Systemic circulatory adjustments to acute hypoxia and reoxygenation in unanesthetized sheep. Role of renin, angiotensin II, and catecholamine interactions

Natriuretic peptides in hormonal regulation of hypoxia responses

The possibility that natriuretic peptides' effects are important in hypoxia responses of vertebrates is reviewed. Both the transcription and release of natriuretic peptides are affected by oxygen tension. Furthermore, many of the effects observed in hypoxia, such as diuresis and a reduction of plasma volume, are also caused by treatment of the animal with natriuretic peptides. Also, several clinical observations about changes in natriuretic peptide levels in, e.g., sleep apnea and cyanotic congenital heart disease, are consistent with the idea that hypoxia is involved in the etiology of conditions, in which natriuretic peptide levels increase. Virtually all published information on the relationship between oxygen and natriuretic peptides is based on human studies. Because hypoxic conditions are more common in aquatic than terrestrial environments, future studies about the possible role of natriuretic peptides in hypoxia, as well as the role of hypoxia in the evolution of natriuretic peptides, including the different subtypes, should increasingly involve also aquatic organisms.

Multi-organ system model of O2 and CO2 transport during isocapnic and poikilocapnic hypoxia

A multi-organ systems model of O(2) and CO(2) transport is developed to analyze the control of ventilation and blood flow during hypoxia. Among the aspects of the control processes that this model addressed are possible mechanisms responsible for the second phase of the ventilatory hypoxic response to mild hypoxia, i.e., hypoxic ventilatory decline (HVD). Species mass transport processes are described by compartmental mass balances in brain, heart, skeletal muscle, and "other tissues" connected in parallel via the circulation. In pulmonary and systemic capillaries and in the vasculature connecting the systemic tissues, species transport processes are represented by a one-dimensional, convection-dispersion model. The effects of bicarbonate acid-base buffering, hemoglobin, and myoglobin on the transport processes are included. The model incorporates feedback control mechanisms through a cardiorespiratory control system in which peripheral and central chemoreceptors sense O(2) and CO(2) partial pressures. Model simulations of the ventilatory responses to isocapnic and poikilocapnic hypoxia show two phases with distinct dynamics. A fast phase is discernable immediately after switching from normoxic to hypoxic conditions, while a delayed slow phase (HVD) typically becomes manifested after several minutes. Model simulations allow quantitative evaluation of several proposed mechanisms to account for HVD. Under isocapnic hypoxia, simulations indicate that an increase in brain blood flow has no effect on HVD, but that HVD can be entirely described by central ventilatory depression (CVD). Under poikilocapnic hypoxia, the hypocapnia caused by hypoxic hyperventilation has no effect on HVD.

Assessment of gut mucosal perfusion and colonic tissue blood flow during abdominal aortic surgery with gastric tonometry and laser Doppler flowmetry

The objective of this study was to investigate the effect of infrarenal aortic cross-clamping and unclamping on gut mucosal perfusion by gastric tonometry and on sigmoid colonic tissue blood flow by laser Doppler flowmetry during abdominal aortic surgery. This was a prospective before-and-after intervention comparison study in a university hospital of 8 male patients, aged 57-87, undergoing elective infrarenal abdominal aortic surgery. Each patient was pretreated with ranitidine. Following general anesthesia, a nasogastric tonometer was inserted into the stomach. The balloon of the tonometer was filled with 2.5 mL of normal saline for gas tension and pH analysis. This process was repeated before and after aortic cross-clamping and unclamping. Gastric mucosal pHi was calculated with the Henderson-Hasselbalch equation from the arterial Hco3- and the tonometrically measured mucosal Pco2. A laser Doppler flow probe was placed in contact with the serosa of the sigmoid colon against the mesentery after the abdomen was opened. Sigmoid colonic tissue blood flow (SCBF) was assessed by the laser Doppler flowmeter. Gastric mucosal pHi by gastric tonometry and colonic tissue blood flow by laser Doppler flowmetry were measured before and after aortic cross-clamping and unclamping. Gastric mucosal pHi decreased significantly 30 minutes after aortic cross-clamping (7.37 +/-0.07) (p < 0.01), 60 minutes after aortic cross-clamping (7.39 +/-0.08) (p < 0.05), and 30 minutes after aortic unclamping (7.37 +/-0.08) (p < 0.01), compared with pHi before aortic cross-clamping (7.50 +/-0.06). Gastric mucosal pHi increased to the original level 60 minutes after aortic unclamping (7.46 +/-0.08). Since a gastric mucosal pH above 7.35 is considered normal, these mean values of pHi were clinically insignificant. However, gastric mucosal pHi decreased below 7.32 in 5 patients during abdominal aortic surgery. Gastric mucosal pHi decreased further to 7.30 in 1 patient following aortic cross-clamping and below 7.30 in 3 patients 30 minutes after aortic unclamping. SCBF decreased significantly after aortic cross-clamping (28.1 +/-4.8 mL/min/100 g) compared with the value before aortic cross-clamping (51.9 +/-11.3 mL/min/100 g) (p < 0.01). Following aortic unclamping, SCBF returned to 41.7 +/-7.4 mL/min/100 g. It is concluded that transient episodes of significant intestinal mucosal ischemia may have been encountered occasionally in patients undergoing abdominal aortic surgery, but a sigmoid colonic tissue blood flow of 41.7 +/-7.4 mL/min/100 g was sufficient to prevent postoperative ischemic colitis regardless of whether there was ligation or no ligation of inferior mesenteric artery among the studied population since none of the patients developed clinically significant ischemic colitis.

