Minimal role for H1 and H2 histamine receptors in cutaneous thermal hyperemia to local heating in humans

The precise mechanism(s) underlying the thermal hyperemic response to local heating of human skin are not fully understood. The purpose of this study was to investigate a potential role for H-1 and H-2 histamine-receptor activation in this response. Two groups of six subjects participated in two separate protocols and were instrumented with three microdialysis fibers on the ventral forearm. In both protocols, sites were randomly assigned to receive one of three treatments. In protocol 1, sites received 1) 500 mu M pyrilamine maleate (H-1-receptor antagonist), 2) 10 mM L-NAME to inhibit nitric oxide synthase, and 3) 500 mu M pyrilamine with 10 mM N-G-nitro-L-arginine methyl ester (L-NAME). In protocol 2, sites received 1) 2 mM cimetidine (H-2 antagonist), 2) 10 mM L- NAME, and 3) 2 mM cimetidine with 10 mM L- NAME. A fourth site served as a control site ( no microdialysis fiber). Skin sites were locally heated from a baseline of 33 to 42 degrees C at a rate of 0.5 degrees C/5s, and skin blood flow was monitored using laser-Doppler flowmetry (LDF). Cutaneous vascular conductance was calculated as LDF/mean arterial pressure. To normalize skin blood flow to maximal vasodilation, microdialysis sites were perfused with 28 mM sodium nitroprusside, and control sites were heated to 43 degrees C. In both H-1 and H-2 antagonist studies, no differences in initial peak or secondary plateau phase were observed between control and histamine-receptor antagonist only sites or between L- NAME and L- NAME with histamine receptor antagonist. There were no differences in nadir response between L- NAME and L- NAME with histamine-receptor antagonist. However, the nadir response in H-1 antagonist sites was significantly reduced compared with control sites, but there was no effect of H-2 antagonist on the nadir response. These data suggest only a modest role for H-1-receptor activation in the cutaneous response to local heating as evidenced by a diminished nadir response and no role for H-2-receptor activation.