Stimulation-induced peripheral nerve hyperemia: mediation by fibers innervating vasa nervorum?

We studied the influence of graded nerve trunk stimulation on endoneurial blood flow (NBF), oxygen tension and pH in the rat sciatic nerve. The purpose of the investigation was to explore changes in the in vivo microenvironment of axons which result from their metabolic activation. Using endoneurial microelectrodes, we made serial measurements before, during and after varying stimulation protocols in curarized animals. NBF was increased by stimulation and the effect was directly related to the frequency, length and intensity of the stimulation train. At an intensity designed to supramaximally recruit myelinated fibers, high frequency (50 and 100 Hz) and prolonged duration (5-15 min) stimulation increased NBF. The effect was partly blocked by infiltration of lidocaine at the stimulating site. High intensity stimulation, designed to also recruit unmyelinated fibers, was particularly effective in enhancing NBF and lowering microvascular resistance. Continuous measurements of endoneurial oxygen and pH tension failed to identify associated declines with stimulation irrespective of the protocol. Both oxygen tension and pH tended to rise with stimulation. These studies confirm the presence of stimulation-induced endoneurial hyperemia but provide no evidence that local hypoxia or acidosis are mechanisms. Stimulation recruitment of vasodilatory axons innervating vasa nervorum may be an alternative explanation.