Transport Mechanisms and Thyroxine Uptake in Pineal Slices

Abstract

After a preliminary study showed conclusively that in vitro pineal cells avidly take up thyroid hormones but not iodide, a series of tissue slice studies using ¹²⁵I-thyroxine were conducted to investigate the mechanism of thyroxine (T₄) uptake in pineal cells. T₄ uptake was unaffected by the presence or absence of glucose and cofactors, nor was uptake inhibited by cyanide. No binding capacity for T₄ could be demonstrated, and the percent uptake of labelled T₄ actually increased with T₄ concentration up to a certain level. Uptake of the thyroid hormone was quite rapid. The temperature coefficient was determined for the uptake, and was found to fall in the range of values reported for diffusion mechanisms. On the basis of these findings, we concluded that energy dependent mechanisms of transport could be eliminated, and that some sort of mediated diffusion was the most likely mechanism for T₄ transport.

Electrical Responses of Pineal Cells to Thyroid Hormones and Parathormone

“…There are a number of observations by other workers which also suggest that thyroxin affects cells in the pineal gland, main ly by enhancing pineal activity. Pineal tissue avidly takes up thyroid hormones both in vivo [29] and in vitro [4]. Indica tions that thyroid hormones may have an excitatory action on pineal hormone synthesis result from observations that administration of potassium iodide leads to pinealocyte hy perplasia and hypertrophy [14] and that thyroxin stimulates histochemically demonstrable enzymes of pinealocytes in vitro (13].…”

“…Thus, thyroid weight in creases following pinealectomy [6,27] and decreases after the administration of pineal substances in adult mammals [16], Since pineal tissue avidly takes up thyroid hormones both in vivo [29] and in vitro [4], it is logical to conclude that the pineal influence on the hypothalamo-hypophyseal-thyroid axis may be under feedback control.…”

“…Moreover, since the uptake of thyroid hor mones by pineal cells in vitro is shown to be enhanced by the pineal indolamine melatonin [4], responses of cells to the application of thyroxin were compared with the effects of melatonin microelectrophoresis on the same units.…”

Thyroxine Metabolism by Pineal Slices

“…As part of our investigation of thyroid-pineal interaction, we recently reported findings that suggest some form of mediated diffusion as the mechanism of T4 transport into pineal cells [Cady et al, 1971]. Our present study deals with the metabolism of T 4 by these cells.…”

“…It has been demonstrated that the pineal takes up thyroxine (T4) in vivo [Ford and G ross, 1958] and in vitro [Cady et al, 1971]. As part of our investigation of thyroid-pineal interaction, we recently reported findings that suggest some form of mediated diffusion as the mechanism of T4 transport into pineal cells [Cady et al, 1971].…”