Effects of alpha2-adrenoceptor blockade and thyrotropin-releasing hormone (TRH) on the cardiovascular system in the rabbit

The effects of two different doses of thyrotropin-releasing hormone on regional blood flows were studied in urethane-anaesthetized rabbits pretreated with the α₂ adrenergic antagonists yohimbine and idazoxan. The effects of yohimbine were also studied using unanaesthetized rabbits. Blood flow measurements were performed using the tracer microsphere method. Thyrotropin-releasing hormone was injected i. v. at a dose of either 0.1 mg kg^-1 or 2.0 mg kg^-1. Yohimbine and idazoxan did not modify the effect of thyrotropin-releasing hormone on mean arterial blood pressure. In the anaesthetized animals, blockade of the α₂ adrenoceptors resulted in a vasoconstriction in several peripheral organs and the vasoconstriction increased after thyrotropin-releasing hormone administration. Pretreatment with yohimbine reduced total cerebral blood flow moderately and in such animals thyrotropin-releasing hormone elicited only minor cerebral blood flow effects. Pretreatment with idazoxan did not reduce the total cerebral blood flow and in such animals it increased from 53± 1 to 75±4 g min^-1 100 g^-1 (P < 0.01) after the administration of the lower dose of thyrotropin-releasing hormone and from 64±5 to 112±17 g min^-1 100 g^-1 (P < 0.01) after the higher dose. In the conscious animals, yohimbine caused an increase in mean arterial blood pressure and heart rate. Vascular resistance increased in several organs. The cerebral blood flow decreased in white matter (P <0.05) and the caudate nucleus (P < 0.05). The results indicate that there is a yohimbine-sensitive mechanism involved in the cerebrovasodilating effect of thyrotropin-releasing hormone in anaesthetized rabbits. There is also an activation of the sympathetic nervous system by thyrotropin-releasing hormone which results in increased vascular resistance and mean arterial blood pressure. Its effect on the vascular resistance may be enhanced by α₂ adrenoceptor blockade. In conscious animals, there seems to be a yohimbine-sensitive mechanism involved in the control of cerebral blood flow.