Abstract
Douglas and coworkers [I] have shown that the induction of secretion in cells of the adrenal medulla is caused by an increase in the intracellular level of calcium. This increase appears to be secondary to a burst of calcium penetration in the cell. Because of its obvious similarity with the stimulus contraction coupling in muscle, this phenomenon has been called the stimulus secretion coupling. This concept has been extended to other cells in which secretion is thought to involve the exocytosis of material contained in intracellular vesicles [I] . Thyroid secretion is believed to involve the endocytosis of thyroglobulin by follicular cells and its digestion in secondary lysosomes with a consequent release of thyroid hormones [2]. Although in this case secretion corresponds to endocytosis and not to exocytosis, the hypothesis has been proposed that this phenomenon may also be triggered by Ca*+ [3] . In these experiments the role of Ca*+ in the stimulations by thyrotropin of glucose oxidation, iodide binding to proteins, colloid endocytosis and secretion in dog thyroid slices has been investigated.
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