Fetal and postnatal pulmonary circulation in the Alto Andino.

Lowland mammals at high altitude constrict the pulmonary vessels, augmenting vascular resistance and developing pulmonary arterial hypertension. In contrast, highland mammals, like the llama, do not present pulmonary arterial hypertension. Using wire myography, we studied the sensitivity to norepinephrine (NE) and NO of small pulmonary arteries of fetal llamas and sheep at high altitudes. The sensitivity of the contractile responses to NE was decreased whereas the relaxation sensitivity to NO was augmented in the llama fetus compared to the sheep fetus. Altogether these data show that the fetal llama has a lower sensitivity to a vasoconstrictor (NE) and a higher sensitivity to a vasodilator (NO), than the fetal sheep, consistent with a lower pulmonary arterial pressure found in the neonatal llama in the Andean altiplano. Additionally, we investigated carbon monoxide (CO) in the pulmonary circulation in lowland and highland newborn sheep and llamas. Pulmonary arterial pressure was augmented in neonatal sheep but not in llamas. These sheep had reduced soluble guanylate cyclase and heme oxygenase expression and CO production than at lowland. In contrast, neonatal llamas increased markedly pulmonary CO production and HO expression at high altitude. Thus, enhanced pulmonary CO protects against pulmonary hypertension in the highland neonate. Further, we compared pulmonary vascular responses to acute hypoxia in the adult llama versus the adult sheep. The rise in pulmonary arterial pressure was more marked in the sheep than in the llama. The llama pulmonary dilator strategy may provide insights into new treatments for pulmonary arterial hypertension of the neonate and adult.

Low-dose inhaled carbon monoxide reduces pulmonary vascular resistance during acute hypoxemia in adult sheep.

Carbon monoxide (CO) is produced by the action of the heme oxygenase (HO) complex through the oxidation of heme. CO, like nitric oxide (NO), is a molecular gas that among other actions stimulates guanylyl cyclase and increases cGMP levels in smooth muscle cells, regulating the vascular tone. Acute hypoxia generates pulmonary hypertension and increases the expression of inducible HO isoform (HO-1) in the vascular endothelium. Inhaled NO causes a potent pulmonary vasodilation. We hypothesized that inhaled CO might produce similar actions as NO on pulmonary vascular resistance (PVR). To test our contention, we studied the effects of inhaled CO (40 ppm) in the augmented PVR observed during hypoxemia. Five chronically instrumented German Merino sheep were submitted to a protocol consisting of 20 min of normoxemia (N), 20 min of isocapnic hypoxemia (H20), 20 min of isocapnic hypoxemia plus CO 40 ppm (H40), and 20 min of recovery (R). In the control protocol, we did not administer inhaled CO. Arterial gases and pH, percentage of carboxyhemoglobin (COHb), systemic and pulmonary arterial pressure, systemic and pulmonary vascular resistance, and cardiac output were measured during each period. During H20 period, there was a significant increase in cardiac output and PVR in sheep submitted to both protocols. The sheep treated with inhaled CO (H40 + CO) showed a modest but significant decrease (16%) in the elevated PVR. Our data indicate that inhaled CO decreases pulmonary vascular resistance associated with acute hypoxemia in adult sheep.

Cardiovascular responses to arginine vasopressin blockade during acute hypoxemia in the llama fetus.

The fetal llama has a marked increase in the peripheral vascular resistance and no augmentation of brain blood flow during hypoxemia. In spite of the substantial plasma arginine-vasopressin (AVP) increase during hypoxemia, up to 8 times greater than in fetal sheep, there are no changes of carotid and femoral blood flows during hypoxemia with a V1 receptor blockade, as is seen in the fetal sheep. The aim of this study was to assess the role of AVP function in mediating the combined ventricular output and organ blood flow in the hypoxemic llama fetus. Six fetal llamas at 0.65 of gestation were instrumented under general anesthesia, and cardiorespiratory responses and blood flows determined under normoxemic and hypoxemic conditions. The AVP effect was determined using a V1 antagonist during normoxemic and hypoxemic conditions. Organ blood flows were measured with the radioactive microsphere technique. No significant differences in organ blood flow or in their vascular resistances were seen between the control and treated fetuses during hypoxemia. We conclude that V1 blockade did not have any important role in the cardiovascular response to acute hypoxemia in the llama fetus, in contrast with lowland fetuses. AVP may be playing a role in other regions, possibly in kidney or lung, during hypoxemia.

Adrenergic and vasopressinergic contributions to the cardiovascular response to acute hypoxaemia in the llama fetus.

