Epoxyeicosatrienoic acids activate a high‐conductance, Ca2+‐dependent K+ channel on pig coronary artery endothelial cells

Abstract

Epoxyeicosatrienoic acids (EETs) have been described as endothelium‐derived hyper‐polarizing factors (EDHFs), based on their stimulatory effects on smooth muscle K⁺channels. In order to reveal a putative autocrine effect of EETs on endothelial channels, we have studied the effects of the four EET regioisomers (5,6‐EET, 8,9‐EET, 11,12‐EET and 14,15‐EET) on the high‐conductance, Ca²⁺‐dependent K⁺ (BK_Ca) channel recorded in inside‐out patches of primary cultured pig coronary artery endothelial cells. Currents were recorded in the presence of either 500 nm or 1 μm free Ca²⁺ on the cytosolic side of the membrane. In 81% of experiments, EETs at < 156 nm, applied on the cytosolic side of the membrane, transiently increased BK_Ca channel open state probability (P₀) without affecting its unitary conductance, thus providing evidence for direct action of EETs, without involvement of a cytosolic transduction pathway. The four EET regioisomers appeared to be equally active, multiplying the BK_Ca channel P_o by a mean factor of 4.3 ± 0.6 (n= 15), and involving an increase in the number and duration of openings. The EET‐induced increase in BK_Ca channel activity was more pronounced with low initial P_o. When the BK_Ca channel was activated by 500 nm Ca²⁺, application of EETs increased the initial P_o value of below 0.1 by a factor of 5. When the channel was activated by 1 μm Ca²⁺, application of EETs increased the initial P_o value by a factor of 3. Our results show that EETs potentiate endothelial BK_Ca channel activation by Ca²⁺. The autocrine action of EETs on endothelial cells, which occurs in the same concentration range as their action on muscle cells, should therefore fully participate in the vasoactive effects of EETs, and thus be taken into account when considering their putative EDHF function.