Axon reflex vasodilatation in human skin measured by a laser Doppler technique

This study was designed to investigate whether or not an objective method to measure a blood flow value by laser Doppler technique is possible to study the mechanism of an axon reflex vasodilatation elicited by i.d. injection of histamine or substance P into human skin. The probe of laser Doppler flowmeter was placed 10 mm apart from the injection site in the ventral forearm. The blood flow increased after the i.d. injection of histamine, substance P, but not after injection of Ringer solution. Penetration by the hypodermic needle produced an increase in the blood flow, but this increase was masked by the injection of the Ringer solution. The histamine- or substance P-induced blood flow increase was dose-dependent in both duration and intensity. Pretreatment with diphenhydramine ointment or xylocaine jelly 1 h prior to i.d. injection of histamine or substance P significantly attenuated the blood flow increase elicited by either vasoactive substance. Pretreatment of the skin with capsaicin (1% alcoholic solution) for 2 or 3 days markedly reduced the histamine- or substance P-induced blood flow increase. These results show that the measurement of cutaneous blood flow by laser Doppler flowmeter is useful in studying the mechanism of vasoactive substances-induced axon reflex vasodilatation.