Characteristics of ACh-induced hyperpolarization and relaxation in rabbit jugular vein

BACKGROUND AND PURPOSE The roles played by endothelium-derived NO and prostacyclin and by endothelial cell hyperpolarization in ACh-induced relaxation have been well characterized in arteries. However, the mechanisms underlying ACh-induced relaxation in veins remain to be fully clarified. EXPERIMENTAL APPROACH ACh-induced smooth muscle cell (SMC) hyperpolarization and relaxation were measured in endothelium-intact and -denuded preparations of rabbit jugular vein. KEY RESULTS In endothelium-intact preparations, ACh (≤10(-8) M) marginally increased the intracellular concentration of Ca(2+) ([Ca(2+) ](i) ) in endothelial cells but did not alter the SMC membrane potential. However, ACh (10(-10) -10(-8) M) induced a concentration-dependent relaxation during the contraction induced by PGF(2α) and this relaxation was blocked by the NO synthase inhibitor N(ω) -nitro-l-arginine. ACh (10(-8) -10(-6) M) concentration-dependently increased endothelial [Ca(2+) ](i) and induced SMC hyperpolarization and relaxation. These SMC responses were blocked in the combined presence of apamin [blocker of small-conductance Ca(2+) -activated K(+) (SK(Ca) , K(Ca) 2.3) channel], TRAM 34 [blocker of intermediate-conductance Ca(2+) -activated K(+) (IK(Ca) , K(Ca) 3.1) channel] and margatoxin [blocker of subfamily of voltage-gated K(+) (K(V) ) channel, K(V) 1]. CONCLUSIONS AND IMPLICATIONS In rabbit jugular vein, NO plays a primary role in endothelium-dependent relaxation at very low concentrations of ACh (10(-10) -10(-8) M). At higher concentrations, ACh (10(-8) -3 × 10(-6) M) induces SMC hyperpolarization through activation of endothelial IK(Ca) , K(V) 1 and (possibly) SK(Ca) channels and produces relaxation. These results imply that ACh regulates rabbit jugular vein tonus through activation of two endothelium-dependent regulatory mechanisms.