Prevention of hypoxemia-induced renal dysfunction by perindoprilat in the rabbit

The role of angiotensin II, a potent postglomerular vasoconstrictor, in the hypoxemia-induced renal changes is still controversial. The ability of perindoprilat, an angiotensin converting-enzyme inhibitor, to prevent the acute renal effects of hypoxemia was assessed in 22 anesthetized-ventilated rabbits. In 8 untreated rabbits, hypoxemia induced a significant drop in mean blood pressure (MBP) (-12 +/- 2%), glomerular filtration rate (GFR) (-16 +/- 3%) and renal blood flow (RBF) (-12 +/- 3%) with a concomittant increase in renal vascular resistance (RVR) (+18 +/- 5%) and urine flow rate (+33 +/- 14%), and without any changes in filtration fraction (FF) (-4 +/- 2%). This suggests the occurrence of glomerular vasoconstriction during the hypoxemic stress. In 7 normoxemic rabbits, intravenous perindoprilat (20 microg/kg) induced an increase in urine flow rate (+17 +/- 4%) and RBF (+17 +/- 4%), and a decrease in MBP (-6 +/- 1%), RVR (-14 +/- 3%) and FF (-11 +/- 2%) without a significant change in GFR. The drop in FF and the increase in RBF suggests preferential postglomerular vasodilatation. In 7 rabbits, perindoprilat prevented the occurence of the hypoxemia-induced changes in RBF and RVR without improving MBP. FF decreased significantly (-18 +/- 2%), while the drop in GFR (-7 +/- 2%) was partially blunted and the increase in urine flow rate (+25 +/- 9%) was confirmed. These results could be explained by the inhibition of the angiotensin-mediated efferent vasoconstriction and by the inhibition of bradykinin degradation by perindoprilat. These data confirm the ability of converting-enzyme inhibitors to prevent the renal hypoperfusion induced by acute hypoxemia.

Angiotensin II contributes to cerebral vasodilatation during hypoxia in the rabbit

BACKGROUND

and Purpose Hypoxia increases cerebral blood flow (CBF). Hypoxia also exerts a major influence on the renin-angiotensin system. In addition to the circulating renin-angiotensin system, a local renin-angiotensin system appears to be present in the brain, and angiotensin II receptors have been identified in cerebral blood vessels. In this study we tested the hypothesis that endogenous angiotensin II attenuates dilatation of the cerebral vessels during hypoxia.

METHODS

Pentobarbital-anesthetized rabbits were prepared for measurement of blood flow (microspheres) and assigned to one of two groups: in group 1 (n = 11), rabbits were subjected to 30 minutes of stable hypoxia (PaO2 = 34 +/- 1 mm Hg, mean +/- SD) followed by 15 minutes of reoxygenation (PaO2 = 177 to 200 mm Hg). Blood flow was measured four times: under control conditions, after 15 and 30 minutes of hypoxia, and after 15 minutes of reoxygenation. This was a control group to characterize changes in CBF during hypoxia. In group 2 (n = 11), blood flow was measured as in the previous group except that an infusion of the angiotensin II receptor antagonist saralasin (1 microgram.kg-1.min-1 IV) was started with the onset of hypoxia and continued through reoxygenation to the end of the experiment. The goal of this group was to examine whether endogenous activation of receptors for angiotensin II influences increases in CBF during hypoxia. In a separate series of experiments we examined the influence of the angiotensin-converting enzyme (ACE) inhibitor captopril on the hypoxic response. Thus, in one group of rabbits we measured CBF in the same manner as in group 1 (n = 13). In another group of rabbits we also measured blood flow as in group 1 except that rabbits received 10 mg/kg of the ACE inhibitor captopril before the control measurement (n = 11). We tested for significant differences between groups using two-way ANOVA.

