[en] Isolated strips of rabbit aorta in a state of drug-induced contraction relax reversibly when irradiated with ultraviolet light. The authors previously found that the magnitude of the photorelaxation progressively diminished as the extracellular Na⁺ ion concentration was reduced from 145 mM to 85 mM. At 85 mM Na⁺, there was minimal photorelaxation, even though the preparation continued to respond to vasoconstricting agents. The reduction in photosensitivity is not an osmotic effect because restoration of osmolarity did not restore photosensitivity. Neither the mechanism underlaying photorelaxation nor its modification by Na⁺ is precisely known. In order to examine these further the authors measured cyclic GMP levels in the absence and presence of UV light at both normal and reduced Na⁺ levels. At 145 mM Na⁺, irradiation resulted in an increase of cGMP from 0.299 to 0.717 fmole/μg protein. At 85 mM Na⁺, the corresponding levels were 0.541 and 1.24 fmole/μg protein. Thus, cGMP levels increase (approximately double) with UV irradiation at both reduced and normal Na⁺ concentrations even though there is little or no photorelaxation in the reduced Na⁺ environment. The reduction in Na⁺ may uncouple a link between cGMP elevation and cytoplasmic calcium in the aortic cell