1. The effects of fetal intravenous treatment with phentolamine or a vasopressinergic V1-receptor antagonist on the fetal cardiovascular responses to acute hypoxaemia in the llama were investigated. 2. Six llama fetuses were surgically prepared between 60 and 70 % of gestation under general halothane anaesthesia with vascular catheters and transit-time ultrasonic flow probes around a carotid artery and a femoral artery. At least 4 days after surgery all fetuses were subjected to a 3 h experiment: 1 h of normoxia, 1 h of hypoxaemia and 1 h of recovery while on slow i.v. infusion with saline. On separate days this experiment was repeated with fetal i.v. treatment with either phentolamine or a V1-receptor antagonist dissolved in saline. 3. During saline infusion all llama fetuses responded to acute hypoxaemia with intense femoral vasoconstriction. Phentolamine during normoxia produced hypotension, tachycardia and vasodilatation in both the carotid and the femoral circulations. During hypoxaemia, fetuses treated with phentolamine did not elicit the pronounced femoral vasoconstriction and all died within 20 min of the onset of hypoxaemia. A V1-receptor antagonist produced a femoral vasodilatation during normoxia but did not affect the fetal cardiovascular responses to acute hypoxaemia. 4. In conclusion, alpha-adrenergic and V1-vasopressinergic mechanisms contribute to a basal vasoconstrictor tone in the femoral circulation in the llama fetus. The enhanced femoral vasoconstriction during acute hypoxaemia in the llama fetus is not mediated by stimulation of V1-vasopressin receptors, but is dependent on alpha-adrenergic receptor stimulation. Such alpha-adrenergic efferent mechanisms are indispensable to fetal survival during hypoxaemia in the llama since their abolition leads to cardiovascular collapse and death.

Chemoreflex contribution to adrenocortical function during acute hypoxemia in the llama fetus at 0.6 to 0.7 of gestation.

This study tested the hypothesis that the fetal llama, a species adapted to the chronic hypoxia of life at high altitude, demonstrates a potent carotid chemoreflex influence on adrenocortical responses during acute hypoxemia. Plasma ACTH and cortisol concentrations, and mesencephalic and adrenal blood flows were measured during a 1-h period of acute hypoxemia in six intact and four carotid sinus-denervated llama fetuses at 0.6-0.7 of gestation. Fetal PaO2 was reduced from approximately 23 to about 14 mm Hg in both intact and carotid-denervated groups during acute hypoxemia. During hypoxemia, fetal plasma ACTH, adrenal blood flow, and, therefore, delivery of ACTH to the adrenals increased to similar extents in both intact and carotid-denervated fetal llamas. Despite this, the increase in plasma cortisol in hypoxemia in intact fetuses was absent in carotid-denervated fetuses. In addition, the increase in delivery of cortisol to the mesencephalon calculated in intact fetuses during hypoxemia did not occur in the carotid-denervated group. These data suggest that the integrity of the carotid chemoreceptors is indispensable to cortisol release during acute hypoxemia in the llama fetus, even at 0.6-0.7 of gestation.

Repeated doses of the perfluorocarbon FC-100 improve lung function of preterm lambs.

Intratracheal administration of a single dose of the perfluorocarbon FC-100 improves lung function in surfactant-deficient animals. In this study we compared the response to repeated doses of FC-100 (3 mL/kg 3% solution, n = 5) with that observed after administration of Exosurf (5 mL/kg, n = 5) to mechanically ventilated preterm lambs of 125 d of gestation. The initial dose of FC-100 rapidly increased arterial PO2, decreased arterial PCO2, and improved arterial pH. Also dynamic lung compliance markedly improved with this agent. Administration of an additional dose of FC-100 resulted in relatively similar changes, albeit of lesser magnitude than those observed with the initial dose. In contrast, Exosurf did not improve these variables even after three doses. All lambs treated with FC-100 survived the 6-h study period, whereas one of the five Exosurf-treated lambs survived (p < 0.05). Mean arterial blood pressure and heart rate decreased in those lambs that received FC-100, but not in surviving lambs that received Exosurf. Our data demonstrate that repeated intratracheal administration of the perfluorocarbon FC-100 improves lung function and survival of surfactant-deficient lambs better than the synthetic surfactant Exosurf. We speculate that tensio-active agents with properties different from surfactant, such as FC-100, might improve lung function in preterm neonates with diseases due to surfactant deficiency.

Carotid blood flow changes with behavioral states in the late gestation llama fetus in utero.

This study tested the hypothesis that in the llama fetus changes in cerebral blood flow are closely associated with changes in cerebral oxidative metabolism such as occur during transitions between electrocortical states. For the first time reported in any species, instantaneous changes in common carotid blood flow, employed as a continuous index of cerebrovascular perfusion, were related to instantaneous changes in electrocortical activity. Three late gestation fetal llamas were surgically prepared under general anesthesia with vascular catheters, a tracheal and amniotic catheter, and with electrodes implanted to monitor the fetal electrocorticogram (ECoG). In addition, Transonic flow probes were placed around a common carotid artery and a femoral artery. At least 4 days after surgery fetal arterial blood, amniotic and tracheal pressures, carotid and femoral blood flows and the fetal ECoG were recorded continuously. Our results suggest a close association between increases in common carotid blood flow and low voltage ECoG in the llama fetus. Close coupling between instantaneous changes in carotid blood flow and electrocortical states together with the lack of an increase in brain blood flow without increased cerebral oxygen extraction during hypoxemia in the llama fetus supports a fall in cerebral oxidative metabolism in this species during hypoxemic episodes.

