Role of endogenous opioid peptides in the acute adaptation to hypoxia

A non-lethal, hypoxic conditioning stimulus has been shown by Rising and D'Alecy to increase hypoxic survival time in mice. To determine if endogenous opioids alter the hypoxic conditioning-induced increase in hypoxic survival time, we administered naloxone (0.1, 1.0 mg/kg i.p.) or saline (0.3 ml i.p.) 5 min prior to conditioning. Sixty percent of the mice received the hypoxic conditioning stimulus consisting of three seqeuntial hypoxic exposures (4.5% oxygen balance nitrogen for 1.5, 2 and 2.5 min) separated by 5 min of room air. The remaining mice did not receive hypoxic conditioning but instead remained in room air for this time. All mice were tested for hypoxic survival by first exposing them to 20 s of 8.5% oxygen balance nitrogen followed by exposure to 4.5% oxygen balance nitrogen. The hypoxic survival time was recorded as the time from the onset of the 4.5% oxygen to the cessation of spontaneous ventilation. Naloxone (1 mg/kg) completely blocked the adaptation to hypoxia induced by hypoxic conditioning (P = 0.003). Morphine (1, 5, 10 and 20 mg/kg) had no effect on hypoxic adaptation; however, 50 mg/kg morphine decreased the adaptation induced by conditioning (P < 0.0001) possibly due to high dose toxicity. These data suggest that endogenous opioids are involved in the protective adaptation to hypoxia induced by prior exposure to non-lethal hypoxia.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29985/1/0000349.pd

Continued circulatory support: effect of epinephrine or dopamine on 24-hour survival and neurologic function in dogs

The effects on 24-h survival and neurologic function were compared following continued postresuscitation circulatory support with epinephrine or dopamine. Cardiopulmonary arrest was induced by ventricular fibrillation. After 10 min, resuscitation efforts were initiated including i.v. infusion of either epinephrine (6 [mu]g/kg per min, 11 dogs) or dopamine (10 [mu]g/kg per min, 14 dogs) for continued circulatory support. There was no difference detected in duration of circulatory support, although dogs receiving epinephrine required more lidocaine (3.3 +/- 0.4 vs. 1.8 +/- 0.3 mg/kg, P = 0.005). Likewise, there was no statistically significant difference detected in MAP or HR between groups at any time tested. However, dogs receiving epinephrine had significantly worse neurologic function at 6 and 12 h postarrest. Mean survival time (20.3 +/- 1.2 vs. 15.3 +/- 1.9 h, P = 0.028) and overall survival (P = 0.027, survival curve analysis) were significantly longer for dogs receiving dopamine. Plasma glucose in the first 6 h postarrest was significantly higher in dogs receiving epinephrine (P = 0.006). These results suggest that the use of epinephrine for continued vasopressor support in cardiopulmonary resuscitation may contribute to decreased survival and poorer neurologic function in this controlled experimental setting. It is reasonable to propose that similar responses to these commonly used circulatory support agents occur clinically. Therefore, continued vasopressor support with dopamine rather than epinephrine may be justified in the setting of cardiac resuscitation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27916/1/0000339.pd

A simplified organ donor model produced by permanent complete central nervous system ischemia in dogs

The physiologic and pathophysiologic changes of the potential organ donor in the brain-dead state are poorly understood. We have developed a canine model of complete brain death by infusing saline into the subdural space and elevating intracranial pressure above systolic arterial pressure. A snare on the inferior vena cave decreased venous return and limited the Cushing response to 1 to 2 minutes. The electroencephalogram became isoelectric as soon as intracranial pressure was elevated and maintained above systolic arterial pressure. The brain-dead state was confirmed by several means. No return of electroencephalogram activity was seen throughout the study. Neurologic examination failed to document any cranial nerve or spinal cord reflexes after the induction of brain death. Pharmacologic challenges failed to document any baroreceptor function or autonomic cardiovascular control in this state. Blood flow to the central nervous system using the microsphere technique was determined at 1 hour post-brain death. The mean calculated flows were not statistically different than zero. The model provides a simple, controlled, consistent, and relatively noninvasive model of complete brain death. It should facilitate the investigation of the mechanisms leading to somatic death in the brain-dead state and ultimately lead to improved clinical supports.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29518/1/0000605.pd

