1. Vasoactivity of human A- and B-calcitonin-gene-related peptides was studied in normal subjects, using reflectance plethysmography to measure relative changes in blood flow produced by superficial subcutaneous injections of the peptides.

2. Injection of 10−11 mol of either peptide caused an immediate 200% increase in local blood flow and prolonged hyperaemia lasting up to 3 h. The hyperaemic response to 10−13 mol of each peptide was significantly (P < 0.01) smaller and shorter in duration than that elicited by 10−1 mol of the same peptide, and 10−15 mol of both peptides produced no hyperaemia other than that attributable to needle insertion alone.

3. At all three dosages examined, there were no significant differences between A- and B-calcitonin-gene-related peptides in magnitude or time course of the hyperaemic response.

4. Human A- and B-calcitonin-gene-related peptides are therefore potent vasodilators in man, causing comparable dose-related vasodilatation in the superficial tissues.

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