The effect of lowered extracellular Na+ concentration on ultraviolet light-induced relaxation of vasoconstricted rabbit isolated thoracic aorta

The effect of lowering extracellular ion concentration on ultraviolet (UV) light-induced photorelaxation of norepinephrine(NE)-constricted rabbit isolated thoracic aorta was investigated. The magnitude of the photorelaxation response (similar to acetylcholine-induced, but not nitroprusside-induced, relaxation) progressively declined, in the absence of an effect on NE-induced vasoconstriction, as the total extracellular ion concentration was progressively reduced. This diminution in the photorelaxation response was duplicated by isosmotic lowering of the extracellular concentration of Na+, but not other ions, from 145 to 25 mM and was not restored by the replenishment of the Na+ deficiency by equimolar amounts of mannitol or Li+. In contrast, choline fully substituted for Na+. These findings suggest a fundamental difference in the ion dependency (and, hence, the mechanisms) of UV-induced photorelaxation and the vasorelaxations induced by acetylcholine or sodium nitroprusside.