Vascular effects of singlet oxygen (¹O₂) generated by photo-excitation on adrenergic neurotransmission in isolated rabbit mesenteric vein

Abstract

It has been suggested that reactive oxygen species (ROS) may be involved in the regulation of vascular tone. However, the nature of ROS effects on vascular sensitivity remains to be elucidated. The present study was designed to investigate the effects of ROS, especially ¹O₂, on neurotransmission at the sympathetic neurovascular junction. Basal noradrenaline (NA) release, release of NA induced by electrical stimulation (ES), and resting NA release at the sympathetic nerve terminals were determined using a superfusion technique. The amount of NA was determined by HPLC; isometric tension changes evoked by ES were also recorded simultaneously. ¹O₂ was generated from Rose Bengal by photo-activation. The generation of ¹O₂ in the superfusate was monitored by electron spin resonance (ESR) using the spin trap 2,2,6,6-tetramethyl-4-piperidinol throughout the experimental time course. The ESR results confirmed that ¹O₂ was generated by photo-activation of Rose Bengal via the formation of 2,2,6,6-tetramethyl-4-hydroxyl-piperidinyloxy. Exposure of helical strips of rabbit mesenteric vein to ¹O₂ induced a significant increase in tension and NA release during the basal period, but had no effect on ES-induced release. L-histidine, an ¹O₂ scavenger, significantly inhibited the observed effects on vascular tension and NA release in response to ¹O₂. These results suggest that ¹O₂ may induce NA-mediated vasoconstriction at the postjunctional site, and may be associated with Ca²⁺-independent NA release from the prejunctional site of adrenergic neurotransmission.