EFFECT OF PROSTAGLANDIN SYNTHESIS INHIBITORS ON THE PITUITARY-THYROID AXIS IN THE RAT
1977 Volume 27 Issue 4 Pages 245-258
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1977 Volume 27 Issue 4 Pages 245-258
It has been postulated that prostaglandin (PG) s affect the pituitary-thyroid axis in man and experimental animals. However, the role of endogenous PGs in mediating the effect of thyrotropin (TSH) is unclear. Thus, the present investigation was designed to determine the effect of indomethacin (Ind) and aspirin, PG synthesis inhibitors, on the thyroid and TSH-secretory mechanism in the rat. Animals were injected sc daily with Ind (1. 5mg/100g bw) or aspirin (8mg/100g bw) for 3 days.
The plasma T4, and T3 levels, determined by radio-immunoassay, were consistently lower in the Ind or aspirin treated groups than in the controls. Both thyroid 131I uptake and numbers of intracellular colloid droplet formation in the thyroid after TSH stimulation were also reduced by the Ind treatment. The basal TSH levels appeared to decrease in the Ind or aspirin treated groups, with significant decreases in plasma T4 and T3. However, Ind treatment significantly potentiated the TSH response to synthetic thyrotropin-releasing hormone (TRH) in intact rats. The pituitary TSH content was higher in the Ind group than in the controls. In contrast, aspirin inhibited the TSH response to TRH, when the pituitary TSH content decreased significantly.
On the other hand, the compensatory rise in plasma TSH following thyroidectomy was inhibited by the Ind treatment. Nevertheless, the higher plasma TSH response to TRH was observed in thyroidectomized rats treated with Ind than in the controls. The pituitary TSH content was markedly increased by the Ind, while the hypothalamic TRH content in the two groups was not different.
The increased sensitivity of plasma TSH response to exogenous TRH in the Ind treated group is presumably due to higher pituitary TSH content than in the controls. The action of Ind appears to be mediated, at least in part, at the pituitary level. In addition, there is a dissociation between the action of Ind and aspirin in the TSH response to TRH.
