Gordon G. Power

Inhaled nebulized nitrite is a hypoxia-sensitive NO-dependent selective pulmonary vasodilator

The blood anion nitrite contributes to hypoxic vasodilation through a heme-based, nitric oxide (NO)–generating reaction with deoxyhemoglobin and potentially other heme proteins. We hypothesized that this biochemical reaction could be harnessed for the treatment of neonatal pulmonary hypertension, an NO-deficient state characterized by pulmonary vasoconstriction, right-to-left shunt pathophysiology and systemic hypoxemia. To test this, we delivered inhaled sodium nitrite by aerosol to newborn lambs with hypoxic and normoxic pulmonary hypertension. Inhaled nitrite elicited a rapid and sustained reduction (∼65%) in hypoxia-induced pulmonary hypertension, with a magnitude approaching that of the effects of 20 p.p.m. NO gas inhalation. This reduction was associated with the immediate appearance of NO in expiratory gas. Pulmonary vasodilation elicited by aerosolized nitrite was deoxyhemoglobin- and pH-dependent and was associated with increased blood levels of iron-nitrosyl-hemoglobin. Notably, from a therapeutic standpoint, short-term delivery of nitrite dissolved in saline through nebulization produced selective, sustained pulmonary vasodilation with no clinically significant increase in blood methemoglobin levels. These data support the concept that nitrite is a vasodilator acting through conversion to NO, a process coupled to hemoglobin deoxygenation and protonation, and evince a new, simple and inexpensive potential therapy for neonatal pulmonary hypertension.

Nifedipine: Effects on fetal and maternal hemodynamics in pregnant sheep

We investigated the effects of nifedipine, a calcium entry blocker, on the fetal and maternal circulation. Nifedipine was administered intravenously for 30 minutes to chronically instrumented pregnant ewes. Infusion of 5 micrograms.kg-1.min-1 resulted in a 30% to 50% increase in total and regional fetal cerebral blood flow to the brain stem, watershed, and subcortical regions (p less than 0.05), without a significant change in fetal oxygenation or cardiac output. Infusion of 10 micrograms.kg-1.min-1 decreased uterine blood flow by 21% (p less than 0.001) and fetal arterial oxygen content by 15% (p less than 0.01), with no further increase in fetal cerebral blood flow. Maternal arterial pressure decreased and heart rate increased (p less than 0.001) without variation of arterial blood gases. Significant plasma levels of nifedipine were detected in the fetal and maternal circulations. In view of the potential adverse effects on the fetus, further studies are needed before nifedipine is considered for use in human pregnancy.

Key Neuroprotective Role for Endogenous Adenosine A1 Receptor Activation During Asphyxia in the Fetal Sheep

BACKGROUND AND PURPOSE The fetus is well known to be able to survive prolonged exposure to asphyxia with minimal injury compared with older animals. We and others have observed a rapid suppression of EEG intensity with the onset of asphyxia, suggesting active inhibition that may be a major neuroprotective adaptation to asphyxia. Adenosine is a key regulator of cerebral metabolism in the fetus. METHODS We therefore tested the hypothesis that infusion of the specific adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), given before 10 minutes of profound asphyxia in near-term fetal sheep, would prevent neural inhibition and lead to increased brain damage. RESULTS DPCPX treatment was associated with a transient rise and delayed fall in EEG activity in response to cord occlusion (n=8) in contrast with a rapid and sustained suppression of EEG activity in controls (n=8). DPCPX was also associated with an earlier and greater increase in cortical impedance, reflecting earlier onset of primary cytotoxic edema, and a significantly smaller reduction in calculated cortical heat production after the start of cord occlusion. After reperfusion, DPCPX-treated fetuses but not controls developed delayed onset of seizures, which continued for 24 hours, and sustained greater selective hippocampal, striatal, and parasagittal neuronal loss after 72-hour recovery. CONCLUSIONS These data support the hypothesis that endogenous activation of the adenosine A1 receptor during severe asphyxia mediates the initial suppression of neural activity and is an important mechanism that protects the fetal brain.

