Cellular Signal Transduction Pathways for Anesthetic-induced Cardioprotection

Cardiovascular disease is a major healthcare problem in the US. The presence of this disease significantly affects the outcome of both cardiac and non-cardiac surgery, and peri-operative cardiac morbidity is one of the leading causes of death following anesthesia and surgery. The considerable incidence of myocardial infarction, congestive heart failure, myocardial ischemia, or serious dysrhythmias during the intra-operative or post-operative periods has led many studies to examine medical factors and interventions for decreasing cardiac risk in patients with cardiovascular disease. An extensive amount of work has focused on whether any one anesthetic agent or technique is particularly beneficial for patients with coronary artery disease (CAD). Experimental studies conducted in the laboratory have clearly demonstrated that volatile anesthetics exert profound cardioprotection against myocardial ischemia and reperfusion injury. The purpose of this overview is to summarize the interaction of volatile anesthetics with ischemic myocardium and briefly discuss the underlying mechanisms of cardioprotection against ischemia and reperfusion injury.

Discussion

Myocardium is sensitive to hypoxia and ischemia, which can lead to extensive and irreversible tissue injury. However, short periods of ischemia, called ischemic pre-conditioning (IPC), prior to a prolonged ischemic insult, can reduce the severity of cardiac injury. IPC has been studied extensively for the past 20 years and has resulted in the publication of more than 2,000 articles, which have significantly increased our understanding of the cellular mechanisms of ischemia and reperfusion injury. IPC was initially described in 1986¹ by Murray et al., who demonstrated that four five-minute periods of coronary artery occlusion interspersed with five-minute periods of reperfusion before a prolonged coronary occlusion produced a reduction in myocardial infarct size in dogs. Pre-conditioning can also be induced in humans undergoing percutaneous transluminal coronary angioplasty with a coronary occlusion of two minutes' duration.²

Certain pharmacological agents can mimic IPC. Experimental and clinical data indicate that volatile anesthetics may pre-condition the myocardium against ischemia and infarction by activating endogenous protective cellular mechanisms. This is anesthetic-induced pre-conditioning (APC).³ Myocardial protection by volatile anesthetics is of clinical relevance for patients with CAD because of their high morbidity and because of their mortality, which is secondary to peri-operative myocardial ischemia. The beneficial effects of APC in patients have also been reported.^4-10

The mechanisms by which volatile anesthetics exert protective actions on ischemic myocardium have previously been attributed to an improvement in the relationship between myocardial oxygen supply and demand. For example, our research has reported that volatile anesthetics are negative inotropes that decrease left ventricular contractility and depress sino-atrial nodal function. These actions decrease myocardial oxygen demand.^11,12 Although a portion of the cardioprotective effects of volatile anesthetics following ischemia and reperfusion may be attributed to a reduction in energy requirements and the ultimate preservation of energy-dependent vital cellular processes, other studies have demonstrated that this is not necessary for cardioprotection. For example, volatile anesthetics exert cardioprotective effects even when administered during cardioplegic arrest and at reperfusion. Many components of the signaling pathway leading to cardioprotection are shared by IPC and APC. Laboratory and other studies have identified a signaling cascade, as shown in Figure 1, that involves reactive oxygen species (ROS), G-protein-coupled receptors, protein kinases, and adenosine triphosphate (ATP)-sensitive potassium channels.^3,13-40

APC is as powerful as IPC in reducing infarct size, and ATP-sensitive potassium (K_ATP) channels have long been considered essential components of this process. There are two types of K_ATP channels in cardiac myocytes—mitochondrial (mitoK_ATP) channels, which are located in the inner mitochondrial membrane, and sarcolemmal (sarcK_ATP) channels, which are located in the plasma membrane. After the discovery of the mitoK_ATP channels in the inner mitochondrial membrane and development of selective mitoK_ATP and sarcK_ATP channel inhibitors, evidence suggested that mitoK_ATP channels may play a more important role in cardioprotection than sarcK_ATP channels. Some of the recent studies that involved the use of sarcK_ATP-specific inhibitors and genetic models of disrupted or knocked-out sarcK_ATP-channel sub-units indicate that sarcK_ATP channels may have a role in cardioprotection and should not be ignored.^41,42

