Mary L. Tod

Relative Effects of Cyclooxygenase and Nitric Oxide Synthase Inhibition on Vascular Resistances in Neonatal Lamb Lungs

Effective attenuation of pulmonary vasoconstriction is essential during early postnatal development when increased pulmonary vascular resistance (PVR) may lead to a resumption of right-to-left shunting across fetal channels. In addition, modulation of venous resistance contributes to normal lung fluid balance. This study was designed to identify the relative modulating effects of endothelium-derived nitric oxide (EDNO) and dilator prostaglandins (PG) on normoxic and hypoxic pulmonary vasomotor tone in young newborns. Total and segmental PVR were measured using inflow-outflow and double occlusion techniques in isolated lungs of 6-h-old lambs studied under control conditions or after blocking PG and/or EDNO synthesis with indomethacin and/orNω-nitro-L-arginine, respectively. During normoxia, both indomethacin and Nω-nitro-L-arginine were required to increase total PVR, but EDNO appeared to have the greater modulating effect. Indomethacin markedly enhanced hypoxic pulmonary vasoconstriction of large and small arteries and small veins, whereasNω-nitro-L-arginine caused a lesser, but significant, increase in hypoxic pulmonary vasoconstriction of small arteries and veins, suggesting that dilator PG played the dominant modulating role during hypoxia. In addition, PG synthesis appeared to be enhanced after inhibition of EDNO synthesis. In contrast, indomethacin caused a decrease in venous resistance, suggesting that constrictor prostanoids had a greater effect than dilator PG on this segment. EDNO had a modest modulating effect on venous resistance in these lungs. These data suggest that dilator PG and EDNO exert complementary effects in attenuating total and segmental PVR during normoxia and hypoxia in 6-h-old lamb lungs.

Indomethacin prevents ventilation-induced decreases in pulmonary vascular resistance of the middle region in fetal lambs.

ABSTRACT: Previously, we reported that the major site of pulmonary vascular resistance in fetal lambs occurred in the middle region defined by vascular occlusion, and that this region exhibited the greatest decrease upon ventilation with O2. To assess the relative individual contributions of ventilation and oxygenation to this decrease, we determined the distribution of pressures across the pulmonary circulation in isolated perfused lungs from 20 fetal lambs (131–137 d gestation) by inflow and outflow vascular occlusions. A membrane oxygenator was included in the extracorporeal circuit to control the Po1 at 4 kPa (30 torr) in the unventilated fetal lungs. Half of the fetal lungs were ventilated first without changing the initial gas tensions, and the others were oxygenated first by changing the initial gas tensions to a hyperoxic mixture [Po2 = 26.6 kPa (200 torr)] without ventilation. Finally, both groups of lungs were ventilated and oxygenated. In addition, indomethacin was added to the perfusate (0.112 mM, or 40 μg/mL) in half of the preparations in each group to determine the effect of prostaglandins on the distribution of pressures during these conditions. The decrease in the total pulmonary vascular resistance with ventilation and/or oxygenation was primarily due to changes in the middle pressure gradient (ΔPm). In fetal lungs without indomethacin, ventilation without oxygenation reduced ΔPm from 6.1 ± 0.8 to 2.5 ±1.0 kPa, or 74% of the total ventilation- and oxygenation-induced decrease in ΔPm (final value = 1.2 ± 0.6 kPa). In contrast, oxygenation without ventilation produced a decrease in ΔPm from 5.5 ± 0.7 to 3.8 ± 0.5 kPa, only 40% of the total decrease in ΔPm (1.2 ± 0.4 kPa). Furthermore, in fetal lungs with indomethacin, only oxygenation first caused a reduction in the resistance of the middle region, suggesting that dilator prostaglandins are not involved in the response to increased oxygen. We conclude that recruitment and/or distension of the small pulmonary vessels functionally located in the middle region by the mechanical effect of ventilation is dependent on dilator cyclooxygenase products, and that this mechanical effect is a major factor involved in the decrease in pulmonary vascular resistance occurring at birth.