Fetal lamb cerebral blood flow (CBF) and oxygen tensions during hypoxia: a comparison of laser Doppler and microsphere measurements of CBF

This study was undertaken to compare microsphere and laser Doppler flowmetry techniques for the measurement of cerebral blood flow, to assess the effect of probe implantation at the tip of the sensing probe and to measure brain tissue PO2 (tPO2) in response to acute hypoxia. Fetal sheep of ≈131 days gestation (n= 8) were chronically instrumented with bilateral laser Doppler probes in the parietal cortices and catheters for injection of fluorescent microspheres. Five days after surgery fetuses were subjected to 1 h periods of baseline control breathing, hypoxia and recovery. Microspheres were injected 10 min prior to and 10, 30, 50 and 120 min after initiation of hypoxia. Microspheres were counted in four 12 mm3 tissue samples from each hemisphere, the tip of the laser Doppler probe being positioned in the centre of one of the cubes. The cube containing the probe tip was also subdivided into 4 mm3 pieces of tissue. In response to hypoxia, fetal arterial PO2 declined from 21 ± 2 to 12 ± 1 Torr and brain tissue PO2 fell from 10 ± 1 to a nadir of 1 ± 1 Torr. Each method detected a significant increase in CBF that reached a maximum after 30‐45 min, although the increase of flow measured by laser Doppler flowmetry was less than that measured by spheres after 10 and 30 min (P < 0.05). Microspheres did not detect altered flow at the probe tip or heterogeneity of flow in surrounding volumes of cortical tissue. In summary, laser Doppler flowmetry is a useful measure of continuous relative changes of CBF in the chronically instrumented fetal sheep. Flow compensations in acute hypoxia are not adequate to sustain O2 delivery, and other compensations, including reduced metabolic rate, are possible.

The relationship between uterine artery Doppler velocimetry and umbilical venous adenosine levels in pregnancies complicated by preeclampsia

OBJECTIVE The aim of this study was to evaluate the relationship between uteroplacental circulatory insufficiency and the fetoplacental release of adenosine in pregnancies complicated by preeclampsia. STUDY DESIGN We performed uterine artery Doppler velocimetry and calculated the pulsatility index of the uterine artery, to detect uteroplacental circulatory insufficiency, immediately before cordocentesis in 39 pregnant women complicated by preeclampsia. Umbilical venous blood obtained by cordocentesis was then analyzed for blood gases, pH, and plasma adenosine levels. Increased plasma adenosine was taken to signal its increased release from the placenta and fetus relative to its rate of disappearance. RESULTS The mean umbilical venous plasma adenosine level in the abnormal pulsatility index group was 1.78 +/- 0.17 mumol/L (mean +/- SEM, n = 25), significantly higher than in the normal pulsatility index group 0.58 +/- 0.14 mumol/L (n = 14, p < 0.001). Furthermore, in the abnormal pulsatility index group the elevation of plasma adenosine levels in the umbilical vein was found even in normoxic fetuses. CONCLUSION Fetal plasma adenosine increases before uteroplacental circulatory insufficiency becomes severe enough to cause generalized fetal hypoxemia. We postulate that enhanced adenosine formation in the fetus, umbilical cord vessels, and particularly the placenta may, at least in part, contribute to control and maintenance of placental blood flow.

Respiratory Function of the Placenta as Determined with Carbon Monoxide in Sheep and Dogs

