Fetal and adult cerebral artery K_ATP and K_Ca channel responses to long-term hypoxia

High-altitude long-term hypoxia (LTH) alters cerebral vascular contractile and relaxation responses in both fetus and adult. We tested the hypotheses that LTH-mediated vascular responses were secondary to altered K⁺ channel function and that in the fetus these responses differ from those of the adult. In middle cerebral arteries (MCA) from both nonpregnant adult and fetal (∼140 days gestation) sheep, which were either acclimatized to high altitude (3,820 m) or sea-level controls, we measured norepinephrine (NE)-induced contractions and intracellular Ca²⁺ concentration ([Ca²⁺]_i) simultaneously, in the presence or absence of different K⁺ channel openers or blockers. In adult MCA, LTH was associated with ∼20% decrease in NE-induced tension and [Ca²⁺]_i, with a significant increase in Ca²⁺ sensitivity. In contrast, in fetal MCA, LTH failed to affect significantly NE-induced contraction or [Ca²⁺]_i but significantly decreased the ATP-sensitive K⁺ (K_ATP) channel and Ca²⁺-activated K⁺ (K_Ca) channel-mediated relaxation. The significant effect of K_ATP and K_Ca channel activators on the relaxation responses and the fact that K⁺ channels play a key role in myogenic tone support the hypotheses that K⁺ channels play an important role in hypoxia-mediated responses. These results also support the hypothesis of significant developmental differences with maturation from fetus to adult.