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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 $6 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 C[O.sub.2] production from glucose and palmitate (decrease in glucose oxidation induced by addition of 5.5, 11, 22, and 44 mmol/1 amino acids: - 16 [+ or -] 3, -25 [+ or -] 7, -44 [+ or -] 4, -62 [+ or -] 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 [+ or -] 22, +21 [+ or -] 33, +13 [+ or -] 12%; all non-significant (NS)] or glycogen synthesis (+1 [+ or -] 6, -5 [+ or -] 6, -9 + 8, +6 [+ or -] 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 [+ or -] 4%; with rapamycin, -50 [+ or -] 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. mammalian target of rapamycin; ribosomal protein p70 $6 kinase; glucose transport; glucose oxidation; fatty acid oxidation; species difference; mitochondrion

Aims: To investigate the mechanism of action of 55P0251, a novel multiflorine‐derived substituted quinazolidine that augments insulin release and lowers blood glucose in rodents, but does not act via mechanisms addressed by any antidiabetic agent in clinical use. Materials and Methods: Using male mice, we determined the effects of 55P0251 on glucose tolerance, insulin secretion from isolated islets and blood oxygen saturation, including head‐to‐head comparison of 55P0251 to its inverted enantiomer 55P0250, as well as to other anti‐hyperglycaemic multiflorine derivatives discovered in our programme. Results: 55P0251 was clearly superior to its inverted enantiomer in the glucose tolerance test (area under the curve: 11.3 mg/kg 55P0251, 1.19 ± 0.04 min*mol/L vs 55P0250, 1.80 ± 0.04 min*mol/L; P <.0001). For insulin release in vitro, this superiority became visible only under concomitant adrenergic background stimulation (glucose‐stimulated insulin release, fmol*islet−1*30 min−1: without α2‐adrenoceptor agonist: 500 μmol/L 55P0251, 390 ± 34, vs 55P0250, 459 ± 40, nonsignificant; with α2‐adrenoceptor agonist: 250 μmol/L 55P0251, 138 ± 9, vs 55P0250, 21 ± 6; P <.0001). Since receptor binding assays suggested antagonism at α2A‐adrenoceptors as a potential mechanism of action, we measured oxygen saturation in capillary blood from the tail as a surrogate of vasoconstriction, which supported α2‐antagonistic action in vivo (90 mg/kg 55P0251, 83 ± 3%, vs 55P0250, 57 ± 3%; P <.0001). Lack of association between glucose‐lowering activities and α2A‐adrenoceptor binding affinity arising from comparison of multiflorine derivatives was attributed to differences in their pharmacokinetic properties. Conclusions: Our findings suggest that 55P0251 and related multiflorine derivatives are to be categorized as α2‐adrenoceptor antagonists with potential to lower blood glucose by blocking α2A‐adrenoceptors on pancreatic β cells. [ABSTRACT FROM AUTHOR]

Aims: To investigate the mechanism of action of 55P0251, a novel multiflorine-derived substituted quinazolidine that augments insulin release and lowers blood glucose in rodents, but does not act via mechanisms addressed by any antidiabetic agent in clinical use., Materials and Methods: Using male mice, we determined the effects of 55P0251 on glucose tolerance, insulin secretion from isolated islets and blood oxygen saturation, including head-to-head comparison of 55P0251 to its inverted enantiomer 55P0250, as well as to other anti-hyperglycaemic multiflorine derivatives discovered in our programme., Results: 55P0251 was clearly superior to its inverted enantiomer in the glucose tolerance test (area under the curve: 11.3 mg/kg 55P0251, 1.19 ± 0.04 min*mol/L vs 55P0250, 1.80 ± 0.04 min*mol/L; P < .0001). For insulin release in vitro, this superiority became visible only under concomitant adrenergic background stimulation (glucose-stimulated insulin release, fmol*islet -1 *30 min -1 : without α 2 -adrenoceptor agonist: 500 μmol/L 55P0251, 390 ± 34, vs 55P0250, 459 ± 40, nonsignificant; with α 2 -adrenoceptor agonist: 250 μmol/L 55P0251, 138 ± 9, vs 55P0250, 21 ± 6; P < .0001). Since receptor binding assays suggested antagonism at α 2A -adrenoceptors as a potential mechanism of action, we measured oxygen saturation in capillary blood from the tail as a surrogate of vasoconstriction, which supported α 2 -antagonistic action in vivo (90 mg/kg 55P0251, 83 ± 3%, vs 55P0250, 57 ± 3%; P < .0001). Lack of association between glucose-lowering activities and α 2A -adrenoceptor binding affinity arising from comparison of multiflorine derivatives was attributed to differences in their pharmacokinetic properties., Conclusions: Our findings suggest that 55P0251 and related multiflorine derivatives are to be categorized as α 2 -adrenoceptor antagonists with potential to lower blood glucose by blocking α 2A -adrenoceptors on pancreatic β cells., (© 2019 The Authors. Diabetes, Obesity and Metabolism published by John Wiley & Sons Ltd.)

The pharmacology of thiazolidinediones (TZDs) seems to be driven not only by activation of peroxisome proliferator-activated receptor-γ (PPARγ), but also by PPARγ-independent effects on mitochondrial function and cellular fuel handling. This study portrayed such actions of the novel hydrophilic TZD compound BLX-1002 and compared them to those of conventional TZDs. Mitochondrial function and fuel handling were examined in disrupted rat muscle mitochondria, intact rat liver mitochondria, and specimens of rat skeletal muscle. BLX-1002 was superior to most other TZDs as an inhibitor of respiratory complex 1 in disrupted mitochondria, but had less effect than any other TZD on oxygen consumption by intact mitochondria and on fuel metabolism by intact tissue. The latter finding was obviously related to the hydrophilic properties of BLX-1002, because high potentials of individual TZDs to shift muscle fuel metabolism from the aerobic into the anaerobic pathway were associated with high ClogP values indicative of high lipophilicity and low hydrophilicity (e.g., % increase in lactate release induced by 10 μmol/l of respective compound: BLX-1002, ClogP 0.39, +10 ± 8%, not significant; pioglitazone, ClogP 3.53, +68 ± 12%, P < 0.001; troglitazone, ClogP 5.58, +157 ± 14%, P < 0.001). The observed specific properties of BLX-1002 could result from relatively strong direct affinity to an unknown mitochondrial target, but limited access to this target. Results suggest 1) that impairment of mitochondrial function and increased anaerobic fuel metabolism are unlikely to account for PPARγ-independent glucose lowering by BLX-1002, and 2) that higher lipophilicity of an individual TZD is associated with stronger acceleration of anaerobic glycolysis.