (n = 4/treatment) in concentrations of 2.3, 4.1, and 30 mg/kg dry matter, respectively, for 28 days. The initial concentrations of Cd, Pb, and Hg in serum were significantly increased by the treatment. Exposure to Pb significantly decreased the red blood cell (RBC) count, haemoglobin (Hgb) concentration and the haematocrit (Hct) value. The Znprotoporphyrin concentration did not change as a result of Pb exposure. Pb and Hg loading significantly increased the aspartate aminotransferase (AST) activity. Alanine aminotransferase (ALT) activity was also increased by both Hg and Cd exposure. Comparing the treated and the control rabbits, all the trace elements studied significantly reduced the activity of enzymes in the pancreatic tissues. The haematological results indicate that hyperchromic macrocytic anaemia developed in rabbits treated with Pb. The increased activities of both AST and ALT indicate pathophysiological changes of the liver parenchyma, which was verified by focal fatty infiltration seen histopathologically. Cd exposure could exert a toxic effect on the kidneys, although the slight tubulonephrosis developed would not possibly affect the renal function. The reduced activities of amylase, trypsin, protease and lipase induced by Cd, Pb and Hg suggest toxicity to the pancreas.

Aims/hypothesis The aim of the study was to gain better insight into the mechanisms responsible for impaired glucose metabolism during late pregnancy. We explored the direct effects of progesterone on glucose metabolism of skeletal muscle. Methods Specimens of skeletal muscle from untreated rats were incubated with progesterone and rates of substrate fluxes through the various pathways of glucose metabolism were analysed. Results Progesterone dose-dependently reduced the rates of glucose and pyruvate oxidation (insulin-stimulated rates after 5 h of exposure to 1 and 10 μmol/l progesterone: glucose oxidation, −6±4%, NS, and −39±4%, p<0.001; pyruvate oxidation, −28±2% and −55±4%, p<0.001 each) and increased lactate release (+28±4% and +58±9%, p<0.005 each), which indicated inhibition of mitochondrial respiratory function. Impairment of cell respiration, e.g. by the specific inhibitor rotenone, is known to trigger a compensatory increase in glucose transport, but this response was blunted in the case of progesterone (change of glucose transport in response to 10 μmol/l progesterone vs 60 nmol/l rotenone, both causing a reduction in glucose oxidation by −39%: progesterone, +14±8% vs rotenone, +84±23%, p< 0.03). Further experiments dealt with the underlying mechanisms and revealed a rapid mode of action (50 μmol/l progesterone, reduction in insulin-stimulated glucose oxidation after 30 min: −29±7%, p<0.01) not affected by blockers of gene expression or the nuclear progesterone receptor. Conclusions/interpretation Progesterone inhibits cell respiration and at the same time suppresses a compensatory increase in glucose transport, causing cellular carbohydrate deficiency in isolated rat skeletal muscle. This effect is mediated by a direct, rapid and non-genomic mechanism and could contribute to pregnancy-associated changes in glucose homeostasis.

Zucker diabetic fatty (ZDF) rats are a standard animal model for the study of type 2 diabetes and for pharmacological characterization of insulin-sensitizing drugs. To analyze the agedependent development of their metabolic derangements and the associated changes in their responses to treatment with the insulin sensitizer pioglitazone, groups of 7, 10.5, or 15.5-weekold ZDF rats were treated orally with vehicle or pioglitazone (12 mg/kg/day). Metabolic parameters including circulating concentrations of glucose, insulin, lipids, and adiponectin as well as body weight, tissue glycogen content, and the activity of p70S6 kinase in skeletal muscle were determined. Blood glucose of ZDF rats rose steeply from 5.9 Ϯ 0.4 to 23.7 Ϯ 0.5 mM between 7 and 13 weeks of age and then reached a new steady state, which was associated with increased tissue glycogen content (in 15-week-old ZDF rats versus lean littermates: skeletal muscle, 18.0 Ϯ 0.9 versus 10.5 Ϯ 1.4 mol/g; liver, 181 Ϯ 6 versus 109 Ϯ 14 mol/g; both p Ͻ 0.001). Early intervention with pioglitazone at 7 weeks of age fully prevented the development of hyperglycemia (blood glucose, 6.4 Ϯ 0.4 versus 18.7 Ϯ 1.5 mM after 5.5 weeks of treatment), which was accompanied by a 40% (p ϭ 0.01) reduction of the activity of p70S6 kinase in skeletal muscles. These beneficial effects of pioglitazone were progressively lost, if treatment was initiated at later stages of disease development. Thus, ZDF rats are suitable for preclinical characterization of insulin-sensitizing thiazolidinediones in many aspects, but several important differences versus human type 2 diabetes exist and are to be considered in the use of this animal model.

Two mechanisms have been proposed for the modulation of skeletal muscle glucose metabolism by amino acids. Whereas studies on humans and cultured cells suggested acute insulin desensitization via mammalian target of rapamycin (mTOR) and its downstream target p70 S6 kinase (S6K), investigations using native specimens of rat muscle hinted at impairment of glucose oxidation by competition for mitochondrial oxidation. To better understand these seemingly contradictory findings, we explored the effects of high concentrations of mixed amino acids on fuel metabolism and S6K activity in freshly isolated specimens of rat skeletal muscle. In this setting, increasing concentrations of amino acids dose-dependently reduced the insulin-stimulated rates of CO(2) production from glucose and palmitate (decrease in glucose oxidation induced by addition of 5.5, 11, 22, and 44 mmol/l amino acids:--16 +/- 3, -25 +/- 7, -44 +/- 4, -62 +/- 4%; P < 0.02 each). This effect could not be attributed to insulin desensitization, because it was not accompanied by any reduction of insulin-stimulated glucose transport [+12 +/- 16, +17 +/- 22, +21 +/- 33, +13 +/- 12%; all nonsignificant (NS)] or glycogen synthesis (+1 +/- 6, -5 +/- 6, -9 +/- 8, +6 +/- 5%; all NS) and because it persisted without insulin stimulation. Abrogation of S6K activity by the mTOR blocker rapamycin failed to counteract amino acid-induced inhibition of glucose and palmitate oxidation, which therefore was obviously independent of mTOR/S6K signaling (decrease in glucose oxidation by addition of 44 mmol/l amino acids: without rapamycin, -60 +/- 4%; with rapamycin, -50 +/- 13%; NS). We conclude that amino acids can directly affect muscle glucose metabolism via two mechanisms, mTOR/S6K-mediated insulin desensitization and mitochondrial substrate competition, with the latter predominating in isolated rat muscle.