Ventilated Piglets Research Articles

Modification of the Glutamate Binding Site of the NMDA Receptor During Graded Hypoxia in the Cerebral Cortex of Newborn Piglets. † 183

We have previously shown that during hypoxia the glutamate binding site of the N-methyl-D-aspartate (NMDA) receptor is altered in newborn piglet brain. The present study tests the hypothesis that hypoxia-induced alteration of the glutamate site of the NMDA receptor is progressive and correlates with the decrease in cerebral energy metabolism induced by hypoxia. Studies were performed in 8 ventilated piglets, 4 normoxic and 4 hypoxic. Varying degrees of hypoxia were achieved by exposure to decreased oxygen at different concentrations and confirmed by brain tissue ATP and phosphocreatine (PCr) levels. 3[H]glutamate binding was performed in P2 membrane fractions at concentrations of 25-1000 nM. Specific NMDA-displaceable3[H]-glutamate binding was determined with 100μM NMDA. Non-specific binding was determined with 1mM unlabelled glutamate. Bmax (receptor number) and Kd (dissociation constant) were calculated from Scatchard plots. ATP(μmol/g brain), PCr (μmol/g brain) and Bmax (fmol/mg protein) were as follows: (5.86, 2.77, 402), (4.77, 2.96, 388), (4.50, 2.05, 364), (4.03, 2.01, 332), (3.37, 0.96, 276), (2.49, 0.24, 253), (1.96, 0.77, 322), and (0.24, 0.46, 174). Bmax decreased in a linear relationship as both ATP (r=0.92) and PCr (r=0.83) decreased, but Kd did not. The data show that the number of glutamate binding sites decreased in a linear relationship as oxidative phosphorylation decreased during hypoxia. A decline in oxidative phosphorylation may lead to a reduction in glutamate binding sites by several mechanisms. Since phosphorylation regulates NMDA receptor distribution in neuronal membranes, we speculate that a decrease in oxidative phosphorylation as seen during hypoxia may lead to a loss in number of glutamate binding sites through changing the location and density of NMDA receptors in brain cell membranes. Alternatively, we speculate that dephosphorylation of the receptor may cause an allosteric or conformational change in the receptor preventing glutamate binding and may lead to lipid peroxidation and cell death.

Effect of Hyperglycemia on Brain Cell Membrane Function and Energy Metabolism During Hypoxia-Ischemia in Newborn Piglets. † 840

The present study tests the hypothesis that hyperglycemia ameliorates changes in brain cell membrane function and preserves cerebral high energy phosphates during hypoxia-ischemia in newborn piglets. 41 ventilated piglets were divided into 4 groups: Gr 1 (normoxic/ normoglycemic, n=9), Gr 2(normoxic/hyperglycemic, n=8), Gr 3 (hypoxic-ischemic [HI]/normoglycemic, n=13) and Gr 4 (HI/hyperglycemic n=11). Cerebral HI was induced by 8% oxygen and occlusion of bilateral common carotid arteries for 30 min. Hyperglycemia(blood glucose 20 mmol/L) was maintained for 90 min before and throughout HI. Changes in cytochrome aa3 (Cytaa3) were continuously monitored using near infrared spectroscopy. Blood and CSF glucose and lactate were monitored. Na+,K+-ATPase activity, lipid peroxidation products (conjugated dienes [CDs]) and tissue high energy phosphates (ATP and phosphocreatine[PCr]) levels were determined biochemically in the cerebral cortex. During HI glucose levels in blood and CSF were significantly elevated in Gr 4 (21.1± 3.7, 13.7 ± 2.0) compared with Gr 3 (6.7 ± 2.4, 4.1± 0.8 mmol/L, mean ± SD p<0.05), but lactate levels in blood and CSF were not different between Gr 3 and Gr 4. In HI Gr 3 and 4, Δ Cyt aa3, Na+,K+-ATPase activity, [ATP] and [PCr] values were significantly decreased compared with normoxic Gr 1 and 2 but were not different between Gr 3 and 4. [CDs] were significantly increased in Gr 3 and 4 compared with Gr 1 and 2, and were significantly elevated in Gr 4 than Gr 3(0.30±0.11 vs 0.09±0.02 μmol/g protein, p<0.05). We conclude that hyperglycemia is not neuroprotective and does not preserve cerebral high energy phosphates during HI in piglets. We speculate that hyperglycemia may be harmful during HI due to increased levels of lipid peroxidation. (Funded by SBRI C-95-019-2 and HMP-96-M-2-107).

Venous Oxygen Saturation (SvO2) During Alveolar Hypoxia and Anemic Hypoxia. 115

Background: We studied SvO2 and markers of tissue hypoxia to assess the value of SvO2 during two types of hypoxia. Subjects: Two groups of ventilated neonatal piglets. Intervention: We induced alveolar hypoxia by giving FiO2: 0.1 in one group (ALV) and anemic hypoxia by exchanging blood with plasma in the other group (ANEM). SvO2 was measured by a fiberoptic catheter (Oximetrix, Abbott Lab). Aorta flow (Qt), arterial and venous bloodgases, hemoglobin and lactate were measured. O2 delivery (DO2), percent change O2 consumption (perc.ch VO2) and percent change lactate(perc.ch LAC) were calculated. Mann-Whitney U test was applied. Results: *: p = 0.002; Table

