Differential sensitivity to hydroxyl radicals of pre- and postjunctional neurovascular transmission in the isolated canine mesenteric vein

Abstract

In some pathophysiological conditions, the first target of reactive oxygen intermediates is the vascular system. Superoxide anions, when generated in the vascular circulation, may then escape into the extracellular space via an anion channel and, following dismutation to hydrogen peroxide (H 2O 2), form hydroxyl radicals (HO •). In an attempt to understand the role of HO • in the regulation of transmission at the sympathetic neurovascular junction, the effect of HO • at nerve terminals was examined by measuring the amount of noradrenaline (NA) released from isolated, spirally cut, superfused canine mesenteric vein during basal and electrical stimulation (ES; 5 Hz, 2 ms, 9 V); tension development evoked by ES was also recorded simultaneously. HO • was generated from Fenton's reagent (1.5×10 −4 M H 2O 2 plus 10 −4 M FeSO 4); generation of HO • from H 2O 2/FeSO 4 in the superfusate was monitored by electron spin resonance spectroscopy using the spin-trap 5,5-dimethyl-1-pyrroline- N-oxide throughout the experimental time course. Exposure to HO • of the helical strips produced an irreversible decrease in tension development evoked by ES with no effect on NA release, suggesting that the observed effect is elicited postjunctionally. The susceptibility of the processes of NA-mediated contraction to HO • may differ greatly from that of the NA release mechanism at the prejunctional site. Exposure of the strip preparation to HO • leads to a substantial stimulation of basal release of NA without affecting ES-evoked NA release, possibly due to enhanced non-exocytotic Ca 2+-independent release elicited by HO •. A direct demonstration of this concept was obtained by showing a significant increase in the basal response of NA release in Ca 2+-free solution. The major conclusion of the present study is that HO • can damage NA-mediated contraction of the vascular preparations at the postjunctional site, and may selectively induce a non-exocytotic release of NA from the prejunctional site of sympathetic neurotransmission.