Direct monitoring pressure overload predicts cardiac hypertrophy in mice

Pressure overload (POL) is a classical model for studying cardiac hypertrophy, but there has been no direct measure of hemodynamics in a conscious ambulatory mouse model of POL. We used abdominal aortic constriction to produce POL and radiotelemetry to measure the blood pressure and heart rate for three weeks. The cardiac size correlated with the systolic pressure in the last week is better than other hemodynamic parameters. Cardiac fibrosis was more correlated to the cardiac size than to the systolic pressure. The expression of the cardiac genes that are typically associated with cardiac hypertrophy was correlated with both cardiac size and systolic pressure. In conclusion, the systolic pressure is the major determinant of cardiac hypertrophy in the murine POL model. In contrast, cardiac fibrosis shows the influence of other factors besides systolic pressure. The combination of the POL model with continuous direct measurements of hemodynamics represents a significant technological advance and will lead to an extended usefulness of POL methodologically.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/58150/2/pm7_11_001.pd

Packed Red Blood Cells Are an Abundant and Proximate Potential Source of Nitric Oxide Synthase Inhibition

<div><p>Objective</p><p>We determined, for packed red blood cells (PRBC) and fresh frozen plasma, the maximum content, and ability to release the endogenous nitric oxide synthase (NOS) inhibitors asymmetric dimethylarginine (ADMA) and monomethylarginine (LNMMA).</p><p>Background</p><p>ADMA and LNMMA are near equipotent NOS inhibitors forming blood’s total NOS inhibitory content. The balance between removal from, and addition to plasma determines their free concentrations. Removal from plasma is by well-characterized specific hydrolases while formation is restricted to posttranslational protein methylation. When released into plasma they can readily enter endothelial cells and inhibit NOS. Fresh rat and human whole blood contain substantial protein incorporated ADMA however; the maximum content of ADMA and LNMMA in PRBC and fresh frozen plasma has not been determined.</p><p>Methods</p><p>We measured total (free and protein incorporated) ADMA and LNMMA content in PRBCs and fresh frozen plasma, as well as their incubation induced release, using HPLC with fluorescence detection. We tested the hypothesis that PRBC and fresh frozen plasma contain substantial inhibitory methylarginines that can be released chemically by complete <i>in vitro</i> acid hydrolysis or physiologically at 37°C by enzymatic blood proteolysis.</p><p>Results</p><p><i>In vitro</i> strong-acid-hydrolysis revealed a large PRBC reservoir of ADMA (54.5 ± 9.7 µM) and LNMMA (58.9 ± 28.9 μM) that persisted over 42-d at 6° or -80°C. <i>In vitro</i> 5h incubation at 37°C nearly doubled free ADMA and LNMMNA concentration from PRBCs while no change was detected in fresh frozen plasma.</p><p>Conclusion</p><p>The compelling physiological ramifications are that regardless of storage age, 1) PRBCs can rapidly release pathologically relevant quantities of ADMA and LNMMA when incubated and 2) PRBCs have a protein-incorporated inhibitory methylarginines reservoir 100 times that of normal free inhibitory methylarginines in blood and thus could represent a clinically relevant and proximate risk for iatrogenic NOS inhibition upon transfusion.</p></div

Hyperglycemia exacerbates and insulin fails to protect in acute renal ischemia in the rat

Hyperglycemia worsens ischemic injury in several ischemic models. To determine whether renal lactate accumulation was associated with hyperglycemia-exacerbated postischemic renal dysfunction and mortality, halothane-anesthetized rats underwent right nephrectomy and 45 min of left renal artery and vein occlusion. Prior to ischemia, rats received saline (n = 22), glucose (2 g/kg, n = 22), or insulin (4 U/kg, n = 18). Sham-operated glucose-treated rats (2 g/kg, n = 4) underwent right nephrectomy and no vascular occlusion. As anticipated, glucose pretreatment elevated plasma glucose, while insulin pretreatment lowered plasma glucose; both were significantly different from values in saline controls. Creatinine was unchanged in sham-operated rats but was significantly higher in glucose-treated rats at 24 and 48 hr postischemia compared to saline controls. No statistical differences in creatinine were found when comparing saline controls to insulin-treated rats. Eighteen percent of glucose-treated rats survived to 72 hr postocclusion, while 45% of insulin-treated rats, 73% of saline control rats, and 100% of sham-operated rats survived this period. In a separate but identical treatment protocol, renal tissue was serially sampled and lactate content was determined in rats pretreated with saline (n = 7), glucose (n = 6) or insulin (n = 6) or sham-operated (n = 2) and receiving identical operation. Tissue lactate concentration did not change during serial sampling in the sham group. During ischemia, lactate was significantly higher in glucose-treated rats and significantly lower in insulin-treated rats as compared to saline controls. The adverse effects of exogenous glucose and attendant hyperglycemia on renal function during normothermic renal ischemia are demonstrated. Increased anaerobic metabolism of glucose with marked lactate accumulation may increase the severity of injury. However, a direct link between tissue lactate and ischemic damage is not fully supported since insulin reduced renal lactate but failed to lessen morbidity and mortality.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27924/1/0000348.pd

