Effect of propylthiouracil, and of bacterial endotoxin (LPS), on thyroid hormones, respiratory rate, cutaneous and renal blood flow in rabbits.

Rabbits in a warm environment reacted to i.v. injections of 10 mg/kg propylthiouracil (PTU) with an immediate fall of serum triiodothyronine (T3) concentration, associated with decreases of respiratory rate and cutaneous blood flow. Simultaneously renal blood flow increased, while arterial blood pressure fell slightly. A rise in the animals' core temperature by 1.1 degree C, on average, contributed to the impression that PTU mimicked the stimulation of the normal thermoregulatory response pattern of cold defence. The cardiorespiratory responses to PTU were found to be augmented 6-8 days after thyroidectomy, but were completely abolished 16-20 days after thyroidectomy or chronic PTU treatment. In chronically thyroidectomized rabbits, i.v. injections of T3, but not of T4, elicited panting and cutaneous vasodilatation. The acute effects of injecting i.v. bacterial endotoxin (LPS) into rabbits in a warm environment consisted of cutaneous vasoconstriction and a decrease in respiratory rate, i.e. in an autonomic cold defence response, which was associated with a sustained increase in serum T3 concentration and caused core temperature, T3 serum concentration decreased again, whilst simultaneously the autonomic activity pattern changed to that of heat defence, comprising a rise in respiratory rate and skin vasodilatation. The results suggest the hypothesis that, similarly to a decrease of serum T3, LPS activates neurones in the CNS which secrete the thyrotropin-releasing hormone (TRH). This, in turn, elicits cardiorespiratory adjustments similar to those observed in the cold, while the opposite response occurs if these TRH secreting neurones are inhibited.