Hepatic responsiveness to gluconeogenic substrates during insulininduced hypoglycemia was investigated. For this purpose, livers were perfused with a saturating concentration of 2 mM glycerol, 5 mM Lalanine or 5 mM L-glutamine as gluconeogenic substrates. All experiments were performed 1 h after an ip injection of saline (CN group) or 1 IU/kg of insulin (IN group). The IN group showed higher (P<0.05) hepatic glucose production from glycerol, L-alanine and L-glutamine and higher (P<0.05) production of L-lactate, pyruvate and urea from L-alanine and L-glutamine. In addition, ip injection of 100 mg/kg glycerol, L-alanine and L-glutamine promoted glucose recovery. The results indicate that the hepatic capacity to produce glucose from gluconeogenic precursors was increased during insulin-induced hypoglycemia.

The time-course changes of the responsiveness of glycogen breakdown to a-and ß-adrenergic agonists during insulin-induced hypoglycemia (IIH) were investigated. Blood glucose levels were decreased prior to the alteration in the hepatic responsiveness to adrenergic agonists. The activation of hepatic glucose production and glycogenolysis by phenylephrine (2 µM) and isoproterenol (20 µM) was decreased in IIH. The changes in the responsiveness of glycogen catabolism were first observed for isoproterenol and later for phenylephrine. Hepatic ß-adrenergic receptors showed a higher degree of adrenergic desensitization than did a-receptors. Liver glycogen synthase activity, glycogen content and the catabolic effect of dibutyryl cyclic AMP (the ß-receptor second messenger) were not affected by IIH.

The effect of glucagon and isoproterenol (ß-adrenergic agonist) on hepatic glycogenolysis and glycolysis in isolated perfused liver was compared. The levels of isoproterenol and glucagon which promoted the maximal activation of glycogenolysis were 20 µM and 1nM respectively. However, glucagon (1 nM) not only increased glycogenolysis but also inhibited glycolysis. Because adenosine-3'-5'-cyclic monophosphate (cAMP) is a common second messenger to glucagon and isoproterenol, the level of cAMP that simulates the effect of these substances were investigated. The concentration of cAMP that inhibited glycolysis was five times higher (15 ìM) than that which stimulated glycogenolysis (3 ìM). Similar inhibition of glycolysis was obtained with cAMP agonists resistant to phosphodiesterases, i.e., 8-Br-cAMP and N 6 -monobutyryl-cAMP (6-MB-cAMP) at the concentration of 3 ìM. Thus, apparently glucagon could produce higher cellular levels of cAMP than that obtained with the activation of ß-adrenergic receptors. The higher amount of cAMP could be enough to overcome the action of phosphodiesterases and penetrate in the cytosol creating a favourable gradient to inhibit the enzymes of glycolysis.

The responsiveness of glycogen breakdown to cAMP was investigated in isolated perfused liver from male Wistar fed rats (200-220 g) with insulin-induced hypoglycemia. The activation of glycogenolysis by 3 µM cAMP was decreased (P<0.05) in livers from rats with hypoglycemia induced by the administration of insulin or during the direct infusion of insulin into the isolated liver. The direct effect of insulin on glycogen catabolism promoted by 3 µM cAMP occurred as early as 3 min after starting insulin infusion. In contrast, the cAMP agonists resistant to phosphodiesterases, 8Br-cAMP and 6MB-cAMP, used at the same concentration as cAMP, i.e., 3 µM, did not modify the effect of insulin. The data suggest that the decreased hepatic responsiveness of glycogen breakdown during insulin-induced hypoglycemia is a direct effect of insulin decreasing the intracellular levels of cAMP.

ABSTRACT:The anti-hyperglycemic effect of wood powder of Quassia amara (QA) was evaluated in normal and in alloxan diabetes-induced rats. After a 12 h fast and glycemic check, the animals were orally given 0.9% of saline (control group), metformin (500 mg/kg) or QA (200 mg/kg) and, 30 minutes later, they received an oral glucose dose (1g/kg). The blood glucose level was measured after 30, 60, 90 and 120 minutes. From the oral glucose dose, QA showed anti-hyperglycemic effects, similar to metformin, only in the diabetic animals (p<0.01) when compared to the control group. Although the anti-hyperglycemic mechanism of action of QA was not investigated, a mechanism similar to metformin can be suggested, since both presented similar results for the conditions tested, that is, normal and diabetic rats. It is believed that the use of QA in diabetics could help to control the blood glucose levels and be useful as an alternative therapy.Keywords: alloxan, anti-hyperglicemic effect, diabetes, Quassia amara RESUMO: Efeito anti-hiperglicêmico de Quassia amara (Simaroubaceae) em ratos normais e diabéticos. O efeito anti-hiperglicemiante do pó do lenho de Quassia amara (QA) foi avaliado em ratos normais e diabéticos aloxana induzidos. Após jejum de 12 horas e verificação da glicemia, os animais receberam administração oral de salina 0.9% (grupo controle), metformina (500 mg/kg) ou QA (200 mg/kg) e 30 minutos depois carga oral de glicose (1g/kg). A glicemia foi medida nos próximos 30, 60, 90 e 120 minutos. A partir da carga oral de glicose, a QA mostrou efeito anti-hiperglicemiante, similar a metformina, somente nos animais diabéticos (p<0.01) quando comparados ao grupo controle. Embora o mecanismo de ação anti-hiperglicemiante da QA não tenha sido investigado, podemos sugerir um mecanismo semelhante à metformina, visto que ambos apresentaram resultados similares nas duas condições testadas, ou seja, animais normais e diabéticos. Acredita-se que o uso de QA, em diabéticos, possa auxiliar no controle da glicemia e servir como terapia alternativa. Palavras-chave: aloxana, efeito anti-hiperglicemiante, diabetes, Quassia amaraRev. Bras. Pl. Med., Campinas, v.15, n.3, p.368-372, 2013.