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1. The function of angiotensin converting enzyme was investigated in twenty-four healthy men. Forearm blood flow was measured under basal conditions and during administration of enalaprilat (a converting enzyme inhibitor) and/or peptide substrates of converting enzyme into the left brachial artery. Blood flow was compared in the two arms. 2. Enalaprilat had no effect on basal blood flow. The concentration of enalaprilat in venous blood from the control arm was low, and plasma renin activity was not increased, indicating that systemic inhibition of converting enzyme did not occur. 3. Effects of angiotensin and of bradykinin, administered intra-arterially, were limited to the infused arm. Enalaprilat (13 nmol min-1) inhibited converting enzyme in the infused arm, in which it caused approximately a 100-fold reduction in sensitivity to angiotensin I, while having no effect on the vasoconstriction caused by angiotensin II. Enalaprilat increased vasodilatation caused by bradykinin. 4. Aspirin, an inhibitor of cyclo-oxygenase, did not inhibit vasodilatation caused by bradykinin whether infused alone or with enalaprilat, indicating that these responses are not mediated by prostaglandins. 5. We conclude that under basal conditions neither conversion of angiotensin I to angiotensin II nor degradation of bradykinin determines resistance vessel tone in the human forearm. Converting enzyme may affect vascular tone in situations in which intravascular concentrations of peptides are increased over those present under basal conditions.

A method is described for the quantitative study of the life cycle of microaneurysms in diabetic retinopathy. This method was applied to a series of 21 patients, 9 with mild and 12 with severe retinopathy, 6 of whom had pituitary ablation. — The study showed a variable survival of microaneurysms; overall disappearance rates were 3.3%/month in mild and 3.2 %/month in severe retinopathy. The disappearance rate was almost always higher in the early months of the study (11.6% and 9.3 % per month) than later (0.8 % and 1 %/month) indicating more than one population of microaneurysms. — There was no significant difference in the formation and disappearance of microaneurysms between patients with mild and severe retinopathy. — Some microaneurysms showed intermittent perfusion. — Following pituitary ablation the disappearance rate of microaneurysms increased, while the formation rate of new ones decreased significantly.

1. Clonidine (1 μg/kg), given by intracisternal injection to anaesthetized rabbits, lowered mean arterial blood pressure by 33 mmHg and heart rate by 32 beats/min. 2. In animals pre-treated with 6-hydroxydopamine (6-OHD 500 μg/kg intracisternally) 7-10 days before, intracisternal clonidine (1 μg/kg) reduced mean arterial blood pressure by only 2·5 mmHg and heart rate by 4 beats/minute. 3. The hypotensive action of intravenous clonidine was reduced to 49% of control by pre-treatment with intracisternal 6-OHD. In unanaesthetized normal animals intravenous clonidine (30 μg/kg) lowered mean arterial blood pressure by 19·3 mmHg, while after 6-OHD it fell only 9·4 mmHg. 4. These studies suggest that the central hypotensive effect of clonidine is dependent on the integrity of central monoaminergic neurones.