A number of factors which have been implicated in the response of the rabbit ear artery to adrenergic agents and stimuli are reviewed. Aspects discussed include: (a) the method of propagation of vasoconstriction in the arterial smooth muscle; (b) the interrelationship between the intramural distribution of sympathetic nerves and muscle, and the influence of uptake and enzymic inactivation on the response to norepinephrine, and (c) the influence of presynaptic adrenergic and cholinergic receptors on the response to sympathetic nerve stimulation.

Summary. Untreated and nialamide treated rabbit ear arteries were incubated with noradrenaline in high concentration (20 ug/ml) aud the subsequent rates of release of the amine into the bathing medium measured. Nialamide caused a 5-6 fold increase in the release. The eHect was etinally prominent iu arteries treated with cocaine and in chronically denervated arteries. It is conchidet! that extraneuronal monoamine oxitlase plays a major role in the inactivati{)n oi noradrenaline when the latter is present in high iiinct.'ntration in the tissue. INTRODUCTION.Although the only niouoamine oxitlasf (MAO) which ean be detected histocheiuically in the rabbit ear artery is extraneuronal in distribution (de la Lande, Hill. Jellett and McNeil, 1970). a recent pharmacological study has provided strong evidence for the presence of intraneuronal monoamine oxidase and for an important role of this enzynie. but not extranenronal M.AO, in controlling the vasoconstrictor response to noradrenaline (NA) (de la Lande and Jellett. 1971). However, the failure of inhibition of extraneuronal MAO to modify tbe \asocoristrictor response does not exclude the possibility that NA is metabolised by this enxyine. To test this possibility we have taken advantage of the histochemical findings of Avakian and C^illespie ( 1968) that the suiooth muscle cells of the media rapidly accimuilate noradrenaline when the latter is present in high concentrations in the .solution bathing the artery. In the present study we have determined whether the rate at which noradrenaline is released, following its uptake from high concentration, is m<»dified by prior iniiibition of MAO.

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SUMMARY The findings in two cases of medullary carcinoma of the thyroid are reported, the patients being a mother and her daughter. The mother was also found to have previously unsuspected bilateral phæochromocytomas at autopsy. The association of these two tumours represents a rare but well recognized clinical entity. A family study was then undertaken to detect evidence of these conditions in 17 blood relatives. Neither thyroid carcinoma nor phæochromocytoma were detected. However, because of the previously reported familial nature of this syndrome, family and long term follow‐up studies of patients with either medullary thyroid carcinoma or phæochromocytoma, or both of these conditions, are recommended.

The isolated central artery of the rabbit ear is highly sensitive to catecholamines when perfused with Krebs solution containing 5‐hydroxytryptamine. The preparation responds to 1 ng of noradrenaline and is extremely long lasting. It does not discriminate between noradrenaline and adrenaline.