Sciatic nerve blood flow response to carbon dioxide

Sciatic nerve blood flow (NBF) during hypercarbia was examined in unanesthetized decerebrate rats by means of laser Doppler flowmetry (LDF). During inspiration of gas mixtures containing no CO2, followed by either 5, 10 or 20% CO2, arterial pCO2 increased by 13, 18 and 68 mm Hg, respectively. Blood pressure (BP) and the LDF signal, which were measured continuously, increased for 30-40 s following the start of inhalation of CO2 and then decreased. Three min after the start of inhalation of 5 or 10% CO2, BP had returned to the baseline and the LDF signal was increased by 14 and 15%, respectively, compared with preinhalation values. In rats inspiring 20% CO2, systemic BP remained elevated 12% above the baseline and NBF was increased by 18%. The results indicate that dilatation of the vasa nervorum during hypercarbia is less than that at cerebral blood vessels. The nerve vasculature dilates maximally in response to 5% CO2, with a rise in NBF of about 1.1% per mm Hg increase in paCO2.