Research interest:
An important cause of perioperative morbidity and mortality for patients undergoing anesthesia and surgery is cardiac arrhythmias. In both cardiac and non-cardiac patients, the early postoperative period appears to be the highest risk period where consequences of myocardial ischemia are the most severe. There are indications that volatile anesthetics have a pronounced effect on the stress response during the postoperative period, and thus may potentially be a therapeutical tool in managing perioperative ischemia.
Recent studies have clearly demonstrated that volatile anesthetics can be cardioprotective (anesthetic-induced preconditioning, APC), and mimic ischemic preconditioning, whereby a brief, non-lethal period of ischemia protects the myocardium from a subsequent sustained ischemia. However, the cellular and molecular mechanisms underlying APC have not been elucidated. In addition, though cardioprotection by APC has been demonstrated by measuring reduction in infarct size, cardioprotection against arrhythmias is not well established.
Our long-term goals are to characterize the cellular and molecular mechanisms of interaction between volatile anesthetics and cardiac ion channels, and to identify the signal transduction pathways involved in APC. We utilize various configurations of the patch-clamp methodology, a voltage-clamp technique which allows for the measurement of current through a macroscopic collection of channels (whole-cell configuration) or through a single channel protein (excised and cell-attached patch configurations).
ATP-sensitive K channel
A potential target of anesthetic action is the cardiac sarcolemmal ATP-sensitive potassium (sarcKATP) channel whose regulation is dependent on the metabolic state of the cell. The channel is not available under normal physiological conditions, but are activated under pathophysiological conditions such as ischemia, resulting in a hyperpolarizing current flow and the shortening of the action potential. Whether its activation under metabolic impairment is beneficial, for example antiarrhythmic and protective during reperfusion, or detrimental (proarrhythmic) to the heart appears to be dependent on the type of ischemia.
The role of the sarcKATP channel in anesthetic-induced and ischemic preconditioning is unclear. Recent studies suggest a major role for the mitochondrial (mito), rather than the sarcolemmal, KATP channel. However, studies also demonstrated that ischemic preconditioning did not reduce infarct size in sarcKATP-deficient mice. Thus, the relative contributions of the mito- and sarcKATP channels in cardioprotection are unresolved. We have recently shown that a volatile anesthetic, isoflurane, has a potentiating effect on the cardiac sarcKATP channel. An intriguing aspect of our finding is that though isoflurane potentiates sarcKATP channel activity, the anesthetic alone appears unable to elicit opening of a closed channel. Thus, isoflurane is not an effective "potassium channel opener". The potentiation occurs after an initial opening of the channel. In addition, activation of protein kinase C facilitates the anesthetic potentiation of the KATP channel. Ongoing experiments are designed to determine the intracellular components involved in this anesthetic-induced potentiation.
In addition, we are also examining the effects of anesthetics on the cardiac sarcKATP channel heterologously expressed in a mammalian cell line. The expressed KATP channel consists of two structural components: Kir6.2, the pore forming region, and SUR2A, the sulfonylurea receptor. The expression system will allow us to test possible anesthetic interaction sites.
Cardioprotection against arrhythmias
The effects of APC on cardiac electrophysiology are not well characterized. Activation of the sarcKATP channel, hypothesized to occur during APC, can profoundly affect cardiac electrophysiology. Interestingly, functional cardioprotection utilizing adenosine or KATP channel openers may precipitate reentrant ventricular arrhythmias. On the other hand, glibenclamide, a KATP channel antagonist, can oppose arrhythmia due to reentry by preventing action potential shortening (due to sarcKATP channel activation) but has a tendency to enhance triggered arrhythmias. Consequently, additional antiarrhythmic mechanisms likely exist. The intracellular signaling pathways thought to mediate APC, for example protein kinase C, protein tyrosine kinase, and mitogen-activated protein kinase, also modulate the sarcolemmal voltage-gated ion channels. The key question is whether these changes in signal transduction have a "delayed" effect on the ion channels to coincide with the time course of APC.
