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Sulfhydryl reagents are good stimulators of the enzymatic conversion of T4 to T3. Previous studies have suggested that the underlying mechanism for this property is that these reagents may serve as a cofactor in the reaction, or they maintain the enzyme in its active reduced form. We examined another possibility, namely that sulfhydryl reagents affect T4 itself and hence increase T3 generation by the enzyme. The substrate T4 and sulfhydryl reagents dithiothreitol, mercaptoethanol, and reduced glutathione were incubated at 37 C for 1 h and then combined with rat liver homogenate and incubated for additional 15 min to permit the enzymatic formation of T3. For control, substrate T4 was not preincubated with reagents, and those reagents were added just before the incubation. Preincubation of T4 with these reagents markedly enhanced subsequent T3 generation by the homogenate. Without preincubation, however, reduced glutathione and mercaptoethanol did not enhance T3 generation by simultaneous incubation with the homogenate. Dithiothreitol increased T3 generation when introduced into the incubation medium simultaneously with the homogenate and incubated only 15 min. However, the magnitude of stimulation was far less than that obtained from T4 preincubated with this reagent. These procedures did not alter the nonenzymatic generation of T3 significantly. These results suggest that sulfhydryl reagents can affect T4 itself and make it more susceptible to enzymatic outer ring monodeiodination.

Florsheirn reported that large doses of thyroid hormone, when employed for suppression of intrinsic TSH in McKenzie assay mice, reduced the response of the animal to TSH or LATS. Accelerated radioiodine clearance was proposed for the mechanism of this reduction in the response. We sought to determine whether thyroidal responsiveness per se is influenced by the dose of thyroid hormone, adopting radioiodine release and intracellular colloid droplet formation as parameters of thyroid response. A daily dose of T3, 0.5, 10, 25 μg, or T4, 0.5 or 25 μg was injected SC into mice for three days after 125I injection. On the day after the last thyroid hormone injection, they received 0.075 mU TSH or LATS-IgG. Blood radioactivity and response index were obtained on the specimens taken before, 2½ or 8 hr after the injection. Colloid droplet counts were obtained on the thyroid gland removed at ½ hr after TSH or 2 hr after LATS-IgG. The dose of TSH or LATS, which caused sizable and significant extent of radioiodine rele...

The effect of amphotericin B on Na+ and K+ concentration and iodide tissue/medium ratio (T/M I) of beef thyroid slices in vitro has been studied. Amphotericin B in concentrations from 2.5 to 50 μg/ml regularly decreased thyroidal T/M I and K+ concentration and increased thyroidal Na+ concentration. This effect was reversible by washout and by incubation in medium containing 16 mM/l of K+, but was not altered by change in medium Na+. The addition of cholesterol to the medium had no intrinsic effect on T/M I but inhibited the action of amphotericin B on T/M I. Incubation with amphotericin B did not cause reduction in the ouabain-sensitive ATPase of beef thyroid nuclear fraction. It also did not lower O2 consumption of thyroid slices. These findings lead to the conclusions that amphotericin B acts on the thyroid cell membrane and does so in a fashion different from ouabain. This is further evidence against the concept that operation of Na+, Reactivated ATPase is a sufficient condition for thyroidal iodide tr...

LATS and TSH are chemically different thyroid-stimulating agents. To compare their early actions, we examined the effects in mice of LATS and TSH upon 2 thyroidal reresponses related to hormone secretion, namely, intracellular appearance of colloid droplets and release into the blood of preformed colloidstored radioiodine. Intracellular colloid droplets were induced by LATS in a manner qualitatively indistinguishable from the effect of TSH, but with a longer latency. In mice injected with 0.5 mU of TSH, a marked increase in the number of colloid droplets was observed in 10 min and a maximum was reached in 30 min. A dose of LATS which was chosen to match 0.5 mU of TSH in its radioiodine-releasing effect (at 2 hr) caused no increase in colloid droplets in 10 min, a small but significant increase in 30 min, and a maximal increase in 2 hr. When 16 times as much LATS was injected and shorter time intervals were examined, there was a slight increase in colloid droplets at 10 min, but the shape of the time-respo...