Developmental acceleration of bradykinin-dependent relaxation by prenatal chronic hypoxia impedes normal development after birth

Bradykinin-induced activation of the pulmonary endothelium triggers nitric oxide production and other signals that cause vasorelaxation, including stimulation of largeconductance Ca²⁺-activated K⁺ (BK_Ca) channels in myocytes that hyperpolarize the plasma membrane and decrease intracellular Ca²⁺. Intrauterine chronic hypoxia (CH) may reduce vasorelaxation in the fetal-to-newborn transition and contribute to pulmonary hypertension of the newborn. Thus we examined the effects of maturation and CH on the role of BKCa channels during bradykinin-induced vasorelaxation by examining endothelial Ca²⁺ signals, wire myography, and Western immunoblots on pulmonary arteries isolated from near-term fetal (~140 days gestation) and newborn, 10- to 20-day-old, sheep that lived in normoxia at 700 m or in CH at high altitude (3,801 m) for >100 days. CH enhanced bradykinin-induced relaxation of fetal vessels but decreased relaxation in newborns. Endothelial Ca²⁺ responses decreased with maturation but increased with CH. Bradykinin- dependent relaxation was sensitive to 100 mM nitro-L-arginine methyl ester or 10 mM 1H-[1,2,4]oxadiazolo[4,3-α]quinoxalin-1-one, supporting roles for endothelial nitric oxide synthase and soluble guanylate cyclase activation. Indomethacin blocked relaxation in CH vessels, suggesting upregulation of PLA2 pathways. BK_Ca channel inhibition with 1 mM tetraethylammonium reduced bradykinin-induced vasorelaxation in the normoxic newborn and fetal CH vessels. Maturation reduced whole cell BK_Cachannel β1-subunit expression but increased β1-subunit expression. These results suggest that CH amplifies the contribution of BK_Ca channels to bradykinin-induced vasorelaxation in fetal sheep but stunts further development of this vasodilatory pathway in newborns. This involves complex changes in multiple components of the bradykinin-signaling axes.