Endothelial hydrogen peroxide (H₂O₂) generated through eNOS activation or shear stress is believed to play an important role in vasorelaxation as an endothelium-derived hyperpolarising factor (EDHF) in mouse and human mesenteric arteries. In these arteries, exogenous H₂O₂ caused a dose-dependent relaxation which was inhibited by the combination of charybdotoxin and apamin, and blocked by catalase or a high potassium solution (Matoba et al. (2000); Miura et al. (2003)). We assessed whether H₂O₂ had similar effects in the rat mesenteric artery.

Male Wistar rats (250-275g) were killed humanely by cervical dislocation and small resistance mesenteric arteries (i.d. 200-300 mm) were isolated and mounted on a Mulvany-Halpern small vessel wire myograph for measurement of isometric contraction. Results are expressed as mean percentage relaxation ± S.E.M. and statistical significance (P < 0.05) was performed using unpaired Student’s t test.

H₂O₂ (10 nM-1 mM) caused a dose dependent relaxation that was present in both endothelium intact and denuded arteries. 100 µM H₂O₂ consistently gave rise to a full relaxation of the tissue, and this concentration was therefore used for subsequent experiments. This relaxation was almost completely blocked by the combination of charybdotoxin (100 nM) and apamin (100 nM) (n = 3), while partial blockage occurred when these toxins were applied separately in endothelium intact arteries (32 % ± 6 and 46 % ± 6 blockage for charybdotoxin and apamin, respectively, n = 5). However, the combination of apamin and charybdotoxin had no effect in endothelium denuded arteries. The large conductance calcium-activated potassium (BK_ca) channel inhibitor iberiotoxin (100 nM) also inhibited the hydrogen peroxide response in intact arteries (73 % ± 5 inhibition, n = 4, P < 0.001). The application of pertussis toxin (400 ngml^-1) had no effect on the vasodilatation to H₂O₂ indicating that C-type natriuretic peptide (CNP) is not involved (Chauhan et al. 2003); pertussis toxin did however abolish the response to acetylcholine.

In conclusion, we found that hydrogen peroxide causes vasodilatation which appears to involve multiple types of K_ca channels present on both the endothelium and smooth muscle. However, relaxation can also occur in the absence of the endothelium, suggesting that hyperpolarisation of smooth muscle via BK_ca channels is the primary mechanism for hydrogen peroxide induced-relaxation.

This work was supported by the British Heart Foundation