Upregulation of intermediate calcium-activated potassium channels counterbalance the impaired endothelium-dependent vasodilation in stroke-prone spontaneously hypertensive rats

Endothelial dysfunction has been linked to a decrease in nitric oxide (NO) bioavailability and attenuated endothelium-derived hyperpolarizing factor (EDHF)-mediated relaxation. The small (SK_Ca) and intermediate (IK_Ca) calcium-activated potassium channels play a key role in endothelium-dependent relaxation. Because the repressor element 1-silencing transcription factor (REST) negatively regulates IK_Ca expression, we hypothesized that augmented REST and decreased IK_Ca expression contributes to impaired endothelium-dependent vasodilation associated with hypertension. Acetylcholine (ACh) responses were slightly decreased in small mesenteric arteries from male stroke-prone spontaneously hypertensive rats (SHRSPs) versus arteries from Wistar Kyoto (WKY) rats. Incubation with N-nitro-L-arginine methyl ester (L-NAME; 100μmol/L) and indomethacin (100μmol/L) greatly impaired ACh responses in vessels from SHRSP. Iberiotoxin (0.1μmol/L), which is a selective inhibitor of large-conductance K_Ca (BK_Ca) channels, did not modify EDHF-mediated vasodilation in SHRSP or WKY. UCL-1684 (0.1μmol/L), which is a selective inhibitor of SKCa channels, almost abolished EDHF-mediated vasodilation in WKY and decreased relaxation in SHRSP. 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34; 10μmol/L) and charybdotoxin (0.1μmol/L), which are both IKCa inhibitors, produced a small decrease of EDHF relaxation in WKY but completely abrogated EDHF vasodilation in SHRSP. EDHF-mediated relaxant responses were completely abolished in both groups by simultaneous treatment with UCL-1684 and TRAM-34 or charybdotoxin. Relaxation to SK_Ca/IK_Ca channels agonist NS-309 was decreased in SHRSP arteries. The expression of SK_Ca was decreased, whereas IK_Ca was increased in SHRSP mesenteric arteries. REST expression was reduced in arteries from SHRSP. Vessels incubated with TRAM-34 (10μmol/L) for 24h displayed reduced REST expression and demonstrated no differences in IK_Ca. In conclusion, IK_Ca channel upregulation, via decreased REST, seems to compensate deficient activity of SK_Ca channels in the vasculature of spontaneously hypertensive rats.

Abbreviations: Ach, acetylcholine, BK_Ca, large-conductance calcium-activated potassium channels, EDHF, endothelium-derived hyperpolarizing factor, E_max, maximum effect elicited by the agonist, IK_Ca, intermediate calcium-activated potassium channels, K_Ca, calcium-activated potassium channels, L-NAME, N-nitro-L-arginine methyl ester, NO, nitric oxide, pD₂, negative logarithm of the molar concentration of agonist producing 50% of the maximum response, PGI₂, prostacyclin, PSS, physiologic salt solution, REST, repressor element 1-silencing transcription factor, SHRSP, stroke-prone spontaneously hypertensive rat, SK_Ca, small calcium-activated potassium channels, TRAM-34, 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole, WKY, Wistar Kyoto