Anodal current intensities above 40 microA interfere with current-induced axon-reflex vasodilatation in human skin

When using iontophoresis, the 'non-specific' vasodilatation (NSV) that is observed as a result of C-fibre excitation is generally attributed to the local accumulation of protons under the anode. NSV following prolonged 100-microA anodal current application only appears after the current is stopped. Break excitation alone does not explain the delayed onset of this vasodilatation. We hypothesised that this delay could result from an anodal block and thus, that a minimal intensity would be required to achieve hyperpolarisation of primary afferent fibres (mainly C-fibres). Using laser Doppler flowmetry, cutaneous blood flow was recorded in the forearms of 8 healthy volunteers 2 min before current application, during the application and 20 min after stopping the monopolar anodal current. In protocol 1, after 2.5 min of current application at an intensity of 100 microA, the intensity was abruptly decreased to 0-80 microA for a second 2.5-min period. The onset of vasodilatation was only delayed at intensities >30 microA during this second period. In protocol 2, re-application of the current after a 50-second interruption (expected to allow for the occurrence of an axon reflex) did not interfere with the onset of vasodilatation. Thus: (1) the minimal intensity interfering with the axon reflex is far lower than that reported for C-fibre blockade in isolated nerves; (2) the results suggest that current application does not directly interfere with the vasodilator mechanisms induced by the axon reflex at the level of smooth muscle cells.