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Induced Unidirectional Transport of Protein into the Thyroid Follicular Lumen

Department of Anatomy, University of Göteborg, and Division of Clinical Pharmacology, Department of Pharmacology, University of Lund Sweden

Abstract

In normal rats, the apical region of the thyroid follicle cells harbors a heterogeneous population of vesicular elements. Functionally, these are of at least two categories: endocytotic and exocy-totic.

Elimination of TSH secretion, either by hypophysectomy or by suppression with daily doses (of L-thyroxine (T₄), was followed by marked changes in the population of vesicles: the heterogeneity disappeared, and only one type of vesicles remained. These remaining vesicles had a diameter of 1,000–3,000 Å and they were characterized by a very dense content and a bounding membrane of 60 Å. During continuous suppression of TSH secretion, the number of such vesicles decreased, but at a slow rate and they were still present at the sixth day. In contrast, colloid droplets vanished within one day.

In T₄-suppressed animals, the rate of incorporation of leucine-^l4C into thyroid proteins, including thyroglobulin, was slowly affected, being reduced by only 15% after 1 day and by 50% after 6 days. In contrast, plasma protein-bound ¹²⁵I levels decreased by more than 90% within 1 day.

Electron microscopic autoradiography after administration of ³H-leucine to T₄-treated animals showed that label was concentrated in the endoplasmic reticulum of the follicle cells after 0.5 hr, in the apical vesicle region after 1 hr and reached the periphery of the colloid after 3 hr.

After labeling the colloid with ¹²⁵I, no autoradiographic grains appeared over the dense, 1,000–3,000 A apical vesicles whether in normal or in T₄-treated rats. The apical vesicles did not show any his-tochemical reaction for acid phosphatase.

It is concluded that after elimination of TSH secretion protein synthesis and exocytosis in the follicle cell are only slowly affected, whereas en-docytosis and release of thyroid hormone cease almost immediately. This yields a situation during which exocytotic vesicles can be observed and which would be useful for studies on the mechanisms of exocytosis and endocytosis in the thyroid. (Endocrinology 95: 1506, 1974)

Footnotes

¹ Correspondence and requests for reprints to Lars E. Ericson, Department of Anatomy, University of Göteborg, Fack, S-400 33. Göteborg 33, Sweden.

Received April 1, 1973.

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