Relaxation in Different-Sized Rat BloodVessels Mediated by Endothelium-Derived Hyperpolarizing Factor: Importance of Processes Mediating Precontractions

Abstract

To clarify the mechanisms involved in relaxations mediated by endothelium-derived hyperpolarizing factor (EDHF), acetylcholine (ACh)-induced endothelium-dependent relaxations and hyperpolarizations were examined in the rat aorta, the main branch of the mesenteric artery (MBMA) and the first branch of the mesenteric aftery (FBMA). In the presence of 100 μM N^G-nitro-L-arginine (L-NNA) and 10 μM indomethacin, ACh (1 nM to 100 μM) produced no relaxation in the phenylephrine-precontracted aorta. The L-NNA-resistant relaxations by ACh in MBMA precontracted with phenylephrine were eliminated in the presence of 1 μM nifedipine where contractions were independent of L-type Ca²⁺ channel activation. In FBMA precontracted with phenylephrine, the L-NNA-resistant relaxations were only partially inhibited by nifedipine. When vessels had been contracted with 300 nM phorbol-12,13-dibutyrate in the presence of nifedipine, ACh-induced L-NNA-resistant relaxations were observed in FBMA only. Pinacidil produced relaxations in all different-sized blood vessels, although sensitivity was inversely related to vessel size. The extent of the ACh hyperpolarizing responses was much smaller than that by pinacidil in the aorta. The membrane potential changes by ACh and pinacidil were almost the same in FBMA. These results indicate that the contribution of EDHF to endothelium-dependent relaxations increases as the vessel size decreases. This may be partly explained by precontractile processes dependent on Ca²⁺ entry through L-type Ca²⁺ channels, because Ca²⁺ channel deactivation seems to be involved as a major mechanism of EDHF-mediated vasorelaxations. However, EDHF may also generate vasorelaxations by an additional mechanism, probably a reduced Ca²⁺ sensitivity of contractile elements, as proposed for ATP-sensitive K⁺ channel openers.

Vasorelaxing Effect of Mesaconitine, an Alkaloid From Aconitum japonicum, on Rat Small Gastric Artery: Possible Involvement of Endothelium-Derived Hyperpolarizing Factor

“…From the present study, the relaxation mediated by NO (sensitive to L-NNA and indomethacin, but not to charybdotoxin and apamin) accounts for approximately 20% of the maximum relaxation by acetylcholine. This agrees with the idea that EDHF is the most prominent endothelial relaxing factor in small resistance arteries (4,5).…”

“…While prostacyclin is only involved in a limited number of preparations, the relative contributions of NO and EDHF is likely to be dependent on the size of the vessels; the contribution of NO is the largest in large-diameter vessels, whereas the contribution of EDHF increases with decreasing diameter of vessels, EDHF becoming the dominating relaxing factor in microvessels (4,5).…”

Urocortin Induces Endothelium-Dependent Vasodilatation and Hyperpolarization of Rat Mesenteric Arteries by Activating Ca2+-Activated K+ Channels

“…It is generally accepted that NO plays little role in response to endothelium-dependent vasodilators in resistance arteries (Hwa et al 1994;Tomioka et al 1999). Indeed, in this study urocortin-induced hyperpolarization and relaxation were not affected by the presence of N G -nitro-L-arginine (nitro-Larginine), which inhibits NOS activity.…”

The impact of baseline artery diameter on flow-mediated vasodilation: a comparison of brachial and radial artery responses to matched levels of shear stress

“…For example, it is possible that the endothelial cells of smaller vessels release more EDHF relative to larger arteries (28,55,56). Structural differences between differently sized vessels may also account for heterogeneity in the FMD responses (11,12,16,26,48,52).…”