Dynamics of the Skin Blood Flow Response to Histamine

In previous studies, we noticed that intradermal injection of histamine solutions might significantly complicate the interpretation of clinical data and of laser-Doppler flowmetry recordings (LDF). Therefore, we used the histamine dry skin prick test (HPT) for pharmacological studies. In this study, LDF monitoring of the physiological skin response to histamine was made in a randomized, placebo-controlled, double-blind study comparing the in vivo anti-H₁ activity of cetirizine (10 mg) or loratadine (10 mg), 6 h after a single oral intake. As compared with responses recorded after intake of placebo, LDF readings performed at HPT sites (increase in LDF signal) and at 1 cm from HPT (reduction of LDF signal) conclusively illustrate the stable and almost complete blockade of H₁ receptors by cetirizine. In vivo effects obtained with loratadine were considered weaker because (1) there was no significant influence of loratadine on the blood flow recorded at HPT sites, (2) a significant reduction at 1 cm from HPT sites was observed only after a lag phase (≥ 10 min) and (3) there were significantly higher skin perfusion levels at 1 cm from HPT sites between 6 and 10 min after the test under loratadine as compared with cetirizine. Hence, multipoint probing with LDF over time appears as a sensitive method for discriminating response profiles between two anti-H₁ agents. Furthermore, this is the first time that anti-H₁-related changes of the dynamics of expansion of the flare response have been demonstrated.