Redox control of oxygen sensing in the rabbit ductus arteriosus

How the ductus arteriosus (DA) closes at birth remains unclear. Inhibition of O2-sensitive K+ channels may initiate the closure but the sensor mechanism is unknown. We hypothesized that changes in endogenous H2O2 could act as this sensor. Using chemiluminescence measurements with luminol (50 [mu]M) or lucigenin (5 [mu]M) we showed significantly higher levels of reactive O2 species in normoxic, compared to hypoxic DA. This increase in chemiluminescence was completely reversed by catalase (1200 U ml-1). Prolonged normoxia caused a significant decrease in K+ current density and depolarization of membrane potential in single fetal DA smooth muscle cells. Removal of endogenous H2O2 with intracellular catalase (200 U ml-1) increased normoxic whole-cell K+ currents (IK) and hyperpolarized membrane potential while intracellular H2O2 (100 nM) and extracellular t-butyl H2O2 (100 [mu]M) decreased IK and depolarized membrane potential. More rapid metabolism of O2- with superoxide dismutase (100 U ml-1) had no significant effect on normoxic K+ currents. N-Mercaptopropionylglycine (NMPG), duroquinone and dithiothreitol all dilated normoxic-constricted DA rings, while the oxidizing agent 5,5'-dithiobis-(2-nitrobenzoic acid) constricted hypoxia-dilated rings. NMPG also increased IK. We conclude that increased H2O2 levels, associated with a cytosolic redox shift at birth, signal K+ channel inhibition and DA constriction.

Dexfenfluramine elevates systemic blood pressure by inhibiting potassium currents in vascular smooth muscle cells

Appetite suppressants, such as dexfenfluramine (dex), are associated with primary pulmonary hypertension, valvular heart disease, and systemic vascular complications, such as coronary, cerebral, or mesenteric ischemia. These drugs suppress appetite by enhancing release and inhibiting reuptake of serotonin in the central nervous system. The effects of dex on the systemic circulation have not been studied. K(+) channels regulate vascular tone in most vascular beds. We hypothesized that dex is a systemic vasoconstrictor acting primarily by inhibiting K(+) channels, independent of effects on serotonin. The effects of clinically relevant concentrations of dex (10(-6) to 10(-4) M) on outward K(+) current and membrane potential were studied with whole-cell patch clamping in freshly isolated smooth muscle cells from rat renal, carotid, and basilar arteries. Tone was measured in tissue baths. Blood pressure, cardiac output, and left ventricular end diastolic pressure were assessed in open- and closed-chest anesthetized rats. At 10(-4) M, dex inhibits outward K(+) current (50%) and increases membrane potential (by >35 mV), an effect comparable with 4-aminopyridine (5 mM). Furthermore, dex constricts rings and acutely elevates systemic pressure (+17 +/- 3 mm Hg) and systemic vascular resistance in the presence of ketanserin. Dex vasoconstriction is dose-dependent (threshold dose 10(-6) M; 156 microg/ml) and enhanced in L-NAME-fed rats. We conclude that dex causes acute systemic vasoconstriction, at least in part by inhibition of voltage-gated K(+) channels, independent of effects on serotonin. To our knowledge, this is the first time that a commonly prescribed drug with voltage-gated K(+) channel-blocking properties is shown to have significant hemodynamic effects in vivo.

Potassium channel diversity in vascular smooth muscle cells

Several recent observations suggest that the vascular medium is a mosaic of functionally and morphologically unique cell types. This diversity includes differences in cell phenotype and expression of cytoskeletal and contractile proteins as well as heterogeneity of the number and activity of potassium (K+) channel types. K+ channels play a role in the regulation of arterial tone and in the control of cell proliferation. There is evidence for cell to cell, segment to segment, and vascular bed to bed diversity of K+ channels that could explain the varying responses of arterial segments or different arteries to stimuli such as hypoxia, vasoactive drugs, or arterial wall injury. Pulmonary artery vascular smooth muscle cells contain several types of K+ channels, including calcium sensitive (KCa), delayed rectifier (KDR), and ATP gated (KATP). Hypoxic pulmonary vasoconstriction (HPV) is more prominent in the resistance than in the conduit arteries. HPV is initiated by the inhibition of a KDR channel, resulting in membrane depolarization, increase in the intracellular calcium, and contraction. We have shown that some pulmonary artery smooth muscle cells are enriched in KDR channels whereas others have more KCa channels. These cells can be differentiated by their morphology (using light microscopy and electron microscopy) and their electrical properties (using patch-clamp techniques). Although present throughout the pulmonary artery, KDR-enriched cells are more prominent in the distal-resistance segments whereas KCa-enriched cells are more prominent in the proximal-conduit segments. Nitric oxide (NO) causes relaxation in part by activating a KCa channel, causing membrane hyperpolarization and inactivation of the voltage-gated calcium channels. NO is a slightly more potent vasodilator in the conduit than in the resistance pulmonary artery. In summary, the pulmonary artery may be thought of as a mosaic of cells that have different proportions of key proteins, such as K+ channel subtypes, which confer upon the cell an ability to respond to a stimulus (hypoxia or NO) differently than an adjacent cell exposed to the same stimulus. The prevalence of these cells differs from conduit to resistance arteries. Diversity of cell function may be important in physiology and pathophysiology, allowing responses to vasodilators, vasoconstrictors, and proliferative stimuli to vary within or between vascular beds.

