A vasodilator innervation to the central artery of the rabbit ear.

1 The possibility of a vasodilator innervation to the isolated and perfused central artery of the rabbit ear was examined.2 Stimulation of the periarterial nerves in the presence of noradrenaline or other agonist used to maintain a partial constriction of the ear artery, led to a decrease in intraluminal flow followed after the cessation of stimulation by an increase in flow beyond the pre-stimulation level.3 After blockade of adrenergic transmission with bretylium or guanethidine or of the alpha- and beta-adrenoceptors with phentolamine and propranolol, stimulation of the periarterial nerves in the presence of a background tone, led to a clearly detectable vasodilation. This dilatation was not blocked by treatment with atropine or mepyramine; nor was it enhanced by physostigmine.4 Pretreatment of rabbits with reserpine (2 mg/kg) to deplete catecholamine stores, eliminated both the vasoconstrictor and vasodilator responses to nerve stimulation. However, a lower dose of reserpine (0.2 to 0.5 mg/kg) selectively eliminated the vasoconstrictor component of periarterial nerve activation.5 The ear artery dilated in response to low concentrations of prostaglandin E(1), and E(2), in the presence of noradrenaline, but treatment with inhibitors of prostaglandin synthesis, indomethacin, aspirin or eicosa-5,8,11,14-tetraynoic acid did not reduce the vasodilator response. Attempts to extract a prostaglandin in the bathing medium were unsuccessful.6 The involvement of a purine nucleotide appeared unlikely since the ear artery dilated only in response to fairly high concentrations of adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP) and adenosine 5'-monophosphate (AMP). Furthermore, dipyridamole, an inhibitor of adenosine uptake, enhanced dilation due to exogenous ATP but not to periarterial nerve stimulation.7 It is concluded that the central artery of the rabbit ear has a vasodilator innervation but the identity of the transmitter remains to be established.