AHEART March 47/3

Huang, An, Dong Sun, Carolyn J. Smith, Joseph A. Connetta, Edward G. Shesely, Akos Koller, and Gabor Kaley. In eNOS knockout mice skeletal muscle arteriolar dilation to acetylcholine is mediated by EDHF. Am. J. Physiol. Heart Circ. Physiol. 278: H762–H768, 2000.—The mechanisms that account for acetylcholine (ACh)-induced responses of skeletal muscle arterioles of mice lacking endothelial nitric oxide (NO) synthase (eNOS-KO) were investigated. Isolated, cannulated, and pressurized arterioles of gracilis muscle from male eNOS-KO (74.1 6 2.3 μm) and wild-type (WT, 87.2 6 2.1 μm) mice developed spontaneous tone accounting for 63 and 61% of their passive diameter (116.8 6 3.4 vs. 143.2 6 2.8 μm, respectively) and dilated dose-dependently to ACh (1029-1027 M). These dilations were significantly smaller in vessels of eNOS-KO compared with WT mice (29.2 6 2.0 μm vs. 46.3 6 2.1 μm, at maximum concentration) but responses to the NO donor, sodium nitrite (NaNO2, 1026-3 3 1025 M), were comparable in the vessels of the two strains. NG-nitro-L-arginine (L-NNA, 1024 M), an inhibitor of eNOS, inhibited AChinduced dilations by 60–90% in arterioles of WT mice but did not affect responses in those of eNOS-KO mice. In arterioles of eNOS-KO mice, dilations to ACh were not affected by indomethacin but were essentially abolished by inhibitors of cytochrome P-450, clotrimazole (CTZ, 2 3 1026 M) or miconazole (MCZ, 2 3 1026 M), as well as by either high K1 (40 mM) or iberiotoxin [1027 M, a blocker of Ca21-dependent K1 channels (KCa channels)]. On the other hand, in WT arterioles CTZ or MCZ inhibited ACh-induced dilations only by ,10% and only in the presence of L-NNA. These results indicate that in arterioles of eNOS-KO mice, endothelium-derived hyperpolarizing factor (EDHF), synthesized via cytochrome P-450, accounts entirely for the mediation of ACh-induced dilation via an increase in KCa-channel activity. In contrast, in arterioles of WT mice, endothelium-derived NO predominantly mediates ACh-induced dilation in which participation of EDHF becomes apparent only after inhibition of NO synthesis.