A low-protein diet during pregnancy prevents modifications in intercellular communication proteins in rat islets

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Disruption of glucose tolerance caused by glucocorticoid excess in rats is partially prevented, but not attenuated, by arjunolic acid.

Arjunolic acid (AA) obtained from plants of the Combretaceae family has shown anti-diabetic effects. Here, we analyzed whether the diabetogenic effects of dexamethasone (DEX) treatment on glucose homeostasis may be prevented or attenuated by the concomitant administration of AA. Adult Wistar rats were assigned to the following groups: vehicle-treated (Ctl), DEX-treated (1 mg/kg body weight intraperitoneally for 5 days) (Dex), AA-treated (30 mg/kg body weight by oral gavage twice per day) (Aa), AA treatment previous to and concomitant to DEX treatment (AaDex), and AA treatment after initiation of DEX treatment (DexAa). AA administration significantly ameliorated (AaDex) (P>0.05), but did not attenuate (DexAa), the glucose intolerance induced by DEX treatment. AA did not prevent or attenuate the elevation in hepatic glycogen and triacylglycerol content caused by DEX treatment. All DEX-treated rats exhibited hepatic steatosis that seemed to be more pronounced when associated with AA treatment given for a prolonged period (AaDex). Markers of liver function and oxidative stress were not significantly altered among the groups. Therefore, AA administered for a prolonged period partially prevents the glucose intolerance induced by DEX treatment, but it fails to produce this beneficial effect when given after initiation of GC treatment. Since AA may promote further hepatic steatosis when co-administered with GCs, care is required when considering this phytochemical as a hypoglycemiant and/or insulin-sensitizing agent.

Insulin signaling proteins in pancreatic islets of insulin-resistant rats induced by glucocorticoid

Chronic administration of glucocorticoids induces insulin resistance that is compensated by an increase in p-cell function and mass. Since insulin signaling is involved in the control of p-cell function and mass, we investigated the content of insulin pathway proteins in pancreatic islets. Rats were made insulin resistant by daily administration of dexamethasone (1mg/kg, b.w., i.p.) for 5 consecutive days (DEX), whilst control rats received saline (CTL). Circulating insulin and insulin released from isolated islets were measured by radioimmunoassay whereas the content of proteins was analyzed by Western blotting. DEX rats were hyperinsulinemic and exhibited augmented insulin secretion in response to glucose (P < 0.01). The IRa-subunit, IRS-1, Shc, AKT, p-p70S6K, ERK1/2, p-ERK1/2, and glucocorticoid receptor protein levels were similar between DEX and CTL islets. However, the IRp-subunit, p-IRp-subunit, IRS-2, PI3-K, p-AKT and p70S6K protein contents were increased in DEX islets (P < 0.05). We conclude that IRS-2 may have a major role, among the immediate substrates of the insulin receptor, to link activated receptors to downstream signaling components related to islet function and growth in this insulin-resistant rat model.

Increased expression of SNARE proteins and synaptotagmin IV in islets from pregnant rats and in vitro prolactin-treated neonatal islets

During pregnancy and the perinatal period of life, prolactin (PRL) and other lactogenic substances induce adaptation and maturation of the stimulus-secretion coupling system in pancreatic â-cells. Since the SNARE molecules, SNAP-25, syntaxin 1, VAMP-2, and synaptotagmins participate in insulin secretion, we investigated whether the improved secretory response to glucose during these periods involves alteration in the expression of these proteins. mRNA was extracted from neonatal rat islets cultured for 5 days in the presence of PRL and from pregnant rats (17th-18th days of pregnancy) and reverse transcribed. The expression of genes was analyzed by semi-quantitative RT-PCR assay. The expression of proteins was analyzed by Western blotting and confocal microscopy. Transcription and expression of all SNARE genes and proteins were increased in islets from pregnant and PRL-treated neonatal rats when compared with controls. The only exception was VAMP-2 production in islets from pregnant rats. Increased mRNA and protein expression of synaptotagmin IV, but not the isoform I, also was observed in islets from pregnant and PRL-treated rats. This effect was not inhibited by wortmannin or PD098059, inhibitors of the PI3-kinase and MAPK pathways, respectively. As revealed by confocal laser microscopy, both syntaxin 1A and synaptotagmin IV were immunolocated in islet cells, including the insulin-containing cells. These results indicate that PRL modulates the final steps of insulin secretion by increasing the expression of proteins involved in membrane fusion.

