Linolenic acid enhances contraction induced by phenylephrine in isolated rat aorta

This study examined the effect of linolenic acid on the contraction of isolated endothelium-intact and -denuded rat aorta induced by phenylephrine and its underlying mechanism. This was conducted in the presence or absence of N^W-nitro-L-arginine methyl ester (L-NAME), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), methylene blue, and calmidazolium. The effects of linolenic acid on contraction induced by calcium chloride in calcium-free Krebs solution containing 60 mM potassium chloride were also examined. Moreover, the effect of linolenic acid on the association between intracellular calcium level ([Ca²⁺]_i) and tension induced by phenylephrine was examined. Finally, we examined the effects of linolenic acid on cGMP formation and endothelial nitric oxide synthase (eNOS) phosphorylation induced by phenylephrine. Linolenic acid (5 × 10^-5 M) increased phenylephrine-induced contraction in endothelium-intact aorta (standardized mean difference [SMD] of log ED₅₀: 2.23), whereas it decreased this contraction in endothelium-denuded aorta (SMD: 1.96). L-NAME, ODQ, methylene blue, and calmidazolium increased phenylephrine-induced contraction in endothelium-intact aorta. Linolenic acid decreased contraction induced by calcium chloride in calcium-free Krebs solution containing 60 mM potassium chloride in endothelium-denuded aorta. Linolenic acid caused an increase in [Ca²⁺]_i (SMD at 3 × 10^-7 M phenylephrine: 1.63) and calcium sensitivity induced by phenylephrine in endothelium-intact aorta. Conversely, linolenic acid decreased [Ca²⁺]_i (SMD: 0.99) induced by phenylephrine in endothelium-denuded aorta. Linolenic acid decreased cGMP formation and eNOS phosphorylation induced by phenylephrine. These results suggest that linolenic acid increases phenylephrine-induced contraction, which is attributed to linolenic acid inhibition of endothelial NO release rather than its decrease of [Ca²⁺]_i in vascular smooth muscle.