Palmar digital vessel relaxation in healthy horses and in horses given carbohydrate

Objective-To compare in vitro smooth muscle relaxation of palmar digital ve ssels from healthy horses with those from horses in the prodromal stage of experimentally (carbohydrate) induced laminitis. Animals-16 adult horses. Procedure-Segments of palmar digital vessels were obtained from 5 healthy h orses and 6 horses given carbohydrate. Vascular rings from the palmar digit al artery and vein were suspended in individual organ baths containing buff er solution and indomethacin; isometric tension was recorded, and contracti on and relaxation were compared. Smooth muscle contraction in response to c umulative addition of phenylephrine was recorded in the absence and presenc e of 1 mu M N-G-nitro-L-arginine methyl ester L-NAME). After wash out, vasc ular rings were preconstricted with phenylephrine (0.3 mu M), and cumulativ e endothelium-dependent (acetylcholine-induced) and independent (nitropruss ide-induced) smooth muscle relaxations were recorded in the absence or pres ence of L-NAME. Results-Phenylephrine increased vascular smooth muscle tone in ring prepara tions of palmar digital arteries and veins. Addition of acetylcholine or ni troprusside induced relaxation of palmar digital artery and vein ring prepa rations. Use of L-NAME (1 mu M) significantly reduced maximal relaxation in duced by acetylcholine, but not by nitroprusside. Maximal relaxation induce d by acetylcholine, but not by nitroprusside, was reduced in vascular rings prepared from carbohydrate-overloaded horses. Conclusion and Clinical Relevance-Reduced endothelium-dependent relaxation of palmar digital vessels may have a role in the pathophysiology of acute l aminitis after carbohydrate overload in horses.