Deficiência nutricional e diabetes mellitus / Nutritional deficiency and diabetes mellitus

Distúrbios nutricionais estao relacionados ao desenvolvimento de diabetes (DM). Em países subdesenvolvidos milhares de indivíduos apresentam um tipo de DM relacionado à alimentaçao pobre em proteína e calorias. Existem indícios de que dietas hipoprotéicas e hipocalóicas induzam a uma perda funcional da célula beta pancreática e, conseqüentemente, levem a um desequilíbrio na homeostase da glicose. Outra possibilidade seria uma potencializaçao, pelo quadro de desnutriçao, de efeitos diabetogênicos ambientais, imunológicos ou genéticos preexistentes. Finalmente, a carência nutricional protéico-calórica poderia simplesmente modular o curso de um quadro preexistente de DM. Outro aspecto importante da influência do padrao nutricional sobre o controle dos níveis sangüíneos da glicose pode ser observado em indivíduos submetidos à carência nutricional durante as primeiras fases de vida, e que a seguir passam a se alimentar com dietas hipercalóricas. Tal mudança no padrao alimentar leva muitas vezes à obesidade e a uma maior incidência de diabetes. Em humanos e modelos animais de desnutriçao protéico-calórica observa-se uma reduzida secreçao de insulina em resposta a um estímulo de glicose. Aparentemente a carência de insulina é compensada, ao menos em parte, por uma maior sensibilidade periférica ao hormônio pancreático, a qual se deve a uma modulaçao nas etapas iniciais da sinalizaçao da insulina. Tal compensaçao garante a homeostase da glicose na maior parte dos indivíduos, entretanto diversos fatores podem romper este equilíbrio e desencadear o aparecimento de DM.

Canais de potassio sensíveis ao ATP e a secreçäo de insulina: aspectos fisiopatológicos da hiperinsulinemia familiar / Potassium channels sensibles to ATP and insulin secretion: physiopathological aspects of the familial hyperinsulinism

Glicose provoca a secreçao de insulina através do aumento da relaçao ATP/ADP no citoplasma das células beta. Isto leva ao bloqueio de canais de K+ sensíveis ao ATP (KATP), reduçao da saída deste cátion da célula, despolarizaçao celular, ativaçao da permeabilidade ao Ca2+ sensível à voltagem, entrada e acúmulo deste cátion nas células e consequente secreçao de insulina. O canal KATP parece ser composto por duas unidades distintas; uma delas, denominada Kir6,2, constitui o canal propriamente dito, por onde fluem as correntes de K+. A outra é o receptor de sulfoniluréias (SUR1), que é provida de sítios de ligaçao para o referido fármaco, para ATP, MgADP e diazoxida, atuando como unidade regulatória. Neste artigo, fazemos uma breve revisao da fisiologia dos canais KATP, considerando também sua importância na fisiopatologia do processo secretório.

Acoplamento da estimulaçäo-secreçäo de insulina pelas células beta pancreáticas / Compling of insulin stimulation-secretion by pancreatic beta cell

O metabolismo da glicose pelas células beta pancreáticas, através do aumento da relaçäo ATP/ADP na célula, induz, na membrana plasmática, bloqueio de um canal específico para o K+. O acúmulo relativo deste cátion na célula provoca despolarizaçäo da membrana e consequente ativaçäo de uma permeabilidade ao Ca2+, dependente de voltagem. Ions Ca2+ penetram a célula por gradiente eletroquímico, aumentando a concentraçäo citoplasmática do mesmo, o que ativa o mecanismo secretor composto por microtúbulos, microfilamentos e membranas vesicular e plasmática. Esse mecanismo é potencializado pelo próprio Ca2+, que ativa a adenilil-ciclase e a fosfolipase C, ambas enzimas de membrana e que, através dos seus respectivos segundos-mensageiros (AMPc, IP3 e DAG), sensibilizam o aparelho secretor ao próprio Ca2+. É através da ativaçäo desses dois sistemas enzimáticos que substâncias endógenas näo metabolizáveis pelas células B, tais como neurotransmissores e hormônios do eixo êntero-insular, potencializam a secreçao de insulina. Cabe lembrar que a maioria dos dados sobre o mecanismo de secreçäo de insulina foram obtidos em esperimentos realizados com animais. Portanto, a extrapolaçäo dessas informaçöes para a espécie humana deve-se revestir da necessária cautela.