In the systemic circulation, epoxyeicosatrienoic acids (EETs) act as endothelium-derived hyperpolarizing factors by stimulating TRPC6 channel translocation to the membrane and the subsequent activation of Ca²⁺-activated K⁺ channels. In the pulmonary circulation however, acute hypoxic vasoconstriction has been linked to the activation of a cytochrome P450 epoxygenase and to the inhibition, rather than the activation, of large conductance Ca²⁺-activated K⁺ (BK_Ca) channels. To address this apparent controversy we assessed the consequences of the deletion of the ß1 subunit of the BK_Ca channel on pulmonary responsiveness to 11,12-EET as well as to acute hypoxia in the isolated, buffer-perfused mouse lung.

In lungs from wild-type mice, authentic 11,12-EET (0.01–3mM) increased pulmonary artery pressure in a concentration-dependent manner and significantly enhanced hypoxic pulmonary vasoconstriction (HPV;1.5-fold, P<0.05). Iberiotoxin (0.1mM) shifted the 11,12-EET dose-response curve to the right and markedly decreased HPV after 11,12-EET application. Moreover, 1-adamantyl-3-cyclohexylurea, an inhibitor of the soluble epoxide hydrolase (the enzyme that metabolizes EETs to their less active diols), potentiated acute hypoxic pulmonary vasoconstriction (HPV; 1.4-fold, P<0.05); an effect that was abolished by pre-treatment with iberiotoxin. In lungs from BK_Caß-/- mice, HPV was smaller than in lungs from wild-type mice, and soluble epoxide hydrolase inhibition failed to potentiate the response. Moreover, the 11,12-EET-induced pulmonary vasoconstriction was significantly attenuated in lungs from BK_Caß-/- mice.

To assess the mechanisms underlying these responses we loaded rat pulmonary artery smooth muscle cells with the potential sensitive indicator di-8-ANEPPS. 11,12 EET (10mM) elicited a pronounced smooth muscle cell depolarization, an effect significantly diminished in the presence of iberiotoxin.

Taken together, these data indicate that EETs enhance HPV in the mouse lung by inducing smooth muscle cell depolarization and that the EET-induced pulmonary vasoconstriction under normoxic and hypoxic conditions requires the expression of the ß1 subunit of the BKCa channel.