David A. Mei

Role of adenosine in isoflurane-induced cardioprotection

Background: This investigation tested the hypothesis that adenosine (A1) receptor blockade modulates the cardioprotective effects of isoflurane. Methods: Hemodynamics and percentage segment shortening (%SS) in the left anterior descending coronary artery (LAD) perfusion territory were evaluated in barbiturate‐anesthetized dogs (n = 31) at selected intervals after pretreatment with the selective A1 receptor antagonist (8‐cyclopentyl‐1,3,dipropylxanthine; DPCPX 0.8 mg/kg, intravenously) or drug vehicle in the presence or absence of 1 minimum alveolar concentration (MAC) isoflurane. Dogs were subjected to five 5‐min occlusions and reperfusions of the LAD, followed by 180 min of final reperfusion. Isoflurane was administered for 30 min before and during LAD occlusions and reperfusions and was discontinued at the onset of final reperfusion. Two other groups of dogs (n = 17) were used to measure interstitial concentrations of purines in the LAD region using a microdialysis technique in the presence and absence of isoflurane. Results: Dogs receiving drug vehicle or DPCPX exhibited no recovery of %SS after 180 min of reperfusion (‐5 +/‐ 7 and 5 +/‐ 11% of baseline, respectively, +/‐ SEM). In contrast, dogs receiving isoflurane alone demonstrated complete recovery of %SS at 60 min after reperfusion. DPCPX pretreatment partially attenuated isoflurane‐induced enhancement of recovery of %SS (34 +/‐ 11% of baseline 180 min after reperfusion; P <0.05). Interstitial purine concentrations were increased during multiple occlusions and reperfusions of the LAD in dogs not receiving isoflurane, but they were unchanged by coronary artery occlusion and reperfusion in dogs receiving isoflurane. Conclusions: The results indicate that isoflurane‐induced cardioprotection in stunned myocardium is partially mediated by adenosine type 1 receptor activation and is accompanied by decreases in endogenous adenosine release.

Effects of monophosphoryl lipid A on myocardial ischemia/reperfusion injury in dogs.

Summary We wished to determine if the previously observed cardioprotective effects of monophosphoryl lipid A (MLA, 65 μg/kg intravenously, i.v.), an endotoxin derivative, were time related and mediated by an enhancement of antioxidant defense mechanisms, i.e., myocardial catalase and superoxide dismutase (SOD) activities and neutrophil infiltration as assessed by myeloperoxidase (MPO) activity. We also wished to study the effect of pretreatment with MLA on vascular endothelial and smooth muscle function in vivo and in vitro. Barbital-anesthetized dogs were subjected to 60-min left circumflex coronary artery (LCX) occlusion followed by 5-h reperfusion. Myocardial catalase, SOD, and MPO activities were measured at the end of 5-h reperfusion. Pretreatment with MLA 24 h before ischemia produced a significant reduction in myocardial infarct size as measured by triphenyltetrazolium staining (15.3 ± 4.4 vs. 30.9 ± 5.2% in controls, p < 0.05), but 1-h pretreatment with MLA had no protective effect. MLA pretreatment for 24 h resulted in marked reduction (p < 0.05) in MPO activity in the border zone surrounding the infarct. Although a trend indicated an increase in catalase activity in the 24-h pretreatment group, no significant changes were observed in either catalase or SOD activities among the three groups. The cardioprotection produced by MLA was independent of differences in collateral blood flow to the ischemic region assessed by radioactive microsphere technique, systemic hemodynamics, myocardial oxygen demand, and ischemic bed size. Responses of the LCX bed to intracoronary acetylcholine (ACh) or nitroglycerin (NTG) in vivo and responses of isolated femoral artery rings to the endothelium-dependent vasodilators, ACh, A23187, bradykinin, or the nonendothelium-dependent vasodilator, sodium nitroprusside (SNP) in vitro were significantly decreased in the MLA 1-h pretreatment group but not in the 24-h pretreatment group. Incubation of the femoral artery rings from the MLA 1-h pretreatment group with 3 mM L-arginine for 1 h reversed the decreased endothelium-dependent responses to ACh and A23187, but not those to bradykinin. These results indicate that (a) the MLA-induced myocardial infarct size reduction was pronounced when MLA was administered for 24 h but was not evident at 1-h pretreatment; (b) a decrease in neutrophil infiltration into the site of ongoing tissue damage might be partially responsible for the protection; (c) vascular endothelial and/or smooth muscle function were transiently decreased by MLA administration and returned to nearly normal levels 24 h after treatment; and (d) the effect of MLA on endothelium-dependent responses might be mediated by the L-arginine/nitric oxide (NO) pathway.

