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

Abstract

SUMMARY1. 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 763 % 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 (EC,50) 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.

Influence of perivascular peptides on endoneurial blood flow and microvascular resistance in the sciatic nerve of the rat.

“…As in previous work Day, Lagerlund & Low, 1989 Han et al 1990). Several reasons justified higher doses than those employed in direct pharmacological studies of excised and perfused vessel segments (Edvinsson, Ekman, Jansen, Ottosson & Uddman, 1987): (i) capsaicin releases local SP in concentrations greater than 10-8 M (Moskowitz, Brody & Liu-Chen, 1983); (ii) the epineurial arteriolar plexus has varying degrees of connective tissue overlying it, which we did not disrupt; (iii) direct application of lower doses of peptide were not effective, probably because of limited penetration to the site of vasoactive action; (iv) in similar published experiments considerably higher doses of epineurial noradrenaline were required to mimic the vasoconstrictor effects of local intra-arterial infusion of noradrenaline (Kihara & Low, 1990;; (v) the vessels were not pre-contracted, as in experiments on excised vessels (Edvinsson et at. 1987 hCGRP (8-37) was applied 2 min before capsaicin administration (before the clearance curve) to study SP and CGRP antagonism respectively.…”

“…The use of epineurial pharmacological agents allows the study of smaller vessels and provides information about the influence of epineurial arterioles on the endoneurial flow compartment. This experimental strategy has been previously employed in studies of adrenergic epineurial microvessel vasoconstriction (Kihara & Low, 1990).…”

“…Further, in each animal only three to four curves can be obtained (excluding the first) D. W. ZOCHODNE AND L. T. HO which limits pharmacological dose-response data that can be acquired. Also, the normal connective tissue that covers portions of the epineurial plexus means that high doses of pharmacological agents must be applied (as in previously published work, Kihara & Low, 1990) with the possibility of inducing complicating systemic effects. Despite these potential limitations, this approach probably predicts 'in vivo' microvessel behaviour better than 'in vitro' study of excised vessel segments.…”

“…Noradrenergic innervation of vasa nervorum is predominantly epineurial and perineurial, and has 'tonic' activity (Rechthand et al 1986;Zochodne, Huang, Ward & Low, 1990). There is evidence that epineurial adrenergic a-receptors determine 'downstream' endoneurial flow and that some segments of epineurial arterioles are particularly sensitive to applied noradrenaline Kihara & Low, 1990). However, peptidergic innervation could also be important in allowing neurogenic dilatation of epineurial/perineurial 'feeding' arterioles, and may provide 'tonic' dilatation as well.…”

High oxygen tension constricts epineurial arterioles of the rat sciatic nerve via reactive oxygen species

“…Thus exogenous application of vasoactive physiological substances onto sciatic nerves strongly affects local blood flow (16,17,46). In the present study, superfusion of NE onto sciatic nerves of rats caused a dose-dependent constriction of the epineurial arterioles.…”