Cardiovascular, metabolic and neurologic effects of carbon monoxide and cyanide in the rat

Alteration in Cerebral Metabolism in a Rodent Model of Acute Sub-lethal Cyanide Poisoning

INTRODUCTION

Cyanide exposure can occur in various settings such as industry and metallurgy. The primary mechanism of injury is cellular hypoxia from Complex IV (CIV) inhibition. This leads to decreased ATP production and increased reactive oxygen species production. The brain and the heart are the organs most affected due to their high metabolic demand. While the cardiac effects of cyanide are well known, the cerebral effects on cellular function are less well described. We investigated cerebral metabolism with a combination of brain respirometry, microdialysis, and western blotting using a rodent model of sub-lethal cyanide poisoning.

METHODS

Twenty rodents were divided into two groups: control (n = 10) and sub-lethal cyanide (n = 10). Cerebral microdialysis was performed during a 2 mg/kg/h cyanide exposure to obtain real-time measurements of cerebral metabolic status. At the end of the exposure (90 min), brain-isolated mitochondria were measured for mitochondrial respiration. Brain tissue ATP concentrations, acyl-Coenzyme A thioesters, and mitochondrial content were also measured.

RESULTS

The cyanide group showed significantly increased lactate and decreased hypotension with decreased cerebral CIV-linked mitochondrial respiration. There was also a significant decrease in cerebral ATP concentration in the cyanide group and a significantly higher cerebral lactate-to-pyruvate ratio (LPR). In addition, we also found decreased expression of Complex III and IV protein expression in brain tissue from the cyanide group. Finally, there was no change in acyl-coenzyme A thioesters between the two groups.

CONCLUSIONS

The key finding demonstrates mitochondrial dysfunction in brain tissue that corresponds with a decrease in mitochondrial function, ATP concentrations, and an elevated LPR indicating brain dysfunction at a sub-lethal dose of cyanide.

Behavioral toxicity of sodium cyanide following oral ingestion in rats: Dose-dependent onset, severity, survival, and recovery

Sodium cyanide (NaCN) is a commonly and widely used industrial and laboratory chemical reagent that is highly toxic. Its availability and rapid harmful/lethal effects combine to make cyanide a potential foodborne/waterborne intentional-poisoning hazard. Thus, laboratory studies are needed to understand the dose-dependent progression of toxicity/lethality following ingestion of cyanide-poisoned foods/liquids. We developed an oral-dosing method in which a standard pipette was used to dispense a sodium cyanide solution into the cheek, and the rat then swallowed the solution. Following poisoning (4-128 mg/kg), overt toxic signs were recorded and survival was evaluated periodically up to 30 hours thereafter. Toxic signs for NaCN doses higher than 16 mg/kg progressed quickly from head burial and mastication, to lethargy, convulsions, gasping/respiratory distress, and death. In a follow-on study, trained operant-behavioral performance was assessed immediately following cyanide exposure (4-64 mg/kg) continuously for 5 h and again the following day. Onset of behavioral intoxication (i.e., behavioral suppression) occurred more rapidly and lasted longer as the NaCN dose increased. This oral-consumption method with concomitant operantbehavioral assessment allowed for accurate dosing and quantification of intoxication onset, severity, and recovery, and will also be valuable in characterizing similar outcomes following varying medical countermeasure drugs and doses.

Acute carbon monoxide poisoning alters hemorheological parameters in human

Acute carbon monoxide (CO) poisoning seriously hinders oxygen delivery to tissues. This harmful effect of CO may be aggravated by accompanying changes in the viscosity of blood. We had previously reported increased plasma viscosity in people chronically exposed to CO. This study was planned to test our hypothesis that acute CO poisoning increases blood viscosity. For this purpose four main parameters contributing to blood viscosity - hematocrit, erythrocyte deformability, erythrocyte aggregation and plasma viscosity - were determined in patients with acute CO poisoning and compared with healthy controls. Plasma viscosity and erythrocyte aggregation tendency were lower in the CO group (p <  0.05). Erythrocyte deformability was also lower in CO group (p <  0.05). Our results indicate that acute CO poisoning has diverse effects on hemorheological parameters such as attenuating hematocrit value, plasma viscosity, erythrocyte aggregation tendency and erythrocyte deformability.

Value of lactic acidosis in the assessment of the severity of acute cyanide poisoning

OBJECTIVE

To test the hypothesis that plasma lactate concentrations could be of confirmatory value in patients with histories consistent with acute pure cyanide poisoning because immediate laboratory confirmation of suspected cyanide poisoning is rarely possible and because clinicians must rapidly decide whether to administer specific antidotes, which may have severe side effects.

DESIGN

Retrospective clinical study.

SETTING

An intensive care unit in a university-affiliated teaching hospital.

