Vascular potassium channels mediate oxygen-induced pulmonary vasodilation in fetal lambs

Calling into question the future of hyperoxygenation in pregnancy

Maternal hyperoxygenation has been investigated as a potential diagnostic and therapeutic tool since the 1960s. Since then, it has been applied in many obstetric scenarios, both clinically and in the research setting. It is often administered without any determination of pre-hyperoxygenation maternal or fetal oxygen levels. Studies focussing on maternal oxygen therapy for the treatment of fetal growth restriction have been ongoing for over thirty years and there remains no clear evidence of benefit. Studies investigating the potential diagnostic or therapeutic role of maternal oxygen therapy in the setting of fetal congenital cardiac disease have reported varying success rates and some potentially worrying adverse effects. The purpose of this article is to review the effects of maternal hyperoxygenation on fetal and maternal health and to ascertain the safety of undertaking further clinical trials that employ the use of hyperoxygenation in pregnancy.

Fetal pulmonary arterial vascular impedance reflects changes in fetal oxygenation at near-term gestation in a nonhuman primate model

OBJECTIVE

We tested the hypothesis that fetal pulmonary arterial circulation reacts to changes in fetal oxygenation status at near-term gestation.

STUDY DESIGN

A total of 20 rhesus macaques underwent fetal Doppler ultrasonography at near-term gestation. Right pulmonary artery (RPA), umbilical artery (UA), ductus arteriosus (DA), and ductus venosus (DV) blood velocity waveforms were obtained, and pulsatility index (PI) values were calculated. Fetal right and left ventricular cardiac outputs were determined. Ultrasonographic data were collected during 3 maternal oxygenation states: room air (baseline), hyperoxemia, and hypoxemia.

RESULTS

Fetal RPA PI values increased (P < .05) during maternal hypoxemia and decreased (P < .05) during maternal hyperoxemia, compared with baseline. Maternal hyperoxemia increased (P < .05) DA PI values from baseline. Fetal cardiac outputs, UA, and DV PI values were not affected.

CONCLUSIONS

Our results demonstrate that at near-term gestation, fetal pulmonary arterial circulation is a dynamic vascular bed that reflects acute and short-term changes in fetal oxygenation.

Developmental expression of vasoactive and growth factors in human lung. Role in pulmonary vascular resistance adaptation at birth

The factors that mediate the postnatal fall in pulmonary vascular resistance, which is crucial for normal gas exchange, are not fully understood. The endothelium has been implicated in this phenomenon, through the release of vasorelaxant factors such as nitric oxide (NO). Human pulmonary expression of endothelial NO synthase increases up to 31 wk of gestation, together with vascular endothelial growth factor (VEGF), and both factors potently mediate pulmonary angiogenesis and vasorelaxation. During the perinatal period, when pulmonary vasodilatation is maximal, endothelial NO synthase and VEGF are weakly expressed. This raises the involvement of vasorelaxant factors other than NO at birth. One candidate is endothelial-derived hyperpolarizing factor, which induces smooth muscle cell hyperpolarization by activating K(ATP) channels. The marked vasorelaxation induced by activation of these channels in newborn animals, and their strong perinatal expression in the human lung, suggest their involvement during this phase. Another candidate is endothelin (ET)-1, together with its receptors ET-A and ET-B. ET-A receptors are located exclusively on smooth muscle cells and mediate vasoconstriction, whereas ET-B receptors mediate vasoconstriction when located on smooth muscle cells and vasodilatation when located on endothelial cells. ET-B receptors, which are strongly expressed in the human fetal lung both at the end of gestation and after birth, may be involved in perinatal pulmonary vasodilatation. Thus, in human fetal lung, K(ATP) channels and ET-B receptors could be important in mediating the perinatal pulmonary vasodilatation crucial for adapting the pulmonary circulation to extrauterine life.

