Avaliação de parâmetros metabólicos e comportamentais em ratos tratados com ractopamina e submetidos a estresse por imobilização

Abstract

Ractopamine (RAC) is a β-adrenergic agonist used in animal diet as a nutrient repartitioning agent. However, few studies have detailed the biochemical profile involved in the activation of β-adrenergic receptors and their relationship to stress and animal behavior. Thus, this study aimed at evaluating the effect of RAC on metabolism, performance, body composition and behavior using an experimental model in Wistar rats treated with RAC and submitted to stress. The rats were divided into 15 experimental groups with five replicates. The oral dose of 5 ppm of RAC was provided in periods of 07, 14, 21 and 28 days. Protocols for acute (1 hour in a single day) and chronic (one hour per day / 5 times a week / 40 days) restrain stress were performed. The animals were tested on the elevated plus-maze test (EPMT) for behavioral assessment. Insulin and corticosterone levels were determined by ELISA. Total and fraction cholesterol, triglycerides and glucose levels and characteristics of carcass and viscera were evaluated. The determination of insulin dependent glucose transporters (GLUT4) were analyzed by Western Blot epididymal adipocytes. A completely randomized design (CRD) in a factorial 3x5 was employed. The data were submitted to ANOVA and, when necessary, the comparison of means was made by SNK and Regression models. The analysis of the behavior-related variables was done using the Generalized Linear Deviance Model. RAC periods associated with the chronic stress increased, in a time-dependent manner, the protein expression GLUT-4 in adipose tissue. Additionally, periods of stress combined with RAC increased exploration in the maze in a time-dependent manner. An opposite effect was found when RAC was provided to non-stressed animals. Chronic and acute stress reduced locomotive activity in the maze. Acute stress reduced blood supply to certain organs and raised the level of corticosterone. The chronic stress, in turn, reduced weight gain of the animals. However, blood parameters: insulin, glucose, triglycerides, total cholesterol and HDL and VLDL + LDL levels were not affected by stress or by supply of RAC. The low oral dose of 5 ppm of RAC, although sufficient for swine, caused less pronounced effects in rats, suggesting first pass metabolism in the liver of this specie. Even at this dosage, the RAC potentiated the effects of stress in rats. Thus, in extrapolating these findings to animal production, it is suggested that stress may act as a detrimental factor against the possible benefits offered by RAC.