A technique is described for studying the respiratory function of the placenta using carbon monoxide, a gas whose exchange across the placenta between the maternal and fetal circulations is limited by diffusion rather than blood flow. During the steady state before the introduction of CO, the normal concentration of carboxyhemoglobin in the ewe, [COHb](M), is approximately 0.90%, and that in the fetus is 2.9%, the ratio [COHb](F)/[COHb](M) being 3.2. In dogs the corresponding values are 1.9%, 4.8%, and 2.4%. After the introduction of CO into the mother animal, CO diffused across the placenta slowly with an equilibration half-time of approximately 2 hours. The average carbon monoxide diffusing capacity (D(Pco)) of the placenta during maternal to fetal exchange was 0.54 ml per (minute x mm Hg x kg fetal weight) (SD +/- 0.13) in sheep and 0.57 ml per (minute x mm Hg x kg) (SD +/- 0.18) in dogs. The fetal to maternal placental diffusing capacity in two sheep was 0.54 ml per (minute x mm Hg x kg). Calculations considering the relative rates of reaction of O(2) and CO with red cell hemoglobin and the relative rates of diffusion of the two gases suggest that the true D(Po2) should be about 1.2 to 2 times greater than the D(Pco) or 0.65 to 1.1 per (minute x mm Hg x kg). This is about 5 times greater than the reported value of D(Po2) calculated from measurements of P(O2) in the mixed uterine and umbilical venous blood. With a diffusing capacity of this magnitude the maternal and fetal placental end capillary P(O2) would approach equilibrium, becoming too small to measure, and the calculation of D(Po2) would be unreliable. We suggest that the apparent end capillary P(o2) gradients of 15 to 20 mm Hg, obtained from sampling uterine and umbilical venous blood, result from a combination of uneven distribution of maternal and fetal placental blood flow and from placental oxygen consumption.

Uneven Distribution of Maternal and Fetal Placental Blood Flow, as Demonstrated Using Macroaggregates, and Its Response to Hypoxia*

A technique is described for studying the distribution of blood flow to the maternal and fetal placental vessels in sheep and dogs with radioactive labeled macroaggregates of albumin. When the maternal animal breathed room air the distribution of maternal placental blood flow was uneven among the cotyledons as well as within a given cotyledon. Fetal blood flow was also distributed nonuniformly among and within the cotyledons. The relation of maternal to fetal placental blood flow was also markedly uneven (coefficient of correlation, tau = 0.066). After the animal was made hypoxic by breathing 10-12% O(2) the distribution of maternal, fetal, and maternal/fetal placental flows became more uniform. The coefficient of correlation of maternal to fetal flow was high (tau = 0.53, P < 0.01). While the maternal animal breathed room air, after ligation of a major branch of the umbilical artery the distribution of maternal, fetal, and maternal/fetal flows in the remaining two-thirds to three-fourths of the placenta became more uniform. The correlation coefficient for maternal to fetal flow was high (tau = 0.35, P < 0.01).It appears that under normal circumstances with uneven distribution of blood flows there is a considerable portion of the placenta that does not receive blood flow in optimum quantities to promote efficient O(2) exchange. Failure to consider the influence of nonuniform maternal flow/fetal flow will result in overestimation of mean maternal-fetal oxygen tension gradients, and thus underestimation of the placental diffusing capacity for oxygen. In response to maternal hypoxia or compromise of the fetal placental circulation the distribution of maternal, fetal, and maternal/fetal flows becomes more uniform, thereby increasing the efficiency of placental O(2) exchange.

Plasma Adenosine levels and P-selectin expression on platelets in preeclampsia

Objective To measure the correlation of plasma adenosine levels with platelet activation in women with preeclampsia. Methods Plasma adenosine concentration and expression of P-selectin, a marker for platelet activation, were measured in 18 normal pregnant women and 18 preeclamptic women. The effect of 8-sulfophenyltheophylline, an adenosine receptor blocker, on expression of P-selectin on platelets also was measured. Results Plasma adenosine level averaged 0.77 ± 0.11 μM (standard error of the mean [SEM]) in women with preeclampsia, significantly higher than the mean level of 0.47 ± 0.08 μM in women with normal pregnancies (P < .05). Expression of P-selectin on platelets averaged 7.8 ± 1.2% in women with preeclampsia, also significantly higher than the mean level of 4.7 ± 0.7% in normal pregnancy (P < .05). Adenosine receptor blockade significantly increased expression of P-selectin on platelets in women with preeclampsia by 26% (P < .05), which was significantly higher than the 13% increase of activation in those with normal pregnancies (P < .05). Conclusion Adenosine is an established platelet activation suppressor. Increased plasma levels of adenosine in preeclampsia might partially compensate and tend to prevent further excessive platelet activation in women with preeclampsia.

