Pituitary neuromedin B content in experimental fasting and diabetes mellitus and correlation with thyrotropin secretion

Abstract

Fasting and diabetes mellitus in the rat model have been associated with abnormalities of thyrotropin (TSH) secretion. Neuromedin B is a bombesin-like peptide highly concentrated in the pituitary gland that has been shown to have inhibitory action on TSH secretion, acting as an autocrine/paracrine factor. Here, we aimed to determine if the pituitary content of neuromedin B would change in fasted rats (1, 2, 3, and 4 days of food deprivation) and streptozotocin (55 mg/kg body weight)-diabetic rats. The total pituitary content of neuromedin B was decreased in fasted rats, except at 2 days of fasting, as was the total protein content in the gland; however, the concentration of the peptide (femtomoles per milligram protein) did not significantly change until the fourth day of food deprivation, when an abrupt decrease in total protein happened and therefore neuromedin B concentration increased. In rats after 20 days of diabetes induction, pituitary neuromedin B increased. Serum thyroxine (T 4) and triiodothyronine (T 3) decreased in both disorders, whereas serum TSH was normal or decreased in 4-day fasted rats. Therefore, the caloric deprivation of diabetes and fasting changed the pituitary neuromedin B content and concentration, by mechanisms that remain to be elucidated. Since neuromedin B has been shown to act as a local inhibitor of TSH release, the results raise the possibility that increased neuromedin B concentration might be involved in the altered TSH secretion of diabetes mellitus and fasting.