Effects of short-term thyroxine treatment on pancreatic cytology and responses of blood sugar, serum insulin and serum free fatty acids to epinephrine infusion.

The influence of short-term treatment with l-thyroxine on pancreatic histology and on the responses of glycemia, insulinemia and serum free fatty acids to a continuous l-epinephrine infusion in the absence or presence of alpha- and beta-adrenergic blockade was studied in male dogs. l-Epinephrine dosage: 0.06 microgram/kg body weight/min for 55 min. Two experimental groups were studied, one treated for 10 days with sodium l-thyroxine, one daily dose of 100 micrograms/kg body weight, the other of untreated controls. Three alternative treatments were applied to dogs of both groups: 1) no treatment; 2) alpha-adrenergic blockade with phentolamine (2.0 mg/kg body weight, 35 min before starting the l-epinephrine infusion); 3) beta-adrenergic blockade with propranolol (0.3 mg/kg body weight, 20 min before starting the l-epinephrine infusion). Body weight, rectal temperature, heart and respiratory rates were used as guidelines to assess experimental hyperthyroidism. Insulin immunocytolocalization was also studied in the Langerhans islets of T4-treated and control dogs. Body weight decreased and rectal temperature did not vary as a result of thyroxine administration which had no significant effect on respiratory and heart rate. The mean number of breaths from 0 to 120 min from the start of l-epinephrine infusion decreased in both T4-treated and control dogs submitted to propranolol blockage compared to non-blocked animals; phentolamine had no effect on the respiratory rate. Thyroxine treatment did not modify the number of heart beats, but phentolamine blockade had a different effect in T4-treated compared to control dogs whereas propranolol had similar effects in these two groups. Histological examination of the Langerhans islets of dogs submitted to short-term thyroxine treatment showed degranulation though no vacuolation. Most of the beta-granules contained in the B-cells of these islets were found near the cell membrane, thus forming a dark brown line after the immunochemical reaction. Since negative images of B-cell nuclei and vascular spaces were predominant in these specimens, the pancreas of T4-treated dogs presented a mesh structure. In dogs submitted to short-term thyroxine treatment, the hyperglycemic response to l-epinephrine was enhanced and prolonged as compared to untreated controls. In normal dogs, this response is mainly mediated by alpha-adrenergic receptors while beta-receptors hardly influence this response.(ABSTRACT TRUNCATED AT 400 WORDS)