Combinations of the K⁺ channel inhibitors apamin and charybdotoxin (CHTx) (Waldron & Garland, 1994) and apamin and maurotoxin (MTx) (Hinton & Langton, 2001) abolish EDHF-mediated relaxation of rat isolated mesenteric arteries. The site of action of these toxins is on the endothelium (Doughty et al. 1998), suggesting that membrane hyperpolarisation is via opening of endothelial K⁺ channels in these arteries. Apamin inhibits small-conductance calcium-activated K⁺ (SK) channels, while CHTx and MTx inhibit intermediate-conductance calcium-activated K⁺ (IK) channels and some voltage-gated K⁺ channels. In the present study, we examined the identification and expression of IK and SK channels in rat small mesenteric arteries.

Male Wistar rats (200-250 g) were stunned and killed by cervical dislocation, mesenteric arteries of third and forth order branches were dissected free and cleaned of connective tissue. In isometrically mounted arteries contracted with phenylephrine (0.5-2 mM), acetylcholine (ACh, 0.01-10 mM) elicited concentration-dependent relaxation in the presence of N-nitro-L-arginine methyl ester (100 mM) and indomethacin (2.8 mM), inhibitors of NO synthase and cyclo-oxygenase, respectively. This relaxation was abolished by combinations of CHTx plus apamin, and MTx plus apamin (each, 50 nM; n = 6), application of the toxins alone had no effect. The scorpion toxin SK inhibitors, PO5 toxin and TS Kappa toxin (each, 50 nM; n = 6) alone had no significant effects, and in combination with either CHTx or MTX (each 50 nM, n = 6) there was no further effect on EDHF-mediated relaxation.

Western blotting of arterial homogenates using anti-SK2 and SK3 (Alomone Laboratories) demonstrated several bands, none at the predicted molecular weight for SK2 or SK3. Subsequent immunoconfocal microscopy of arterial sections using these antibodies demonstrated diffuse and inconclusive antibody staining throughout the arterial wall.

These data demonstrate the involvement of IK channels in EDHF-mediated relaxation of rat isolated mesenteric arteries. However, the identity and role of endothelial SK channels in these arteries remains unclear.

We thank the BHF for financial support (PG/99/70).

All procedures accord with current UK legislation.