An analysis of the purinergic component of active muscle vasodilatation obtained by electrical stimulation of the hypothalamus in rabbits.

In anaesthetized rabbits, electrical stimulation of the hypothalamus in areas analogous to the defence area in cats produces the 'defence reaction.' This response includes signs of arousal and a large increase in blood flow to skeletal muscle in the hind limb caused by a vasodilatation in the skeletal muscle vasculature. The vasodilatation is a sympathetic response, and it is not dependent upon muscle activity in the hind limb. The muscle vasodilatation is insensitive to alpha-adrenoceptor, beta-adrenoceptor, cholinoceptor and histamine receptor antagonists. Intra-arterial injections of the purinoceptor agonists, adenosine triphosphate (ATP) and adenosine, mimic the vasodilatation produced by electrical stimulation. The P1-purinoceptor blocker, aminophylline, attenuates adenosine-induced vasodilatation, but it does not affect the vasodilatation produced by ATP or hypothalamic stimulation. The P2-purinoceptor blocker, antazoline, attenuates the vasodilatation produced by both ATP and hypothalamic stimulation. Our results suggest that the muscle vasodilatation produced by hypothalamic stimulation is mediated by purinergic nerves which release ATP and act on P2-purinoceptors.