Cardiac function and microsphere blood flow distribution in the brain-dead dog

The mechanisms for the deterioration of the brain-dead organ donor are not clearly defined. We measured myocardial blood flow and function during the 4 hours after the induction of brain death in dogs. Brain death was induced by elevating and maintaining intracranial pressure above systolic arterial pressure and effectively stopping central nervous system blood flow. Multiorgan blood flow and systemic arteriovenous shunting were measured using radioactive microspheres. The mean arterial pressure was found to decrease markedly with the induction of brain death. The initial changes in mean arterial pressure were attributed to a decrease of systemic vascular resistance, with the more terminal changes due to a decrease in cardiac index. There was a marked decrease of left ventricular dP/dt with the induction of brain death and a gradual decrease of stroke volume despite no change in pulmonary capillary wedge pressure. The microsphere calculated blood flows to the left ventricle and septum of the myocardium were significantly lower at the 1- and 4-hour time points relative to control. However, the coronary sinus oxygen extraction ratio was not statistically different from control at 4 hours. Systemic arteriovenous shunts increased after the induction of brain death but remained below 10%. We conclude that despite brain death-induced hypotension, there is little evidence to suggest that marked myocardial hypoxic ischemic changes initiated the deterioration in this model.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29990/1/0000357.pd

Ischemia with intermittent reperfusion reduces functional and morphologic damage following renal ischemia in the rat

Attempts to minimize ischemic injury by interrupting a given ischemic period might be compromised if repeated bouts of reperfusion injury occurred. To determine whether intermittent ischemia improved or worsened functional and morphologic outcome of renal ischemia, halothaneanesthetized rats underwent a right nephrectomy and placement of a snare about the left renal vascular pedicle at 37° C. Eleven animals underwent 45 minutes of continuous renal ischemia (C-ISC), whereas 10 animals received 45 minutes of vessel occlusion interrupted (I-ISC) at 15 and 30 minutes by snare release and 5 minutes of reperfusion. A group of three sham rats underwent the above procedure but did not have the snare tightened. Blood samples were drawn preoperatively and 24, 48, and 72 hours postoperatively for creatinine analysis. At 72 hours the animals were sacrificed and their kidneys morphologically evaluated. The C-ISC group had a significantly higher mean postoperative plasma creatinine ( p <0.01) as well as significantly higher plasma creatinine levels at 24 ( p <0.005) and 48 hours ( p <0.05) than did the I-ISC group. The C-ISC group also demonstrated significantly greater histologic damage than the I-ISC group ( p <0.002) when assessed by a pathologist blinded to the intervention. Sham rats did not demonstrate functional or morphologic damage. These data demonstrate a significantly improved outcome when 45 minutes of renal ischemia is interrupted by periods of reperfusion. We are led to conclude that in this setting reperfusion injury did not overwhelm the salutary effects of interrupting the 45-minute ischemic event.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/41372/1/10016_2005_Article_BF02001009.pd

Infusion of five percent dextrose increases mortality and morbidity following six minutes of cardiac arrest in resuscitated dogs

The aim of this study was to assess the effect of dextrose administration during and following cardiac resuscitation on mortality and morbidity. Thirty-one dogs anesthetized with halothane were subjected to six minutes of ventricular fibrillation and were resuscitated with open chest cardiac message. All dogs were successfully resuscitated. Thirteen received no dextrose infusion and were fully ambulatory, eating and drinking at 24 hours. Ten of the 18 dogs receiving an infusion of 5% dextrose died before 24 hours and the eight that survived were profoundly impaired. Significantly greater neurologic deficits were recorded for dogs with higher blood glucose concentrations. We conclude that the inclusion of dextrose in fluids used in resuscitation increases mortality and morbidity.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26775/1/0000328.pd

Somatosensory evoked potentials of the dog: recording techniques and normal values

Median and tibial nerve somatosensory evoked potentials (SSEPs) of 5 sedated dogs were studied to determine their normal features and optimal stimulation and recording techniques. Cortical potentials were mapped from an extensive array of skull electrodes as each limb was independently stimulated with subdermal needles. The effects of bandpass and stimulus intensity and rate were also assessed. Three cortical components (P1, N1, P2) were evoked by median or tibial nerve stimulation and were localized along the coronal suture at lateral and medial electrodes, respectively. SSEP voltage varied much more than morphology, topography, or latency. The inion was a stable, indifferent reference site. Cortical SSEP frequency content was mostly below 250 Hz. Maximal SSEP voltage was achieved only at stimulus intensities 2-3 times motor threshold. Appropriate methods minimize technical difficulties and consistently yield legible SSEPs.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27990/1/0000423.pd