Relation between adenosine and T-helper 1/T-helper 2 imbalance in women with preeclampsia

OBJECTIVE To evaluate the relationship between changes in plasma adenosine concentration and imbalances in the T‐helper 1/T‐helper 2 ratio in peripheral blood of women with preeclampsia. METHODS Plasma adenosine concentrations and the T‐helper 1/T‐helper 2 ratio were examined in the peripheral blood of 16 preeclamptic and normal pregnant women. The proportion of specific T‐cell marker CD4‐positive cells expressing intracellular cytokines, interferon‐γ derived from T‐helper 1 and interleukin‐4 derived from T‐helper 2 cells, were analyzed by flow cytometry. The ratio of interferon‐γ secreting cells to interleukin‐4 secreting cells was taken as the T‐helper 1/T‐helper 2 ratio in vivo. The effect of the adenosine‐receptor blocker 8‐sulfophenyltheophyl‐line was also measured in vitro. RESULTS Mean plasma adenosine concentration in preeclampsia was significantly higher than that in normal pregnancy (0.68 ± 0.07 μmol/L versus 0.39 ± 0.06 μmol/L, P < .05). The proportions of interferon‐γ secreting cells increased and interleukin‐4 secreting cells decreased significantly in preeclampsia, and the T‐helper 1/T‐helper 2 ratio in preeclampsia was significantly higher than in normal pregnancy (18.1 ± 2.6 versus 9.9 ± 1.5, P < .05). The increase of the proportion of interferon‐γ secreting cells after adenosine receptor blockade in preeclampsia significantly exceeded that of normal pregnancy. The T‐helper 1/T‐helper 2 ratio in preeclampsia was significantly greater than that in normal pregnancy (36% versus 17%, P < .05). CONCLUSION Increased plasma adenosine may be involved in the regulation of imbalances in the T‐helper 1/T‐helper 2 ratio in women with preeclampsia.

Role of Nitric Oxide in Hypoxic Cerebral Vasodilatation in the Ovine Fetus

To investigate the role of nitric oxide (NO) in fetal cerebral circulatory responses to acute hypoxia, near‐term fetal sheep were instrumented with laser Doppler probes placed in the parasagittal parietal cortices and vascular catheters in the sagittal sinus and brachiocephalic artery. After a 3 day recovery period, responses of cortical blood flow (CBF) to hypoxia were compared with and without inhibition of nitric oxide synthase (NOS). After an initial 30 min baseline period, fetuses were given a bolus followed by a continuous infusion of Nω‐nitro‐l‐arginine methyl ester (l‐NAME), or saline vehicle as control. After administration of l‐NAME, CBF decreased by 14 ± 6 % (P < 0.01) despite increases in arterial blood pressure of 15 mmHg, resulting in an ∼60 % increase in cerebrovascular resistance. Thirty minutes following initiation of l‐NAME or vehicle infusion, fetal systemic hypoxia was induced by allowing the ewes to breathe 10–11 % oxygen. In control fetuses CBF increased progressively to 145 ± 9 % of baseline (P < 0.01) after 30 min, while cortical release of cyclic guanylate cyclase (cGMP), an index of NOS activity, increased 26 ± 8 % (P < 0.05). In contrast, CBF in l‐NAME‐treated fetuses increased to only 115 % of the reduced CBF baseline, whereas cortical release of cGMP did not change significantly. In summary, basal levels of NO lower resting cortical vascular resistance by ∼15 % in the fetal sheep. Inhibition of NO synthesis attenuates hypoxic cerebral relaxation but does not completely prevent the characteristic increases in CBF. Hypoxic increases in NO directly increase cortical production of cGMP and inhibition of NO synthesis ablates these changes in cGMP.