Distinct endothelium-derived hyperpolarizing factors emerge in vitro and in vivo and are mediated in part via connexin 40-dependent myoendothelial coupling

The endothelium-derived hyperpolarizing factor (EDHF) contributes critically to the regulation of vascular tone. Its dependency on direct signaling through myoendothelial gap junctions composed of connexins (Cx) is controversially discussed. We studied the impact of Cx40^-/- in EDHF-type dilations in vivo and in vitro (wire and pressure myography) in small arteries (A. gracilis) using different Cx40^-/--deficient mouse models. Acetylcholine induced prominent EDHF-type dilations (inhibition of NO synthase and cyclooxygenase) of 90%(maximum effect) in wild-type and Cx40-deficient vessels (Cx40) in vitro under isobaric conditions. In contrast, under isometric conditions, EDHF-type relaxations were nearly abrogated in Cx40 (9±3%) but only slightly reduced in wild-type vessels (45±4%; P<0.05). Vessels expressing Cx45 instead of Cx40 exhibited similarly reduced relaxations (13±1%), demonstrating that Cx45 cannot replace Cx40 functionally. The necessity of Cx40 in EDHF-type dilations under isometric conditions was verified by the attenuation in vessels being specifically deficient for Cx40 in endothelial cells (Cx40:TIE2-Cre: 17±3%; Cx40-floxed controls: 67±6%; P<0.05). Nevertheless, EDHF-type dilations were Cx40 independent when studied isobarically. The EDHF-type dilation in vivo resembled the isobaric situation, being virtually Cx40 independent and similar powerful. Distinct EDHF mechanisms can be distinguished by their Cx40 dependency. A powerful EDHF is present in vivo and in vitro under isobaric conditions but is lacking in wire myography (isometric conditions). Herein, a less potent EDHF depends on Cx40 and may represent signaling through myoendothelial gap junctions. We suggest that distinct EDHFs (even in the same artery) explain partially the controversy on the role of myoendothelial gap junctions in EDHF signaling.