Regulation of rat nerve blood flow: role of epineurial alpha-receptors.

1. Transperineurial arterioles connect the extrinsic (epineurial) and the intrinsic (endoneurial) microvasculatures. Our goal was to determine whether the extrinsic system regulated nerve blood flow locally and whether subperineurial and centrifascicular endoneurial nerve blood flows were regulated differentially. 2. The local application of noradrenaline resulted in a dose-dependent reduction of nerve blood flow in subjacent endoneurium. Asymptotes of 78.8 and 76.3% vasoconstriction were recorded for subperineurial and centrifascicular endoneurial nerve blood flow, respectively, indicating near-complete closure of capillaries. 3. Near-identical concentrations required to generate 50% vasoconstriction (EC50) and asymptotes are suggestive of the fact that the two areas are not differentially regulated. 4. Local vasoconstriction cannot be due to a systemic effect of noradrenaline since a significant decrease in nerve blood flow occurs despite undetectable increases in plasma noradrenaline and mean blood pressure, or decrease in contralateral sciatic nerve blood flow. 5. There are small and statistically non-significant reductions of the compound muscle action potential and conduction velocity in the sciatic-tibial nerve following nerve ischaemia. 6. These findings suggest that epineurial arterioles control regional nerve blood flow and are primarily responsible for its regulation in subjacent endoneurial tissue.