Kinetics of dog thyroid secretion in vitro

Area of Science:

• Endocrinology
• Cell Biology
• Thyroid Physiology

Background:

• Thyroid hormone synthesis involves the uptake and release of iodine.
• Thyroid-stimulating hormone (TSH) is a key regulator of thyroid function.
• Understanding the kinetics of radioiodine handling is crucial for diagnosing and treating thyroid disorders.

Purpose of the Study:

• To investigate the time course of radioiodine sequestration and secretion in dog thyroid tissue.
• To elucidate the role of TSH in regulating these processes.
• To determine the rate-limiting steps in thyroid hormone secretion.

Main Methods:

• Dog thyroid slices were prelabeled with radioiodine (131I) in vivo.
• Tissues were incubated in vitro with or without TSH or (Bu)2cAMP.
• Endocytosis-blocking drugs were used to suppress TSH-stimulated endocytosis.
• Radioactivity in phagocytic vacuoles and colloid droplets (sequestration) and secreted radioiodine were measured over time.

Main Results:

• TSH induced radioiodine sequestration within 5 minutes and stimulated secretion after 20 minutes.
• The secretion rate was a constant 1% per hour of total slice radioactivity.
• At steady state, the hydrolysis of sequestered radioiodine was the rate-limiting step for secretion.
• Basal secretion (without TSH) was a constant 0.23% per hour and insensitive to cytochalasin B.

Conclusions:

• TSH plays a critical role in the rapid sequestration and subsequent secretion of radioiodine in the thyroid.
• The rate of endocytosis is matched by the rate of secretion at maximal stimulation.
• Hydrolysis of sequestered radioiodine is a key regulatory step in thyroid hormone release.