A B S T R A C T Morphologic observations suggest that the inner layers of the thoracic aorta in man and dog are avascular and the outer layers have vasa vasorum. It appears that vasa vasorum are essential in the thoracic aorta because their interruption produces medial necrosis. These experiments provide the first measurements of blood flow through aortic vasa vasorum and examine physiologic regulation of that flow.During control conditions the outer two-thirds of the media of the thoracic aorta received 10 ml/min per 100 g blood flow through vasa vasorum. Flow to the inner third of the aorta was 1 ml/min per 100 g. Flow to both the inner and outer media of the abdominal aorta was less than 1 ml/min per 100 g. Adenosine increased blood flow to vasa vasorum in the outer media of the thoracic aorta from 7 to 18 ml/min per 100 g, but did not increase flow to the inner layers of the aorta. Hemorrhagic hypotension decreased flow in the outer media of the thoracic aorta from 14 to 2 ml/min per 100 g. Acute hypertension failed to increase blood flow through vasa vasorum, as conductance decreased significantly.These studies indicate that vasa vasorum provide a considerable amount of blood flow to the outer layers of'the thoracic aorta. The vessels are responsive to physiologic stimuli because they dilate during infusion of adenosine and constrict during both hemorrhagic hypotension and acute hypertension. We speculate that the f'ailure of blood flow to the aortic wall to increase during acute hypertension might, if it were suistained, contribute to aortic medial necrosis.

Exposure of the skin of rats to u.v. light (>295 nm) for 30 s or longer elicited a delayed erythema response, the rate of onset increasing with the period of irradiation. The erythema was still present at 24 h and was replaced by scab formation in 48 hours. Both topically applied steroidal and non‐steroidal anti‐inflammatory drugs reduced the erythema formation when administered immediately after u.v. exposure. Propyl gallate, an antioxidant with sun screening properties in man, also possessed topical anti‐erythemic activity. Both steroidal and non‐steroidal anti‐inflammatory drugs, systemically administered 1 h before u.v. exposure, reduced the erythema. However, the steroidal compounds were less effective than the nonsteroids and reduced the intensity of erythema by less than 50%. Antagonists of 5‐hydroxytryptamine (5‐HT) reduced the erythema but several other drugs with different pharmacological activities were ineffective. Neither topical nor systemic treatments of any of the drugs examined suppressed the scab formation at 48 hours. These results and those using other selective blocking agents indicate that in the mediation of the erythema reaction prostaglandins may play a major role and 5‐HT perhaps a minor one but that H1 histamine receptors and α‐ and β‐adrenoceptors have no significant role.

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