Criteria for a mediator or effector of myocardial preconditioning: Do KATP channels meet the requirements?

ConclusionsA set of criteria have been presented which we feel need to be met before one can assign an important role for a particular endogenous mediator, ion channel or enzyme as an important component of ischemic PC and evidence has been presented to suggest that the KATP channel meets most of the criteria listed. Obviously, this list is not all conclusive and other important factors will evolve as we increase our understanding of this fascinating phenomenon of ischemic PC.

Evidence for the involvement of the ATP-sensitive potassium channel in a novel model of hypoxic preconditioning in dogs.

OBJECTIVES The major aims of the present study were to determine if a 5 min period of hypoxic (pO2 = 30-40 mmHg) buffer perfusion of the left anterior descending (LAD) coronary artery 10 min prior to a 60-min LAD occlusion produces myocardial preconditioning (PC) and to determine if hypoxic PC is mediated via activation of ATP-sensitive potassium channels (KATP). Normoxic (pO2 = 500-600 mmHg) buffer perfusion served as a control. METHODS Barbital-anesthetized dogs were subjected to 60 min of LAD occlusion followed by 3 h of reperfusion. Hypoxic PC was produced by 5 min of LAD perfusion with hypoxic buffer followed by 10 min of blood reperfusion prior to a 60-min occlusion. A sham PC group, elicited by 5 min of LAD perfusion with normoxic buffer, served as a control. A final group of animals was treated with glibenclamide (0.3 mg/kg i.v.), a specific KATP channel antagonist, 15 min prior to hypoxic PC and 3 microM of glibenclamide was also added to the hypoxic buffer. Transmural myocardial blood flow (TMBF, ml/min/100 g) was determined by radioactive microspheres 30 min after the initiation of the prolonged 60-min occlusion and infarct size (IF/AAR) as a percent of the area at risk (AAR) was determined by triphenyltetrazolium staining. RESULTS There were no significant differences between groups in hemodynamics, AAR, or TMBF. Five minutes of perfusion with hypoxic buffer prior to the 60-min occlusion produced a marked reduction in myocardial infarct size as compared to control animals (control, 30 +/- 7 to 9 +/- 2%, hypoxic PC, P < 0.05). Five minutes of perfusion with normoxic buffer had no effect on infarct size (30 +/- 6%) and pretreatment with glibenclamide completely blocked the protective effect of hypoxic PC (31 +/- 7%). CONCLUSIONS These results support the hypothesis that a brief period of hypoxic buffer perfusion can precondition the heart and that this cardioprotective effect is dependent on the opening of myocardial KATP channels.

Mathematical modeling of Carbon monoxide exposures from anesthetic breakdown : Effect of subject size, hematocrit, fraction of inspired oxygen, and quantity of Carbon monoxide