Ventilation-induced pulmonary vasodilation at birth is modulated by potassium channel activity

At birth, pulmonary blood flow rapidly increases 8- to 10-fold, and pulmonary arterial pressure falls by 50% within 24 h. The postnatal adaptation of the pulmonary circulation is mediated, in part, by endothelium-derived nitric oxide (EDNO). Recent studies suggest that EDNO may reduce vascular resistance, in part, by activating K+ channels. We hypothesized that K+ channels modulate the changes in pulmonary hemodynamics associated with birth. To test this hypothesis, we studied the effect of K+ channel inhibition on two separate, but interdependent stimuli: 1) mechanical ventilation with low inspired O2 concentrations (designed to maintain normal fetal blood gas tensions) and 2) mechanical ventilation with high inspired O2 concentrations. Tetraethyl-ammonium (TEA, 1 mg/min for 100 min; n = 5), a nonspecific K+ channel blocker, glibenclamide (Gli, 1 mg/min for 30 min; n = 6), an ATP-sensitive K+ channel blocker, or saline (n = 7) was infused into the left pulmonary artery (LPA) of acutely instrumented fetal lambs. The umbilical-placental circulation remained intact, and lambs were ventilated with 0.10 inspired O2 concentration (FIO2) for 60 min, followed by 1.0 FIO2 for 20 min. Neither TEA nor Gli had an effect on basal pulmonary tone. TEA attenuated the increase in LPA flow and decrease in pulmonary vascular resistance in response to mechanical ventilation with 0.10 and 1.0 FIO2; Gli had no effect. These results support the hypothesis that non-ATP-sensitive K+ channels modulate the transition from fetal to neonatal pulmonary circulation.

Oxygen-induced constriction of rabbit ductus arteriosus occurs via inhibition of a 4-aminopyridine-, voltage-sensitive potassium channel

The ductus arteriosus is a vital fetal structure allowing blood ejected from the right ventricle to bypass the pulmonary circulation in utero. Closure of the ductus arteriosus at birth, essential for postnatal adaptation, is initiated by an increase in oxygen (O2) tension. We recently demonstrated the presence of O2-sensitive potassium channels in the fetal and adult pulmonary circulation which regulate vascular tone in response to changes in O2 tension. In this study, we assessed the cellular mechanisms underlying O2-induced constriction of the ductus arteriosus in late-gestation fetal rabbits. We report that O2 reversibly inhibits a 58-pS voltage- and 4-aminopyridine-sensitive potassium channel, causing membrane depolarization, an increase in intracellular calcium through L-type voltage-gated calcium channels, and constriction of the ductus arteriosus. We conclude that the effector mechanism for O2 sensing in the ductus arteriosus involves the coordinated action of delayed rectifier potassium channels and voltage-gated calcium channels.

Oxygen causes fetal pulmonary vasodilation through activation of a calcium-dependent potassium channel

At birth, pulmonary vasodilation occurs as air-breathing life begins. The mechanism of O2-induced pulmonary vasodilation is unknown. We proposed that O2 causes fetal pulmonary vasodilation through activation of a calcium-dependent potassium channel (KCa) via a cyclic nucleotide-dependent kinase. We tested this hypothesis in hemodynamic studies in acutely prepared fetal lambs and in patch-clamp studies on resistance fetal pulmonary artery smooth muscle cells. Fetal O2 tension (PaO2) was increased by ventilating the ewe with 100% O2, causing fetal total pulmonary resistance to decrease from 1.18 +/- 0.14 to 0.41 +/- 0.03 mmHg per ml per min. Tetraethylammonium and iberiotoxin, preferential KCa-channel inhibitors, attenuated O2-induced fetal pulmonary vasodilation, while glibenclamide, an ATP-sensitive K+-channel antagonist, had no effect. Treatment with either a guanylate cyclase antagonist (LY83583) or cyclic nucleotide-dependent kinase inhibitors (H-89 and KT 5823) significantly attenuated O2-induced fetal pulmonary vasodilation. Under hypoxic conditions (PaO2 = 25 mmHg), whole-cell K+-channel currents (Ik) were small and were inhibited by 1 mM tetraethylammonium or 100 nM charybdotoxin (CTX; a specific KCa-channel blocker). Normoxia (PaO2 = 120 mmHg) increased Ik by more than 300%, and this was reversed by 100 nM CTX. Nitric oxide also increased Ik. Resting membrane potential was -37.2 +/- 1.9 mV and cells depolarized on exposure to CTX, while hyperpolarizing in normoxia. We conclude that O2 causes fetal pulmonary vasodilation by stimulating a cyclic nucleotide-dependent kinase, resulting in KCa-channel activation, membrane hyperpolarization, and vasodilation.