Effects of 6-Keto-prostaglandin E1 on Perinatal Pulmonary Vascular Resistance

Abstract The effects of a biologically active metabolite of PGI2, 6-keto-PGE1, were evaluated in fetal goats and newborn lambs using an in situ, constant-flow, isolated lower left lobe preparation. Intrapulmonary injections of 6-keto-PGE1 (0.074-4.41 μg/kg) produced dose-dependent decreases in pulmonary vascular resistance, mean systemic arterial pressure, and heart rate in fetal goats. Fetal systemic responses to 6-keto-PGE1 were significantly less following left atrial injections than after intrapulmonary injections. Newborn lambs also responded to intrapulmonary infusions of 6-keto-PGE1 (0.078-5.15 μg/kg-min) with dose-dependent reductions of pulmonary vascular resistance and systemic arterial pressure. A possible role for 6-keto-PGE1 in the modulation of perinatal pulmonary vascular resistance is discussed.

Discordant effects of alkalosis on elevated pulmonary vascular resistance and vascular reactivity in lamb lungs.

OBJECTIVES After an initial vasodilator response to alkalosis, many children with pulmonary hypertension exhibit marked pulmonary vascular reactivity despite continued alkalosis therapy. This study sought to a) identify the mediator of alkalosis-induced pulmonary vasodilation in isolated lamb lungs; b) determine whether alkalosis-induced pulmonary vasodilation decreases over time in this model; and c) determine whether alkalosis enhanced vascular reactivity to subsequent pressor stimuli. DESIGN Prospective, interventional study. SUBJECTS Isolated perfused lungs from 1-month-old lambs. INTERVENTIONS Hypocarbic alkalosis, hypoxia, and infusion of the thromboxane mimetic agent U46619 MEASUREMENTS AND MAIN RESULTS Pulmonary artery pressure was measured at constant flow, so a change in pressure reflects change in resistance. Hypoxic pulmonary artery pressure was compared after 20 and 100 mins of hypocarbic alkalosis or normocarbia in control and cyclooxygenase-inhibited lungs. Pulmonary artery dose responses to U46619 were then measured in control lungs. Responses to hypoxia and U46619 were also compared after 60-80 mins of hypocarbic or normocarbic normoxia. Hypocarbic alkalosis acutely reduced hypoxic pulmonary vascular resistance, and this was sustained for at least 100 mins. Cyclooxygenase inhibition blocked this vasodilation, suggesting that it was mediated by dilator prostaglandins. However, subsequent reactivity to U46619 was enhanced in hypoxic alkalotic lungs, and both hypoxia and U46619 caused significant vasoconstriction in normoxic alkalotic lungs. CONCLUSIONS Alkalosis caused sustained vasodilation when pulmonary vascular resistance was high but either failed to attenuate or enhanced vascular reactivity to subsequent pressor stimuli.

Effects of Prostaglandin H2 on Perinatal Pulmonary Circulation

ABSTRACT. A pivotal intermediate in prostaglandin (PG) biosynthesis is the endoperoxide PGH2. This endoperoxide is capable of eliciting direct responses in biological systems without undergoing conversion to other PGs. Effects of PGH2 include stimulation of platelet aggregation and vascular smooth muscle contraction in vitro; injections of PGH2 in vivo cause increases in pulmonary arterial pressure. The response of the pulmonary vasculature of perinatal lambs to PGH2 was measured using an in situ pumpperfused left lower lung preparation. Intrapulmonary injections of PGH2 (0.24-0.61 μg/kg) into six unventilated fetal lambs (0.93-0.97 gestation) produced decreases in pulmonary vascular resistance (PVR) of 10-21%. The fall in PVR was rapid in onset, reached a peak at 10 s after injection, and returned to baseline within 35 s. Following ventilation (FIO2=0.21) of fetal lambs, injections of PGH2 (0.24-0.61 μg/kg) caused increases in PVR (ave increase=50% over control PVR). The pulmonary pressor re-sponse to PGH2 in ventilated fetal lambs was depressed almost 50% by inhibition of thromboxane synthetase. Injections of a “heat-inactivated” PGH2 did not affect PVR in ventilated fetuses. We did not observe any effects on systemic blood pressure or heart rate of intrapulmonary arterial injections of PGH2. These findings suggest a me-tabolism of PGH2 to dilator PGs before ventilation and constrictor PGs and thromboxanes after ventilation, and or direct effects of PGH2 on vascular smooth muscle that are dependent on existing vascular tone.