RESULTS

Under control conditions, CBF was similar in all groups and averaged 53 +/- 15 mL.min-1.100 g-1. During hypoxia, CBF increased to a greater extent in the absence versus the presence of saralasin (95 +/- 31 and 104 +/- 30 mL.min-1.100 g-1 versus 72 +/- 24 and 71 +/- 25 mL.min-1.100 g-1, respectively; P = .003). Increase in CBF during hypoxia was also significantly greater in the animals that did not receive captopril versus those that were treated with captopril (100 +/- 24 and 89 +/- 16 mL.min-1.100 g-1 versus 72 +/- 16 and 73 +/- 17 mL.min-1.100 g-1). To rule out the possibility that saralasin produced non-specific attenuation of cerebral vasodilatation, we tested the influence of hypercapnia on CBF in the absence and presence of saralasin. During normocapnia, CBF values were not significantly different in the absence and presence of saralasin (57 +/- 17 and 64 +/- 6 mL.min-1.100 g-1, respectively; P > .05). Hypercapnia increased CBF similarly in the absence and presence of saralasin (81 +/- 22 and 91 +/- 19 mL.min-1.100 g-1; PaCO2 = 61 +/- 2 and 60 +/- 2 mm Hg, respectively; P > .05).

CONCLUSIONS

Because the ACE inhibitor captopril and the angiotensin II receptor blocker saralasin attenuated increased in CBF during hypoxia, the findings suggest that endogenous release of angiotensin II contributes to the increase in CBF during hypoxia.

Ketamine antagonizes hypoxia-induced dopamine release in rat striatum

The purpose of this study is to investigate the hypothesis that ketamine, a non-competitive antagonist of the N-methyl-D-aspartate (NMDA) receptor, attenuates hypoxia-induced striatal dopamine release in vivo. High-speed chronoamperometric recording techniques, using Nafion-coated carbon fiber electrodes, were used to evaluate extracellular dopamine (DA) concentration in the striatum. KCl and DA were locally applied directly to the striatum of urethane-anesthetized Sprague-Dawley rats, in order to measure release and clearance, respectively, of DA. These anesthetized animals were paralyzed with D-tubocurarine and connected to a respirator to allow controlled respiration. Systemic concentrations of oxygen and carbon dioxide were altered by changing the partial pressure of O2, CO2, N2 of inspired air and the rate of the respirator. Our data indicate that lowering the respiratory rate from 90 to 20 times/min for 5 min, in room air, caused a decrease in blood O2 while increasing the CO2 concentration. These changes in blood gas concentration were reversible and reproducible. We also found that lowering the respiratory rates potentiated K(+)-induced DA release but not DA clearance in the striatum. In an attempt to induce hypercapnia, the room air was replaced with high CO2-containing air (15% CO2 + 20% O2 + 65% N2), and this change resulted in increased blood CO2 levels without lowering O2 concentration. The hypercapnia did not alter K(+)-induced DA release in the striatum. Next, we attempted to simulate anoxic hypoxia in the absence of hypercapnia. Respiration with pure N2 for 30 s resulted in lowering blood O2 without increasing CO2 levels. Both basal and K(+)-evoked DA releases were increased during N2-induced anoxic hypoxia. These data suggested that transient hypoxia facilitates DA release in the striatum. It has been suggested that NMDA is involved in many hypoxia-mediated responses. We also found that systemic application of ketamine, which itself did not affect blood O2 or CO2 levels, antagonized hypoxia-induced electrochemical responses. These data suggest that the increase in DA release in vivo during short-term hypoxia may probably be mediated through NMDA receptors.

Endogenous angiotensin II inhibits production of cerebrospinal fluid during posthypoxemic reoxygenation in the rabbit

BACKGROUND AND PURPOSE

The choroid plexus, the major source of cerebrospinal fluid (CSF), contains receptors for angiotensin II and a very high concentration of angiotensin converting enzyme. Circulating angiotensin II decreases blood flow to the choroid plexus and the production of CSF. During recovery from hypoxia, marked increases in circulating angiotensin II have been described in some studies. We tested the hypothesis that CSF production decreases during posthypoxemic reoxygenation and examined related changes in plasma concentrations of angiotensin II. We also determined whether effect of posthypoxic reoxygenation on production of CSF is due to endogenous release of angiotensin II.

