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Summary. The noradrenaline content of the ciliary body, the iris, and the choroid in she cow was 0.3—0.4 μg per g wet weight. Adrenaline was present to 2.6—7.2 per cent of the total catechols. In aqueous humour the noradrenaline was generally less than 0.2 μg per 100 ml. On one occasion an effect corresponding to 1.3 μg noradrenaline per 100 ml was found. Detectable amounts of catechol amines were not found in the lens or the vitreous body. The retina did not contain measurable quantities of noradrenaline or adrenaline (< 0.05 μg/g). Concentrated aqueous humour acquires a red colour on standing at alkaline reaction. The retina of cow and dog contains high amounts of substance P, up to 200 units per gram of dry organ.

It is frequently noted that the rise in blood pressure following single injections of noradrenaline in doses above 5-10 pg in the anaesthetized cat is followed by (1) a fall in blood pressure to subnormal levels, and (2) a temporarily diminished sensitivity to subsequent small doses of noradrenaline.It is common experience that the sudden withdrawal of a noradrenaline infusion in man is sometimes followed by a fall in blood pressure below the pre-infusional level, and this may necessitate a prompt recommencement of the infusion, often in a higher dosage than previously given.Although this secondary fall has been observed by severaI investigators there seems to be no generally accepted explanation of this phenomenon. BLACKET, PICKERING and WILSON (1950) have studied the effect of prolonged infusion of noradrenaline in the unanaesthetized rabbit. With doses not exceeding 3 pglkglmin. they found that the blood pressure in most animals had a tendency to fall and that, on termination of the infusion, there was an abrupt and severe drop in blood pressure, lasting for a day or more. The authors discuss various -possible mechanisms for the observed fall in blood pressure and consider a release of vasodilator substances the most likely explanation of the effect. Similar observations have been made earlier with adrenaline (BAINBRIDGE and TREVAN 1917, ERLANGER and GASSER 1919 and others).