Pressor responses to arachidonic acid in pump-perfused sheep lungs

The reported actions of arachidonic acid in the adult pulmonary circulation are controversial. Some authors reported that arachidonic acid causes only pulmonary vasoconstriction; others have found decreases in pulmonary vascular resistance with low-dose infusions. We have previously reported that arachidonic acid causes only pulmonary vasoconstriction in perinatal lambs during both normoxia and hypoxia. The effects of arachidonic acid on pulmonary vascular resistance were determined in adult sheep using an in situ pump-perfused left lower lung preparation. Arachidonic acid infusions (10.5-31.9 micrograms/kg . min) resulted in pulmonary vasoconstriction. The pulmonary vascular response to arachidonic acid was not altered by hypoxia or by infusion of PGF2 alpha. However, the pulmonary pressor response to hypoxia was increased by concomitant arachidonate infusions. Infusions of arachidonic acid during hypoxia resulted in systemic hypotension. Thus, pulmonary arachidonate metabolism appears to be unaffected by hypoxia or PGF2 alpha; however, hypoxia may enhance the formation of dilator PGs from the lung.

Hemodynamic Effects of Postpulmonary Administration of Prostaglandin D2 in Fetal Animals

Abstract Dose-response relationships in pulmonary vascular resistance (PVR), mean systemic arterial pressure (SAP), and heart rate (HR) to left atrial administration of prostaglandin D2 (PGD2) were determined in five fetal lambs. Fetuses were delivered by cesarean section from chloralose anesthetized ewes with the umbilical circulation maintained intact. Fetuses were prevented from breathing thus maintaining pulmonary vascular tone in the elevated fetal state. Blood was withdrawn from the inferior vena cava and pumped at constant flow into the lower left lobe of the fetal lung. Postpulmonary infusions of PGD2 brought about dose-dependent decreases in pulmonary vascular resistance. Heart rate tended to increase in fetal lambs. Mean systemic arterial pressure increased in the fetal lambs at all doses tested except for the largest dose (44.14 μg/kg-min), which produced slight hypotension. These data demonstrate that exposure to the systemic circulation prior to entering the pulmonary vasculature does not alter the preferential dilator action of PGD2 on fetal pulmonary vessels nor does it produce Significant Systemic hypotension.

RESPONSIVENESS AND SENSITIVITY TO INHALED NITRIC OXIDE CHANGES WITH AGE IN ISOLATED LUNGS OF DEVELOPING NEWBORN LAMBS. † 2059

In a recent study, we found that sodium nitroprusside (SNP) attenuated hypoxic vasoconstriction more in 2 day (d) than 1 month (m) old lamb lungs (J Appl Physiol 79: 824, 1995), suggesting that pulmonary vascular responsiveness to endothelium-derived nitric oxide decreases with age. However, SNP may have different effects than nitric oxide (NO) on vascular smooth muscle. Therefore, in this study we compared pulmonary artery dose-responses to inhaled NO (2.5 - 100 ppm) in indomethacin-treated 1d and 1m lamb lungs ventilated with 4% O2 (hypox). All lungs were perfused with autologous blood at a constant flow of 100 ml/kg.min. Data are expressed as mean pulmonary artery pressure(Ppa in mmHg) ± SEM. Groups were compared by ANOVA (* different at p< 0.05). Inhaled NO caused a significantly greater decrease in hypox Ppa in 1d compared to 1m lungs. Maximal dilation occurred at 100 ppm NO at both ages. 5 ppm NO caused a 49±8.2% decrease in Ppa relative to maximal dilation in 1d lungs, but only a 25.1±13% decrease in 1m lungs. Over 10 ppm NO were required to cause a 50% decrease in Ppa in 1m lungs. Thus, 1d lungs appeared more responsive and sensitive to NO. Whether this was due to higher baseline Ppa in 1d lungs or to age-dependent differences in metabolism of cGMP(or other steps in signal transduction) remains to be determined. Support: American Heart Assoc, MD Affiliate, Inc. Table