Cardiac-specific knockout and pharmacological inhibition of Endothelin receptor type B lead to cardiac resistance to extreme hypoxia

Oxygen plays a central role in cardiac energy metabolism. at high altitude where the ambient oxygen level is low, we found ednrb is associated with human hypoxia adaptation. our subsequent study in global heterozygous knockout mice (ednrb−/+) revealed that cardiac function was conserved in these mice when exposed to extreme hypoxia. the major goal of this study was (i) to determine the functional role of cardiomyocyte ednrb in maintaining cardiac function under hypoxic stress and (ii) to validate the phenotypes we detected in ednrb−/+ mice using ednrb blockers. unlike the global knockouts, cardiac-specific heterozygote (ednrbflox/+) and homozygote (ednrbflox/flox) ednrb knockout mice were phenotypically normal. when treated with graded low levels of oxygen (10% and 5% o2), both ednrbflox/+ and ednrbflox/flox were hypoxia tolerant. the cardiac indexes at 10% and 5% o2 for ednrbflox/+ were significantly higher and lactate levels were significantly lower when compared to the cre-negative controls (p < 0.05). simultaneously, mice treated with bq-788 (ednrb-specific blocker) had a significantly higher cardiac index (p < 0.005) and significantly lower lactate levels (p < 0.0001) than in control mice. a similar result was obtained with mice treated with bosentan (non-specific). these data indicate that a lower level or complete lack of ednrb in the cardiomyocytes significantly improves cardiac performance under extreme hypoxia, a novel role of cardiomyocyte ednrb in the regulation of cardiac function. furthermore, this rescue under extreme hypoxia can also be achieved using ednrb-specific pharmacological agents, e.g., bq-788. this systematically confirms, both genetically and pharmacologically, the protective role of a lower ednrb under extreme hypoxia stress.