Our research utilized patch clamp methodology to directly demonstrate that isoflurane can stimulate K_ATP channels in isolated ventricular myocytes.^{13,31,32,38,39,43,44} In addition, it was demonstrated that the K_ATP channel agonist pinacidil exerts greater effects to open the K_ATP channel in myocytes that were previously exposed to isoflurane. Isoflurane may diminish the sensitivity of the K_ATP channels to inhibition by ATP and therefore increase the probability of this channel being in the open state.⁴⁴ Recent findings suggest that an opening of the mitoK_ATP channels is critically involved in IPC and induces reversible oxidation of the mitochondrial matrix.^45-48 Opening of mitoK_ATP channels can dissipate the inner mitochondrial membrane potential established by the proton pump. The effects of anesthetics on mitochondrial redox state were examined by recording the fluorescence of FAD-linked enzymes. The results indicate that isoflurane and sevoflurane dose-dependently increase flavoprotein fluorescence.^22,26,49,50 Additional studies are being conducted to examine human cardiac K_ATP channels using the artificial lipid bilayer.⁵¹

The role of A₁-receptor stimulation in the enhancement of recovery of contractile function of stunned myocardium by isoflurane was also examined.⁵² These and other studies suggest that isoflurane either directly activates A₁ receptors or indirectly enhances the sensitivity of A₁-receptors to reduced amounts of endogenously released adenosine.⁵³ This protection could be reversed by adenosine receptor antagonists, Gi protein blockers, protein kinase C (PKC) inhibitors, and K_ATP channel blockers. Therefore, it may be hypothesised that inhalational anesthetics could stimulate these receptors, proteins, and signal transduction pathways, including the K_ATP channels. As research has demonstrated that cardioprotection by anesthetics is not accompanied by augmented release of adenosine, it could be postulated that these receptors, signal transduction pathways, and K_ATP channels may be sensitised to protein/channel modulators.

Recent studies indicate that both the sarcK_ATP and the mitoK_ATP channels are essential for the protection of myocytes from damage by oxidative stress since their inhibition completely abolished the protective effects of pre-conditioning by isoflurane. Specifically, activation of the sarcK_ATP channel was required for the cardioprotective effects of APC during the stress period, but not during the pre-conditioning period. In contrast, activation of the mitoK_ATP channel was found to be necessary for protection during both isoflurane pre-conditioning and exposure to oxidative stress. From these results, it appears that although activation of both channels is important for the cardioprotective effects of APC, each channel performs a distinct role—the sarcK_ATP channel acts as an effector, while the mitoK_ATP channel plays a dual role as a trigger and an effector. The role of the mitoK_ATP channel as both a trigger and an effector, and the sarcK_ATP channel as the effector of APC, could be explained by the following sequence of events.

• Exposure to isoflurane can directly activate mitoK_ATP channels, resulting in changes in mitochondrial bioenergetics producing a small burst of ROS.⁵⁴
• The ROS can activate cytosolic mediators such as PKC that translocate to the sarcolemma, phosphorylate the sarcK_ATP channel, and sensitize it to opening.
• The newly primed sarcK_ATP channel opens sooner during subsequent metabolic stress, resulting in a greater K⁺ efflux, a more rapid re-polarization of the cell membrane, and action potential shortening, which leads to a decrease in cytosolic Ca²⁺ loading during ischemia/reperfusion. This can reduce or prevent mitochondrial Ca²⁺ overload, a major trigger of the cell death pathway.
• The mitoK_ATP channel can also open during ischemia/reperfusion, which may result in the depolarization of the inner mitochondrial membrane and thus further decrease a driving force for mitochondrial Ca²⁺ entry and accumulation during ischemia/reperfusion.

Modulation of the K_ATP channels by pharmacological agonists and antagonists may produce additional direct cardiac effects. For example, in response to the K_ATP agonist-induced shortening of action potential duration, a concomitant reduction in calcium influx was associated with prevention of triggered arrhythmias.^55,56 However, K_ATP channel agonists also appear to exacerbate re-entry arrhythmias. Glyburide prevents action-potential shortening and may suppress re-entrant arrhythmias. It may also lead to increased triggered activity.^57,58 Therefore, modulation of the K_ATP channels with pharmacological agonists and antagonists may produce either detrimental or beneficial effects on cardiac electrophysiology.⁵⁹ Nevertheless, in addition to the K_ATP channel modulation, volatile anesthetic actions that involve changes in the intracellular regulation of other major trans-sarcolemmal ionic channels can play a critical role in the protection against ischemia and reperfusion arrhythmogenesis.

Conclusion

The observation that volatile anesthetics protect ischemic myocardium is important and clinically significant for patients with CAD. Their morbidity and mortality that is secondary to peri-operative myocardial ischemia is disproportionately high. Current data indicate that APC occurs, and myocardium can be rendered resistant to ischemia and reperfusion injury for a prolonged period, after a prior, brief exposure to a volatile anesthetic. APC displays many similarities to IPC including a window of protection that has also been confirmed in patients undergoing coronary bypass graft and other surgeries.