PATIENTS

All acute cyanide-poisoned patients admitted to our intensive care unit, excluding fire victims, from 1988 to 1999.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Eleven patients were studied. Before antidotal treatment, the median plasma lactate concentration was 168 mg/dL, the median blood cyanide concentration was 4.2 mg/L. Using Spearman's test, there was a significant correlation between plasma lactate and blood cyanide concentrations ( =.74, =.017). Before antidotal treatment, plasma lactate concentration correlated positively with anion gap and inversely with systolic blood pressure, spontaneous respiratory rate, and arterial pH. During the course of cyanide poisonings, a plasma lactate concentration of >or=72 mg/d/L (8 mmol/L) was sensitive (94%) and moderately specific (70%) for a toxic blood cyanide concentration (>or=1.0 mg/L). The specificity was substantially improved in patients not receiving catecholamines (85%).

CONCLUSIONS

The immediate and serial measurement of plasma lactate concentrations is useful in assessing the severity of cyanide poisoning.

Ischemic tolerance of the heart by adaptation to chronic hypoxia is suppressed by high subchronic carbon monoxide exposure

This study was designed to investigate whether exposure to carbon monoxide (CO) could alter the ischemic tolerance induced by chronic hypoxia. We aimed to determine whether chronic hypoxia-induced cardiovascular adaptation was modified during the return to normoxia or by subchronic CO exposure. The degree of resistance to an in vitro transient ischemia was measured, using the Langendorff method, in hearts from rats previously exposed to chronic hypoxic hypoxia and/or subchronic CO exposure to 600 ppm. Chronic hypoxia decreased ischemic contracture (15.6 +/- 04.9 vs. 60.8 +/- 07.7%) and improved both contractile recovery (59.6 +/- 07.3 vs. 21.8 +/- 06.8%) and ventricular arrhythmia during reperfusion (0 vs. 45%) compared to a control normoxic group. However, in our chronic hypoxia regression model many parameters returned near to control values except for the persistence of cardiomegaly, a significant decrease in both ischemic contracture (22.0 +/- 04.9 vs. 60.8 +/- 07.7%), and ventricular tachycardia (25 vs. 45%). CO exposure alone increased the coronary flow and improved both contractile recovery (42.6 +/- 7.2 vs. 21.8 +/- 6.8%) and ventricular arrhythmia (16.7 vs. 45%) without altering the action potential shape. These two models causing tissue hypoxia induced the same degree of polycythemia or cardiomegaly and provided similar ischemic tolerance. CO exposure after chronic hypoxia exacerbated ischemic contracture (69.3 +/- 10.5 vs. 22.0 +/- 14.5%) and ventricular tachycardia incidence (100 vs. 50%) but with significant alteration in contractile recovery (12.7 +/- 10.5%) compared to the chronic hypoxia or CO exposure. Thus, CO exposure completely suppressed the chronic hypoxia-induced ischemic tolerance.

Cardiovascular effects of subchronically low/high carbon monoxide exposure in rats

This study was designed to determine whether subchronic CO exposure ranging from 15 to 530 ppm induced modifications in the rat cardiovascular system. We investigated the degree of resistance to an in vitro transient ischemia in the hearts exposed in vivo to different CO concentrations for 1-4 weeks. Subchronic CO exposure induced dose and/or time-dependent increases (hematocrit, cardiomegaly and coronary flow). We showed an increase in the ventricular tachycardia (VT) incidence with the passing weeks of exposure, which demonstrated the proarrhythmic activity of CO even in low doses (15 ppm). The contractile recovery decreased owing to a low (50 ppm) or high (530ppm) CO exposure after a 25-min ischemia period. This diminution seems to be dependent on the increased amplitude of ischemic contracture. The present study supports the hypothesis that subchronic CO exposure, even at low levels of CO, can produce cardiovascular changes and could explain the increased risk of myocardial infarction.

Resurgence of carbon monoxide: an endogenous gaseous vasorelaxing factor

Carbon monoxide (CO) is an endogenously generated gas that may play an important physiological role in the regulation of vascular tone. The CO-induced vasorelaxation, as a result of a direct action on vascular smooth muscles, has been demonstrated in many cases. Three major cellular mechanisms are proposed to explain the vasorelaxing effect of CO. These include the activation of soluble guanylyl cyclase, stimulation of various types of K channels, and inhibition of the cytochrome P450 dependent monooxygenase system in vascular smooth muscle cells. An interaction between CO and nitric oxide may also significantly contribute to the fine tuning of vascular tone. Furthermore, alterations in either the endogenous production of CO or the vascular responsiveness to CO have been encountered in several pathophysiological situations. A better understanding of the vascular effects of CO and the underlying cellular and molecular mechanisms will pave the way for the establishment of the role played by CO in vascular physiology and pathophysiology.Key words: carbon monoxide, heme oxygenase, smooth muscles, vasorelaxation.

