Chronic Hypoxia uncouples Ca²⁺ and eNOS in bradykinin‐induced relaxation of Ovine pulmonary arteries

Abstract

Bradykinin‐induced activation of the pulmonary endothelium triggers a rise in intracellular Ca2+, which activates a nitric oxide (NO)‐dependent signaling pathway. This pathway leads to vasodilation, thereby regulating pulmonary blood flow and 02 uptake. This vasodilation process includes stimulation of endothelial nitric oxide synthase (eNOS) and downstream activation of large‐conductance K+ (Maxi K+) channels. Chronic hypoxia (CH) is known to increase pulmonary pressures and restrict arterial relaxation, and can contribute to the development of pulmonary hypertension. We thus examined the effects of CH on bradykinin‐induced Ca2+ signals, bradykinin‐induced vasorelaxation, and the roles of eNOS and Maxi K+ channels in this relaxation. Wire‐myography and confocal microscopy studies were performed on pulmonary arteries (PA) from nonpregnant ewes that lived in a normoxic state at low altitude or a hypoxic state at high altitude (3,801 m) for >100 days. eNOS was inhibited with 100 μM NG‐nitro‐L‐arginine methyl ester (LNAME), and Maxi K+ channels were blocked with 1mM tetraethylammonium (TEA). The data show CH augmented endothelial Ca2+ signals, but restricted bradykinin relaxation. Further, CH caused bradykinin‐induced contraction. LNAME sensitivity was restricted, which suggests eNOS dysfunction is central to the uncoupling of Ca2+ signals and bradykinin relaxation. CH also abolished TEA‐sensitivity in bradykinin relaxation, suggesting there is loss of Maxi K+ function following CH. Overall, these results suggest that CH causes uncoupling of endothelial Ca2+ signaling and eNOS function and mediates major changes in the mechanisms of membrane hyperpolarization. It follows that CH‐induced Ca2+‐eNOS uncoupling and Maxi K+ channel dysfunction are important mechanisms to examine as future therapeutic avenues for pulmonary hypertension.Support or Funding InformationSupport from NIH, NSF and Loma Linda University School of Medicine