Chemoreflex and endocrine components of cardiovascular responses to acute hypoxemia in the llama fetus.

We tested the hypothesis that the llama fetus has a blunted cardiovascular chemoreflex response to hypoxemia by investigating the effects of acute hypoxemia on perfusion pressure, heart rate, and the distribution of the combined ventricular output in 10 chronically instrumented fetal llamas at 0.6-0.7 gestation. Four llama fetuses had the carotid sinus nerves sectioned. In the intact fetuses, there was a marked bradycardia, an increase in perfusion pressure, and a pronounced peripheral vasoconstriction during hypoxemia. These cardiovascular responses during hypoxemia in intact fetuses were accompanied by a pronounced increase in plasma vasopressin, but not in plasma angiotensin II concentrations. Carotid denervation prevented the bradycardia at the onset of hypoxemia, but it did not affect the intense vasoconstriction during hypoxemia. Plasma vasopressin and angiotensin II levels were not measured in carotid-denervated fetuses. Our results do not support the hypothesis that the carotid chemoreflex during hypoxemia is blunted in the llama fetus. However, they emphasize that other mechanisms, such as increased vasopressin concentrations, operate to produce an intense vasoconstriction in hypoxemia. This intense vasoconstriction in the llama fetus during hypoxemia may reflect the influence of chronic exposure to the hypoxia of high altitude on the magnitude and gain of fetal cardiovascular responses to a superimposed acute episode of hypoxemia.

Cardiovascular responses to graded degrees of hypoxaemia in the llama fetus.

The fetal llama exposed to an intense degree of hypoxaemia did not increase cerebral blood flow, but showed a marked peripheral vasoconstriction. The same cardiovascular response is observed in fetal sheep submitted to a extremely severe hypoxaemia, when the initial compensatory vasodilatory mechanisms in brain and heart fail. To investigate whether the fetal llama responses to acute hypoxaemia are adaptive, or whether they are the result of a breakdown of mechanisms of blood flow redistribution that favours the central nervous system, we studied seven fetal llamas (0.6-0.7 of gestation) chronically-catheterized during 1 h of graded and progressive hypoxaemia. Fetal ascending aorta blood gases and fetal cardiac output and its distribution (radiolabelled-microspheres) were measured after 60 min of normoxaemia (B) and at the end of 20 min (H20), 40 min (H40) and 60 min (H60) of hypoxaemia. Data were analysed by ANOVA and Newman-Keuls tests. Each treatment resulted in a lower (P < 0.05) percentage of haemoglobin saturation than hypoxaemia; H40 was lower than H20, and H60 was lower than H20 and H40. No statistical difference was observed among treatments for cardiac output or cerebral blood flow. These results demonstrate that fetal cardiac output and brain blood flow are maintained at all degrees of hypoxaemia, indicating that these cardiovascular responses are an adaptive response in the llama fetus, rather than an index of cardiorespiratory decompensation.

Maternal administration of glucocorticoid and thyrotropin-releasing hormone enhances fetal lung maturation in undisturbed preterm lambs.

OBJECTIVE: We hypothesized that combined treatment with glucocorticoid plus thyrotropin-releasing hormone administered to pregnant ewes with preterm gestation accelerates fetal lung maturation of undisturbed lambs better than single hormonal treatment does. STUDY DESIGN: Twenty-five pregnant ewes at 123 days of gestation were randomized to receive (1) 0.9% sodium chloride (controls), (2) betamethasone (12 mg intramuscularly every 24 hours two times), (3) thyrotropin-releasing hormone (400 micrograms intravenously every 8 hours six times), or (4) thyrotropin-releasing hormone plus betamethasone. After delivery by cesarean section at 125 days fetal lamb lung compliance and alveolar lavage phospholipid content were determined. RESULTS: Betamethasone plus thyrotropin-releasing hormone significantly increased fetal lung compliance expressed as milliliters of air per gram of wet weight at 40 cm H2O and 5 cm H2O (0.82 +/- 0.13 and 0.35 +/- 0.10 ml/gm wet lung, respectively) versus betamethasone (0.37 +/- 0.02 and 0.07 +/- 0.02), thyrotropin-releasing hormone (0.38 +/- 0.02 and 0.14 +/- 0.03), and control (0.25 +/- 0.03 and 0.09 +/- 0.01) groups. Also, total phospholipids and saturated phosphatidylcholine concentrations in alveolar lavage were significantly higher in the combined betamethasone plus thyrotropin-releasing hormone group (27.3 +/- 4.9 and 16.9 +/- 4.3 micrograms/gm wet lung, respectively) versus betamethasone (10.9 +/- 3.5 and 6.7 +/- 2.1), thyrotropin-releasing hormone (15.2 +/- 5.6 and 7.3 +/- 2.0), and control (7.9 +/- 2.4 and 3.6 +/- 1.0) groups. CONCLUSION: Combined maternal administration of betamethasone plus thyrotropin-releasing hormone improves lung maturation in undisturbed fetal lambs at 125 days' gestation more than does either hormone given alone.