Background Carbon monoxide (CO) is produced by reaction of isoflurane, enflurane, and desflurane in desiccated carbon dioxide absorbents. The inspiratory CO concentration depends on the dryness and identity of the absorbent and anesthetic. The adaptation of existing mathematical models to a rebreathing circuit allows identification of patient factors that predispose to more severe exposures, as identified by carboxyhemoglobin concentration. Methods From our companion study, the authors used quantitative in vitro CO production data for 60 min at 7.5% desflurane or 1.5% isoflurane at 1 l/min fresh gas flow. The carboxyhemoglobin concentration was calculated by iteratively solving the Coburn Forster Kane equation modified for a rebreathing system that incorporates the removal of CO by patient absorption. Demonstrating good fit of predicted carboxyhemoglobin concentrations to published data from animal and human exposures validated the model. Carboxyhemoglobin concentrations were predicted for exposures of various severity, patients of different sizes, hematocrit, and fraction of inspired oxygen. Results The calculated carboxyhemoglobin concentrations closely predicted the experimental results of other investigators, thereby validating the model. These equations indicate the severity of CO poisoning is inversely related to the hemoglobin quantity of a subject. Fraction of inspired oxygen had the greatest effect in patients of small size with low hematocrit values, where equilibrium and not the rate of uptake determined carboxyhemoglobin concentrations. Conclusion This model predicts that patients with low hemoglobin quantities will have more severe CO exposures based on the attainment of a higher carboxyhemoglobin concentration. This includes patients of small size (pediatric population) and patients with anemia.

Monophosphoryl lipid A attenuates myocardial stunning in dogs : Role of ATP-sensitive potassium channels

Results of previous studies indicate that monophosphoryl lipid A (MLA) reduces myocardial infarct size when administered 24 but not 1 h before a prolonged period of regional ischemia in dogs and rabbits. This cardioprotective effect of MLA could be reversed by the administration of the adenosine triphosphate (ATP)-sensitive potassium channel (K(ATP)) blockers, glibenclamide, or 5-hydroxydecanoate. MLA also was shown to attenuate myocardial stunning in dogs; however, its mechanism in this model remains unknown. Therefore the major aim of our study was to determine the dose-related effect of MLA to enhance contractile function in stunned myocardium and to determine the role of the K(ATP) channel in mediating its cardioprotective effect. To produce myocardial stunning, barbital-anesthetized dogs were subjected to five cycles of 5 min of left anterior descending (LAD) coronary artery occlusion interspersed with 10 min of reperfusion and finally followed by 2 h of reperfusion. Regional segment shortening (%SS) was determined by sonomicrometers implanted in the subendocardium of the ischemic region. Single intravenous doses of MLA in the range of 10-35 microg/kg given 24 h before ischemia resulted in an improvement in %SS over a 2-h reperfusion period. Similar to results obtained in the canine and rabbit infarct models, cardioprotection against stunning with MLA appears to require activation of K(ATP) channels during ischemia, because glibenclamide (50 microg/kg, 15 min before ischemia) completely blocked the effect of MLA to improve regional %SS during reperfusion. Cardioprotective doses of MLA were without effect on systemic hemodynamics, blood gases, and pH throughout the experiment. No treatment-related effects on regional myocardial blood flow were observed during ischemia or reperfusion. These results suggest that MLA improves %SS at doses of 10-35 microg/kg by an ATP-sensitive potassium channel-dependent process, and that MLA may mimic the antistunning effects observed during the second window of ischemic preconditioning.

Myocardial Preconditioning Via ATP-Sensitive Potassium Channels: Interactions with Adenosine

Publisher Summary This chapter provides an overview of myocardial preconditioning via adenosine triphosphate (ATP)-sensitive potassium channels. ATP is broken down to adenosine diphosphate (ADP), and under aerobic conditions, ADP is rapidly reconverted to ATP. Under ischemic, hypoxic, or metabolically challenged conditions, ADP is hydrolyzed to adenosine monophosphate (AMP). Under severe ischemic or hypoxic conditions, AMP is converted to inosine monophosphate (IMP) by adenylate deaminase; however, a portion of AMP is also hydrolyzed to adenosine via the activity of nucleotidases. Adenosine is rapidly metabolized to inosine, and then both inosine and IMP are metabolized to hypoxanthine. Hypoxanthine is metabolized to xanthine, and xanthine to uric acid. In the ischemic myocardium, the metabolic products of ATP hydrolysis rapidly accumulate in the interstitial space, and it is thought that the accumulation of adenosine in the interstitial space may initiate myocardial PC.