THE ATP-SENSITIVE POTASSIUM-CHANNEL INHIBITOR GLIBENCLAMIDE IMPROVES OUTCOME IN AN OVINE MODEL OF HEMORRHAGIC SHOCK

This study was designed as a prospective laboratory experiment to evaluate the effects of the ATP-sensitive potassium-channel inhibitor glibenclamide on hemodynamics and end-organ function in an ovine model of hemorrhagic shock. Twenty-four adult sheep were anesthetized and surgically prepared to measure hemodynamics of the systemic and pulmonary circulation. The anterior surface of the abdominal aorta was exposed at a location 6 cm superior to the iliac bifurcation. After a 60-min period of stabilization, this location was punctured with a 14-G needle. To induce a hemorrhagic hypotension (mean arterial pressure [MAP] less than 50 mmHg) via bleeding, the needle was left in place for 15 s to insure good blood flow. Thereafter, it was removed, and the abdomen closed. The animals were then randomized to receive either glibenclamide (4 mg/kg over 15 min) or an equal volume of the vehicle, started 1 h postinjury. Hemodynamic variables were measured every 30 min. Compared with the control group, MAP and systemic vascular resistance index (SVRI) were significantly higher in the intervention group throughout the entire 6-h study period. Ileal pH and urine output were higher in treated than in control animals (4 h, ileal pH 7.29 +/- 0.31 vs. 7.17 +/- 0.6; 6 h, urine output 36 +/- 9 vs. 7.5 +/- 2 mL; P value less than 0.05 each). Because glibenclamide improved both hemodynamics and organ function, it may be a beneficial component in the acute treatment of hemorrhagic shock.

Oxygen dilation in fetal pulmonary arterioles: role of K(+) channels

BACKGROUND

Oxygen is a potent stimulus for pulmonary vasodilation. Potassium channels have been implicated as both sensors and effectors for oxygen-induced changes in pulmonary vascular tone. We have examined the effect of potassium channel blockers on oxygen-induced vasodilation in isolated pulmonary arterioles from fetal rats at term.

MATERIALS AND METHODS

Third generation pulmonary arterioles were isolated from fetal rats on Day 22 of gestation, cannulated, pressurized at constant distending pressures, and preconstricted by suffusion with a salt solution bubbled with a "hypoxic gas" mixture (pO(2) <or=50 mm Hg). Oxygen-induced vasodilation was measured as percentage reversal of the "hypoxic" vasoconstriction after 30 min of suffusion with "normoxic" solution (pO(2) 90-145 mm Hg). Responses were recorded in the absence of blockers (controls) or in the presence of a voltage-gated K(+) channel (K(v)) blocker, 4-aminopyridine; an ATP-sensitive K(+) channel (K(ATP)) blocker, glibenclamide; a Ca(2+)-activated K(+) channel (K(Ca)) blocker, charybdotoxin; or a nonspecific K(+) channel blocker, tetraethylammonium.

RESULTS

In control arterioles, normoxic suffusion for 30 min reversed hypoxic preconstriction by 83 +/- 19%. 4-aminopyridine significantly attenuated (44 +/- 9%), and glibenclamide and charybdotoxin had no effect (80 +/- 16 and 79 +/- 20%) on the magnitude of normoxic vasodilation.

CONCLUSIONS

Our results are consistent with a contribution of K(v) channels, but not K(ATP) or K(Ca) channels, to oxygen-induced vasodilation in third generation pulmonary arterioles from term fetal rats.

Adenosine and ATP cause nitric oxide-dependent pulmonary vasodilation in fetal lambs

We investigated the hypothesis that the purine nucleotide ATP and its nucleoside adenosine cause pulmonary vasodilation in fetal lambs by the release of nitric oxide (NO). We also investigated the potential role of K(+)(ATP) channels in mediating the effects of ATP and adenosine on NO. We surgically prepared 28 fetal lambs to measure pulmonary and systemic pressures and pulmonary flow. We investigated the effects of glibenclamide and pinacidil (inhibitor and agonist, respectively, for K(+)(ATP) channels), N-nitro-L-arginine (N-LA) and its methyl ester, N-nitro-L-arginine methyl ester (L-NAME) (inhibitors of endothelium-derived NO synthesis), and U46619 (a thromboxane mimetic) on pulmonary vasodilation caused by adenosine and ATP. Adenosine decreased the pulmonary artery pressure and pulmonary vascular resistance (PVR) at doses of 0.08-2.5 microM/kg/min and increased the left pulmonary flow at doses of 0.3-2.5 microM/kg/min in control experiments. N-LA, L-NAME and glibenclamide attenuated the effects of adenosine at doses of < 2.5 microM/ kg/min and pinacidil potentiated its effects. ATP decreased the pulmonary artery pressure and PVR and increased the pulmonary flow at doses of 0.15-2.5 microM/kg/min in control experiments. N-LA and L-NAME attenuated the effects of ATP at doses of < 2.5 microM/kg/min, whereas glibenclamide and pinacidil had no effect on the response to ATP. U46619 increased the basal pulmonary vascular tone, but did not significantly alter the vasodilative responses to ATP and adenosine. In conclusion, adenosine and ATP cause NO-dependent pulmonary vasodilation in fetal lambs. The activation of K(+)(ATP) channels plays a role in adenosine-induced pulmonary vasodilation. The mechanism by which ATP causes NO release and pulmonary vasodilation requires further investigation.