K⁺ has been shown to be an endothelium-derived hyperpolarizing factor (EDHF) in rat hepatic and mesenteric arteries (Edwards et al. 1998). However, a number of studies have failed to agree with this hypothesis and the identity of EDHF remains controversial. Previous data from this laboratory have provided circumstantial evidence that K⁺ is an EDHF in rat skeletal muscle small arteries (Bund et al. 2001). The purpose of the present study was to determine whether endogenous K⁺ contributes to the non-nitric oxide, non-prostanoid component of acetylcholine-mediated, endothelium-dependent relaxation and as such remains a candidate EDHF in rat skeletal muscle small arteries.

Male Wistar rats (294-420 g body weight) were killed by a stunning blow to the head followed by cervical dislocation. Small femoral artery branches (i.d. 225-319 µm) were dissected free post-mortem and mounted in a myograph for assessment of isometric tension development. The physiological saline solution was bubbled with 5 % CO₂ in air, pH 7.4. Arteries were precontracted with noradrenaline (1 µM) and challenged with acetylcholine (1 nM-10 µM) either in the combined presence of the cyclo-oxygenase inhibitor indomethacin (10 µM) and the nitric oxide synthase inhibitor L-NNA (0.1 mM), or in the presence of indomethacin and L-NNA plus charybdotoxin (100 nM) and apamin (100 nM) to inhibit calcium-activated potassium channels (K_Ca).

Maximum relaxations (%, mean ± S.E.M. (n)) were significantly reduced in arteries with K_Ca inhibition (58 ± 10 (4) vs. 88 ± 7 (8), P < 0.05, two-tailed t test). Relaxations across the concentration response relationships were also depressed during K_Ca inhibition (P < 0.01, repeated measures ANOVA). These data provide further functional evidence that endogenous K⁺ effluxing through K_Ca channels is an endothelium-derived hyperpolarizing factor in rat skeletal muscle small arteries.