Exercise hyperaemia is believed to be elicited by vasoactive metabolites released from the active skeletal muscle. In a previous study we found that small increases in extracellular potassium (K+; 1, 2 or 3 mM) cause profound relaxation of isolated gluteal muscle arteries. Since several vasoactive factors might interact during the hyperaemia response, we investigated the influence of a hyperosmotic environment on K+ induced relaxations.
Gluteal arteries (mean ± S.E.M.; diameter: 247.51 ± 5.78 µm) from humanely killed female Wistar rats were isolated and mounted in an organ bath filled with Krebs Ringer solution for isometric tension recording. After precontraction with norepinephrine (10-6 M), 1, 2 or 3 mM K+ was added in both control and hyperosmotic (HO, 60 mM sucrose) conditions. Endothelial removal and the addition of ouabain, BaCl2, NPPB (5-nitro-2-(3-phenylpropylamino) benzoic acid) or glibenclamide were used to study the underlying mechanisms. A two way repeated measures analysis with follow-up paired univariate tests was used to evaluate statistical significance. This study was approved by the local ethics committee of the Faculty of Medicine and Health Science (Ghent University).
The K+ induced relaxations were significantly (F = 251.05, P < 0.001, n = 6) increased in the presence of 60 mM sucrose (Control: K1: 18.22 ± 6.38; K2: 66.50 ± 8.55; K3: 52.07 ± 10.40; HO: K1: 62.97 ± 9.51, P < 0.05; K2: 92.38 ± 2.17, P < 0.05; K3: 93.56 ± 3.70, P < 0.05). Endothelial removal and the addition of the KATP blocker glibenclamide (10-5 M) or the Na+/K+ pump inhibitor ouabain (5 X 10-5 M) did not reduce the HO-induced increased sensitivity to K+. The application of the KIR-channel blocker BaCl2 (3 X 10-5 M) significantly (F = 1.02, ns, n = 6) abolished the influence of HO on the K+-induced relaxations (BaCl2: K1: 14.55 ± 6.10; K2: 14.08 ± 3.46; K3: 36.93 ± 5.87; BaCl2 + HO: K1: 18.84 ± 7.18; K2: 25.91 ± 10.93; K3: 40.22 ± 5.70). NPPB (10 µM), a volume-regulated anion channel (VRAC) -blocker, mimicked the influence of HO (F = 6.01, P < 0.05, n = 8) by increasing the K+ induced relaxations (Control: K1: 19.81 ± 5.82; K2: 33.39 ± 8.22; K3: 67.34 ± 11.18; NPPB: K1: 54.54 ± 10.62, P < 0.05; K2: 74.13 ± 8.57, P < 0.005; K3: 67.21 ± 10.60, ns).
In conclusion, HO increases the sensitivity of the rat gluteal skeletal muscle arteries to the vasodilating effect of K+. This is not due to activation of KATP-channels or the Na+/K+ pump. It is hypothesized that HO inhibits VRAC’s causing smooth muscle hyperpolarization. This possibly sensitizes the KIR channels which are known to be involved in the K+ induced relaxations in this type of artery.