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Bovine pineal slices were incubated in Krebs-Ringer phosphate buffer together with ¹²⁵I-thyroxine and varying concentrations of epinephrine, norepinephrine, serotonin, melatonin, TSH, and aldosterone. Norepinephrine in concentrations of 10^–5 m and 10^–6 m significantly (p = 0.01, 0.05) decreased the uptake of thyroxine by the pineal slices, whereas epinephrine at 10^–5 m slightly increased (p = 0.l0) the uptake. Melatonin at 10^–3 m increased the uptake of and 10^–5 m thyroxine (p = 0.01, 0.01), but serotonin was without effect. TSH and aldosterone also showed no effect upon the uptake of thyroxine by pineal slices.

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.

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