Iodine metabolism in spotted munia, Lonchura punctulata

Abstract

Radioactive iodine metabolism was studied in both sexes of Spotted Munia, Lonchura punctulata, during different periods of the year. A characteristic “avian pattern” of thyroidal radioiodine retention was observed, which appeared to be due to a rapid turnover, as evident from a high uptake, rapid thyroxinogenesis, and release of hormone into blood. Monoiodotyrosine (MIT), diiodotyrosine (DIT), and thyroxine (T4) were the three iodoamino acids identified. Triiodothyronine (T2) was never detected. Thyroxinogenesis was of the typical vertebrate type. With time MIT:DIT ratio, which was always less than one, registered a decline, indicating a precursor-product relationship with the hormone. A pronounced seasonal variation was demonstrated in thyroidal 131I uptake, thyroxine production, and serum PBI, which were all minimal during the peak phase of reproductive activity in this bird. Effect of starvation, exogenous iodide (KI), thyrotropin (TSH), and propylthiouracil (PTU) was also studied. Starvation increased 131I uptake and favored high MIT formation, resulting in MIT:DIT ratios greater than one, indicating economic handling of iodine. KI drastically depressed 131I uptake and favored rapid conversion of MIT to DIT. But T4 production was not markedly altered by these two treatments. TSH stimulated 131I uptake, organification of iodide, conversion of MIT to DIT and thyroxine production, pointing toward an active pituitarythyroid axis. PTU led to increase in DIT and a 5-fold decrease in T4, indicating that in wild birds, as in mammals, the DIT to T4-step may be the most sensitive to PTU treatment. The annual fluctuations in thyroid activity, which do not coincide with natural fluctuations in temperature and daylength, may be influenced by extra intake of iodide, by contributing to the extrathyroidal pool, and endogenous thyrotropin secretion.