Vasodilatation in hind-limb skeletal muscle evoked as part of the defence reaction in the rat.

Electrical stimulation of the hypothalamic and mid-brain defence area of the rat's brain elicits a consistent cardiovascular pattern of response of which vasodilatation in the skeletal muscle is an integral component: the mechanisms mediating this vasodilatation were investigated. It is not sensitive to atropine in this species, but it was substantially affected, particularly its later stage (the prolonged tail-end of the response), by either beta-adrenoreceptor antagonists (propranolol, sotalol) or bilateral adrenalectomy. A major part of the hind-limb vasodilatation can therefore be attributed to the action of catecholamines released from the adrenal glands. The initial part of the vasodilatation, which still remained after beta-adrenoreceptor blockade and adrenalectomy, was abolished by intravenous injection of guanethidine and phentolamine and seems therefore due simply to withdrawal of vasoconstrictor tone. In confirmation, stimulation of the sympathetic outflow to the hind-quarters after phentolamine and guanethidine had been used to block vasoconstriction did not reveal a sympathetic vasodilator nerve supply to the hind-limb vasculature.