Recombinant bovine somatotropin (rBST) administration in Simbrasil calves and its effects on muscle fibers and T3, T4 and IGF-I hormonal pattern Somatotropina bovina recombinante (rBST) em bezerros Simbrasil e seus efeitos sobre as fibras musculares e o padrão hormonal de T3, T4 e IGF-I

In order to characterize the muscle fibers and alterations of the thyroid hormones and IGF-I, 64 Simbrasil male and female calves, from 60 until 210 days-old, were supplemented in creep-feeding, with recombinant bovine somatotropin (rBST). A dose of 0.15 mg of rBST/kg of body weight was administrated every 14 days in the animals with rBST. The muscle fibers were characterized, and the proteins were quantified. Alterations hormones and IGF-I were also evaluated. It was not observed any increase in the average diameter of the three types of muscle fibers. The myofibril protein content of the muscle from the rBST female calves increased, and the same were observed in male calves, independent of rBST administration. It was not found any influence of rBST administration in the T3 hormone, although the female calves (2.37ng/mL) had greater T3 concentration than the male calves (1.95ng/mL). Regarding the T4 hormone, female calves had greater concentration than male calves, in which the rBST administration resulted in a decrease of T4 concentration. The female calves had lower IGF-I concentration than the male calves, but rBST administration caused an increase in the IGF-I concentration in both gender. In conclusion, the rBST administration did not promote an increase in the muscle fiber diameter, and it happened with the myofibril protein content of the muscle from the females calves. Also, rBST did not influence T3 concentration, reduced T4 in male calves and increased IGF-I concentration in both gender.Com o objetivo de caracterizar as fibras musculares e avaliar as alterações dos hormônios tireoidianos e IGF-I, em bezerros suplementados em creep-feeding, com administração de somatotropina bovina recombinante (rBST), realizou-se o experimento com 64 machos e fêmeas, Simbrasil. O experimento iniciou-se aos 60 dias e foi até os 210 dias de idade. Foram administrados 0,15mg de rBST/kg de peso vivo, em intervalos de 14 dias. Caracterizaram-se morfologicamente as fibras musculares e quantificaram-se as proteínas do tecido muscular, aos 150 e 210 dias de idade, além de terem sido avaliadas as alterações dos hormônios tireoidianos e IGF-I, a cada administração de rBST. Não foi observado aumento no diâmetro médio dos três tipos de fibras musculares. Houve um aumento nos teores de proteína miofibrilar nas fêmeas tratadas com rBST e nos machos. Em relação ao hormônio T3, não foi detectada influência do uso de rBST, embora haja diferença entre as fêmeas (2,37ng/mL) e os machos (1,95ng/mL). Quanto aos valores do hormônio T4, as fêmeas apresentaram concentração mais elevadas que os machos, e entre esses houve redução nos animais com rBST. Observou-se menor concentração de IGF-I nas fêmeas e aumento nos níveis desse hormônio, em ambos os sexos com rBST. O uso de rBST não aumentou o diâmetros das fibras musculares, o que, contudo, ocorreu para os teores de proteína miofibrilar das fêmeas. Ainda, o uso de rBST não influenciou a concentração de T3, reduziu a de T4 no machos e promoveu aumento de IGF-I em ambos os sexos