Low-dose carbon monoxide does not reduce vasoconstriction in isolated rat lungs

Recent studies have demonstrated that nitric oxide (NO) acts as a pulmonary vasodilator when inhaled in low concentrations. Due to the physicochemical similarities between NO and carbon monoxide (CO), it was speculated that low concentrations of CO would have similar effects in the isolated rat lung. The purpose of this study was to determine the role of CO (200 and 1000 ppm) in modulating hypoxia- and angiotensin II (AII)-induced pulmonary vasoconstriction, using isolated salt-perfused lungs of normotensive (CON) or pulmonary hypertensive male rats. Pulmonary hypertensive rats (ALT), induced by simulated altitude exposure (4572 m; 430 mm Hg for 32-48 days), were studied to determine the actions of low-dose CO in a remodeled pulmonary vascular bed. Right ventricular hypertrophy and polycythemia were evident in the ALT rats, suggesting that simulated altitude exposure induced pulmonary hypertension and consequent pulmonary vascular remodeling. CO did not significantly affect pulmonary vascular responses to acute hypoxia (6% CO2, balance N2) in either CON or ALT rats. There were also no significant differences in pulmonary pressor responses to AII injections (0.2 or 0.4 micrograms) in CON or ALT lungs either immediately following or during an acute hypoxia + CO exposure. Therefore, acute low-dose CO exposure (< 1000 ppm) does not appear to attenuate pulmonary vasoconstriction in isolated rat lungs.

Metabolic, cardiovascular, and neurologic aspects of acute cyanide poisoning in the rat

Acute cyanide (CN) toxicity was investigated in the Sprague-Dawley rat. Conscious, loosely restrained rats received sodium CN solution at varying dose rates through a jugular cannula (low CN, 0.077-0.155 mg/kg/min; high CN, 0.157-0.204 mg/kg/min). Blood glucose concentration was significantly increased 45 min after initial CN treatment in both the low and the high CN groups compared to the saline controls. Blood lactate concentration was significantly increased only in the high CN group after 45 min. Lactate increased directly with CN dose rate in surviving high CN rats. In rats that succumbed during CN infusion, lactate concentration reached nearly 150 mg/dl. Body temperature decreased modestly at low CN dose rates, but increased markedly at high CN dose rates. Heart rate was relatively constant in the low CN group, but decreased rapidly in the high CN group with increasing CN dose rate. In rats surviving CN treatment, no significant alterations in either cerebral cortical water content or neurologic status were detected. This contrasts with another potent poison, carbon monoxide, which produces marked neurologic deficit and cerebral edema in this animal model. The mean lethal CN dose was 4.6 mg/kg (range 4.25-4.90 mg/kg). Expressed on the basis of CN infusion rate, the lethal zone was from 0.16 to 0.21 mg/kg/min, a surprisingly narrow range. Assuming that extrapolations are possible to other species, the data provide strong evidence that greatly elevated blood lactate may be a useful marker for CN poisoning very near or within the lethal zone.

Electrocardiographic responses to carbon monoxide and cyanide in the conscious rat

Carbon monoxide (CO) and cyanide (CN), commonly found in exhaust fumes and smoke, act as hypoxic agents in eliciting morbid and lethal effects. This study explored the effects of these two toxicants on the ECG in a controlled and well-characterized animal model. Levine-prepared awake female rats were treated with 1500 and 2400 ppm CO for 90 min, CN at 4 mg/kg, or 1500 ppm CO plus 4 mg/kg CN. As in past studies, CO initially induced hyperglycemia and many-fold increases in blood lactate concentration, and rebound increases in blood glucose during recovery. CN produced hyperglycemia, however, there was no glucose rebound, nor was there a significant increase in lactate. CN plus 1500 ppm CO produced glucose changes similar to that of CO alone. CO exposure also induced hypothermia and hypotension, while CN produced little change in these parameters. CO increased heart rate, while CN tended to decrease heart rate. PR interval was increased significantly 4.5-17.0 ms by exposure to CO, with or without combination with CN, while CN alone produced minimal change in the PR interval. QT interval was increased up to 20 ms by exposure to CO, with or without combination with CN. CN alone produced no change in the QT interval. T wave duration was increased up to 22.5 ms by exposure to 1500 ppm CO, with or without combination with CN. CN alone produced minimal changes in T wave duration. There were no changes in duration of the (Q)RS complex or of the R wave. QT interval lengthening was positively correlated with the decrease in systolic blood pressure (0-30 min, r = 0.657, P < 0.05; 0-60 min, r = 0.704, P < 0.05). Hypothermia was correlated with increase in lactate concentration (r = 0.73, P < 0.05) and with decrease in blood pressure (r = 0.69, P < 0.05). No correlation between body temperature and QT interval was observed. The results indicate that CO at the concentrations used in the Levine-prepared rat has major effects on the ECG in slowing AV conduction and ventricular repolarization. In contrast, CN at 4 mg/dl has little or no effect on either conduction or repolarization in this animal model. These findings are discussed in light of past animal and human studies.