Role of the heterogeneity of thyroglobulin in the secretion of thyroid hormone in mice.

The rates of thyroglobulin hydrolysis and iodothyroine release from mouse thyroid glands were studied in vitro. Recently iodinated thyroglobulin ('new pool') had been labelled during life by injection of 131I 3 h before removal of the thyroid, 'old pool' thyroglobulin had been labelled by the administration of 125I in the drinking water for 1 week starting 3 weeks earlier. Chromatogrphic analysis of pronase digests of the thyroid glands showed that the iodothyronine content of the old and new pools were 19.5 and 7.4 per cent respectively. In the basal state the rate of thyroglobulin hydrolysis was lower from the old pool but the rate of hormone secretion was similar from both pools. Thyrotrophin (TSH) increased the rate of thyroglobulin hydrolysis and hormone release from both pools by up to four to six times the basal rate, the effect being maximal 2 h after administration of TSH and lasting for 6-8 h. The rate of thyroglobulin hydrolysis after TSH was similar in both pools but the rate of release of labelled iodothyronines was significantly higher from from the old pool. These studies have indicated that although hydrolysis of thyroglobulin paroceeds faster in the new pool than in the old ('last come, first served' hypothesis) neverrtheless there is no difference in the rate of hormone secretion from the two pools, and hydrolysis in both pools is affected by TSH.