Effect of baseline lung compliance on the subsequent response to positive end-expiratory pressure in ventilated piglets with normal lungs

To determine the pulmonary function and hemodynamic effects of incremental positive end-expiratory pressure in two groups of normal ventilated newborn piglets with different baseline dynamic lung compliance. Prospective, controlled, intervention study. Animal laboratory. One group of piglets (inflation group) was prepared with 3 cm H2O (0.29 kPa) positive end-expiratory pressure and a maximal lung inflation to increase baseline lung compliance as compared with the other group (no-inflation group), prepared by 3 hrs of ventilation at zero end-expiratory pressure. Both groups were then subjected to a sequence of incremental positive end-expiratory pressures from 0 to 12 cm H2O (0 to 1.18 kPa) in 2-cm increments for 15-min periods at each level followed by a 60-min recovery period at zero end-expiratory pressure. Pulmonary function, hemodynamic and blood gas data were collected at each positive end-expiratory pressure value and at 15-min intervals during recovery. Baseline dynamic lung compliance was 5.2 +/- 0.3 mL/cm H2O (53.04 +/- 3.06 mL/kPa) in the inflation group and 2.5 +/- 0.1 mL/cm H2O (25.5 +/- 1.02 mL/kPa) in the no-inflation group. No differences were found in any other pulmonary function, hemodynamic or blood gas value at baseline. Incremental positive end-expiratory pressure resulted in a decrease in dynamic lung compliance and an increase in end-expiratory lung volume in both groups of piglets; dynamic lung compliance was greater in the inflation group at all times. No differences were found in end-expiratory lung volume between groups. Hemodynamic changes in both groups of piglets included: decreased cardiac output and increased pulmonary vascular resistance and systemic vascular resistance. The changes in cardiac output (-23% vs. -32%), pulmonary vascular resistance (+53% vs. +95%), and systemic vascular resistance (17% vs. 51%) were less in the inflation group as compared with the no-inflation group. Baseline dynamic lung compliance is an important determinant of the subsequent effect of positive end-expiratory pressure on pulmonary function and hemodynamics in the ventilated piglet with normal lungs.

Effect of baseline lung compliance on the subsequent response to positive end-expiratory pressure in ventilated piglets with normal lungs

Effect of baseline lung compliance on the subsequent response to positive end-expiratory pressure in ventilated piglets with normal lungs

The effects of reversing distending pressure sequences in the neonatal piglet.

We studied different sequences of lung inflation in ventilated newborn piglets with normal lungs in order to determine the effects of sequence, magnitude and duration of distending pressure on pulmonary function, and/or hemodynamics. End-expiratory pressure was varied using a continuous negative extrathoracic pressure (CNEP) device. Three groups of ventilated piglets with normal lungs were exposed to 2 cmH2O increments of CNEP from -2 to -12 cmH2O, and to decrements from -12 to -2 cmH2O, or to only -6 cmH2O. Lung inflation sequence, magnitude of inflation pressure, and duration of inflation had significant effects on end-expiratory lung volume and lung compliance at numerically equivalent pressure levels. End-expiratory lung volume and lung compliance varied (at four and five of six inflation pressures studied) by as much as 68% and 104%, respectively. Hemodynamic effects of the lung inflation sequence were more variable; those found to be different at numerically equivalent pressure levels were associated with changes in lung compliance and ventilation. Differences in pulmonary mechanics can best be explained by the effects of lung inflation on alveolar recruitment versus overinflation.

EFFECTS OF NaHCO3 ON CEREBRAL BLOOD FLOW (CBF) DURING HYPOXIA AND ACIDOSIS IN NEWBORN PIGLETS

A reciprical relationship exists between CBF and arterial O2 content (CaO2) during hypoxic hypoxia. Prolonged hypoxia may be complicated by metabolic acidosis further decreasing CaO2. To investigate if NaHCO3 lowers CBF by increasing pH and CaO2 CBF (microspheres), osmolarity, and arteriovenous differences of O2 content and blood gases were obtained in 14 ventilated piglets. Measurements were obtained during control (C), prolonged isocapnic hypoxia (50 min) to produce metabolic acidosis (H+A), and following infusion of saline (Grl, n=6) or 2mEq/kg NaHCO3 (Gr2, n=8) during hypoxia. At C pH was 7.44±.01(X±SE) in Grl and 7.41±.01 in Gr2. H+A resulted in similar reductions in pH (Grl=7.21±.02 and Gr2=7.21±.03), but differences occurred after the infusions, (Grl=7.15±.03 vs. Gr2=7.30±.03, p<.05). In each Gr CaO2 paralleled pH but did not differ between Gr. CBF (ml/min·100g) in Grl was 120±17 at C, increased to 287±60* during H+A and 259±31* during saline. Similarly, CBF rose in Gr2 from C of 116±12 to 251±23* during H+A, and 221±34* with NaHCO3. Although cerebral O2 delivery, extraction, and uptake changed (due to the extent of CaO2 reduction), both Gr were comparable. No changes in osmolarity occurred. It appears that the magnitude of change in pH induced by 2 mEq/kg NaHCO3 does not alter CBF. It is speculated that even larger increases in pH produced by NaHCO3 may not affect CBF due to changes in O2 affinity. (*p<.05 vs. control).