Orally administered tryptophan and experimental type 2 diabetes

Abstract

There is a link between diabetes and oxidative stress. Hyperglycaemia leads to free radical generation and alterations of endogenous antioxidants. Our aim is to study the effect of orally administered L-tryptophan (TRP), the melatonin precursor, an endogenous antioxidant, on circulating levels of glycaemia, insulin and melatonin, and on the superoxide dismutase and catalase antioxidant systems in non-diabetic (ND) and type 2 diabetic (n5-STZ) male Wistar rats. At 19:30 every day for 15 days, TRP (125 mg/kg body weight) was administered orally. At 09:00 every two days the glycaemia was measured and every day the intake of food and water was recorded. At the beginning and end of treatment (at 09:00; 21:00; 02:00) plasma insulin and melatonin levels were measured, and (at 09:00) the enzymatic activities of catalase and superoxide dismutase (SOD) in erythrocytes were also measured. Glycaemia values were greater (p < 0.01) in n5-STZ rats than in ND rats, while insulin levels were lower (p < 0.05) at all times studied and these parameters were not altered by the TRP administration. Melatonin levels at 02:00 were lower in n5-STZ than in ND rats (p < 0.05). The TRP administration did not modify the circulating melatonin levels in ND rats, but raised (p < 0.01) the levels at 02:00 in the treated n5-STZ group. In ND rats after TRP administration there was a decline in catalase activity (p < 0.05), while in n5-STZ rats there was a rise (p < 0.01) at the end of treatment. However, there were no significant changes in SOD activity. There was increased food intake (g/day) in the treated n5-STZ group (p < 0.01). In conclusion, the oral administration of TRP did not modify glycaemia or insulinaemia levels, but raised melatonin levels in diabetic rats at 02:00, lowered catalase activity in ND rats but raised it in n5-STZ rats, and increased food intake in n5-STZ rats.

Neonatally-induced diabetes: lipid profile outcomes and oxidative stress status in adult rats

“…With respect to SOD, the enzyme responsible for neutralizing superoxide anion levels, this study showed no changes in SOD activity in the n5-STZ rats. These results corroborate with literature, which observed no significant changes in SOD activity in diabetic rats 36,37 . However, there are also conflicting findings in literature.…”

Palliative Effects of Daonil Supplemented with Tryptophan on Alloxan Induced Diabetic Rats

“…It was observed from this study that the daonil supplemented tryptophan resulted in minimal change in both the weight and glucose concentration, but daonil only had significant reduction on the weight and glucose concentration, this was more noticeable on the 7 th day. The oral administration of tryptophan did not change the glycaemia level this was also observed in the course of this study [31]. The results obtained from other investigation suggested that tryptophan suppresses the elevation of blood glucose and lessens the burden associated with insulin secretion from β-cells, this agrees with our findings in which there was a decrease in glucose level on day 7 th in the group treated with tryptophan only.…”

L-Tryptophan Suppresses Rise in Blood Glucose and Preserves Insulin Secretion in Type-2 Diabetes Mellitus Rats

“…The OGTT was performed after 16-h fasting in SD or SDT rats with glucose (2 g/kg body weight (BW)), plus either l-or d-tryptophan (62.5 mg/kg BW) (12). Blood samples were collected from the inferior vena cava at the indicated times after oral administration of glucose.…”

“…The effects of l-tryptophan on blood glucose level have been complex, because not only l-tryptophan itself upregulates gluconeogenesis in liver (8), but also its metabolites including 5-hydroxy-tryptamine and serotonin (5-HT) modulate insulin secretion (9) and glucose uptake into skeletal muscle (10), as well as melatonin-stimulated insulin secretion (11). Recently, it was reported that oral administration of l-tryptophan for 15 d did not modify glycermia or insulinemia, but raised melatonin in T2DM model rats (12).…”