METHODS

We measured production of CSF in chloralose-anesthetized rabbits using ventriculocisternal perfusion of artificial CSF containing blue dextran. After control measurements, rabbits were subjected to one of the following interventions: (1) 30 minutes of hypoxia (PaO2 = 36 +/- 1 mmHg [mean +/- SE]) followed by 90 minutes of reoxygenation; (2) 30 minutes of hypoxia (PaO2 = 37 +/- 2 mmHg) followed by 90 minutes of reoxygenation in the presence of the angiotensin II antagonist saralasin; (3) hypoxia for 120 minutes (PaO2 = 35 +/- 1 mmHg); and (4) infusion of vehicle under normoxic conditions for 120 minutes (time control). Plasma concentrations of angiotensin II were also measured (radioimmunoassay) under control conditions, during hypoxia, and during posthypoxic reoxygenation (first intervention) and at corresponding time intervals in time control animals (fourth intervention).

RESULTS

Under control conditions, the rate of production of CSF averaged 6.7 +/- 0.1 microL.min-1. During posthypoxemic reoxygenation, production of CSF decreased by 31 +/- 4% (P < .05). In the presence of sarlasin, CSF production did not change significantly during posthypoxemic reoxygenation (-12 +/- 6%, P > .05). In time control animals and during prolonged hypoxia, CSF production did not change significantly (-12 +/- 5% [P > .05] and 9 +/- 7% [P > .05], respectively). Plasma concentrations of angiotensin were below the threshold of sensitivity of the assay under control conditions and during interventions in animals that were made hypoxic and then reoxygenated and in time control animals.

CONCLUSIONS

CSF production decreases during posthpoxemic reoxygenation. Since plasma concentrations of angiotensin II did not change during posthypoxic reoxygenation, this effect does not appear to be mediated by increases in circulating angiotensin II. We speculate that endogenous release of angiotensin II, perhaps in the choroid plexus epithelium, decreases production of CSF after hypoxic brain injury.

Angiotensin II decreases the rate of production of cerebrospinal fluid

The choroid plexus, which produces cerebrospinal fluid (CSF), contains receptors for angiotensin II and a very high concentration of angiotensin-converting enzyme. Circulating angiotensin II decreases blood flow to the choroid plexus. The first goal of this study was to examine the hypothesis that angiotensin II decreases the production of CSF. The second goal was to determine whether effects of angiotensin II on the production of CSF were receptor-mediated. Production of CSF was measured in chloralose-anesthetized rabbits using ventriculocisternal perfusion of artificial CSF containing blue dextran. Rabbits received either vehicle, angiotensin II, angiotensin II in the presence of an angiotensin II antagonist (saralasin), or saralasin intravenously. Increases in blood pressure, during administration of angiotensin II, were prevented by withdrawal of blood. Under control conditions, CSF production averaged 7.2 +/- 0.2 microliters/min (mean +/- S.E.). Angiotensin II (100 ng/kg/min i.v.) decreased CSF production by 24 +/- 3% (P < 0.05, n = 8). In the presence of saralasin (1 microgram/kg/min i.v.), angiotensin II had no significant effect on CSF production (-4 +/- 6%, P > 0.05, n = 7). Vehicle did not affect CSF production significantly (-2 +/- 6%, P > 0.05, n = 7). Saralasin alone decreased production of CSF (-21 +/- 5%, P < 0.05, n = 7). To test the specificity of saralasin in blocking effects of angiotensin II receptor stimulation on CSF production, the carbonic anhydrase inhibitor acetazolamide was administered in the presence and absence of saralasin.(ABSTRACT TRUNCATED AT 250 WORDS)

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Lung angiotensin converting enzyme activity in chronically hypoxic rats

A study was carried out to test the hypothesis that the reduced lung angiotensin converting enzyme (ACE) activity which occurs in chronic hypoxia is related to the development of pulmonary hypertension rather than to hypoxia per se. Right ventricular mean systolic pressure (Prvs, mm Hg) and ACE activity (nmol/mg protein/min) in lung tissue homogenates were measured in seven groups of four rats placed in a hypobaric chamber (380 mm Hg; 51 kPa) for two to 24 days. Identical measurements were made on 11 groups of four rats, which were placed in the chamber for 24 days and then allowed to recover in room air for one to 153 days. After two days of hypoxia the mean Prvs (25.5 (SD 3.7] and the mean lung ACE activity (56 (4.6] did not differ significantly from control values. Exposure to hypoxia for four to 24 days caused a progressive increase in mean Prvs to 44.4 (5.9) and a progressive reduction in mean lung ACE activity to 34 (4.0). During recovery lung ACE activity increased and Prvs decreased, so that normal values were achieved by 15 and 56 days respectively. Decreased lung ACE activity may be related to haemodynamic factors associated with pulmonary hypertension rather than to hypoxia.