In order to determine the efficacy of APC to protect against cardiac arrhythmias, we are currently utilizing the Langendorff isolated heart methodology to test the hypothesis that volatile anesthetics have persistent antiarrhythmic effects during ischemia and reperfusion. Furthermore, patch clamp studies are being conducted to determine the ionic mechanisms underlying the antiarrhythmic effects of APC. We are focusing on the Ca, Na, and K channels and characterizing their functional changes observed in ventricular myocytes isolated from hearts that underwent APC.
Delayed rectifier K channel
In elucidating the mechanism of cardioprotection against arrhythmia, it is necessary to understand the interaction of volatile anesthetics with ion channels. Yet, a molecular mechanism underlying volatile anesthetic action on cardiac ion channels has not been described. Volatile anesthetic binding sites have been identified on ligand-gated channels such as the GABAA receptors, but the sites at which anesthetics modulate cardiac voltage-gated channels are not known. To investigate anesthetic action on cardiac ion channels at the molecular level, we are currently characterizing the effects of volatile anesthetics on the delayed-rectifier K channel.
In heart, the delayed-rectifier K current plays a major role in the repolarization of the action potential. Recent studies have shown that mutations in the genes encoding the delayed rectifier K channels underlie an inherited arrhythmia, the long QT syndrome, characterized by its propensity for polymorphic ventricular tachycardia. We are particularly interested in a delayed rectifier K channel categorized as IKs. The IKs channel consists of KCNQ1, a pore-forming tetramer, and KCNE1, a small accessory protein (b-subunit) with a single transmembrane domain. Although KCNQ1 coassembles with KCNE1 to form the IKs channel, KCNQ1 itself is also a functional channel.
Emerging studies have demonstrated the important roles of b-subunits in channel regulation. Preliminary results from my laboratory show that the effects of volatile anesthetics on IKs are modulated by KCNE1. The inhibitory effects of isoflurane and halothane are greater on KCNQ1 alone then on the coassembly of KCNQ1 and KCNE1. This suggests that KCNE1 may modulate an anesthetic interaction site on KCNQ1. We are testing this hypothesis by using fusion proteins of KCNQ1 and KCNE1. Our results may provide a unique model of anesthetic action on a cardiac K channel.
Recent Publications
Stadnicka, A., Bosnjak, Z.J., Kampine, J.P., Kwok, W.M. Effects of sevoflurane on the inward rectifier K+ current in guinea pig ventricular cardiomyocytes. American Journal of Physiology. 273(Heart Circ. Physiol. 42):H324-H332, 1997.
Weigt, H.U., Rehmert, G.C., Bosnjak, Z.J., Kwok, W.M. Conformational state-dependent effects of halothane on cardiac Na+ current. Anesthesiology. 88:1494-1506, 1997.
Schultz, J.J., Kwok, W.M., Hsu, A.K., Gross, G.J. Terikalant, an inward-rectifier potassium channel blocker, does not abolish the cardioprotection induced by ischemic preconditioning in the rat. Journal of Molecular and Cellular Cardiology. 30:1817-1825, 1998.
Stadnicka, A., Kwok, W.M., Hartmann, H.A., Bosnjak, Z.J. Effects of halothane and isoflurane on fast and slow inactivation in human heart hH1a sodium channels. Anesthesiology. 90:1671-1683, 1999.
Stadnicka, A., Bosnjak, Z.J., Kampine, J.P., Kwok, W.M. Modulation of cardiac inward rectifier K(+)current by halothane and isoflurane.Anesthesia and Analgesia. 90:824-833, 2000.
Camara, A.K.S., Begic, Z., Kwok, W.M., Bosnjak, Z.J. Differential modulation of the cardiac L- and T-type calcium channel currents by isoflurane. Anesthesiology 95:515-524, 2001.
Sarantopoulos, C., McCallum, B., Kwok, W.M., Hogan, Q. Gabapentin decreased membrane calcium currents in injured as well as in control mammalian primary afferent neurons. Regional Anesthesia and Pain Medicine 27:47-57, 2002.
Kwok, W.M., Martinelli, A.T., Fujimoto, K., Suzuki, A., Stadnicka, A., Bosnjak, Z.J. Differential modulation of the cardiac adenosine triphosphate-sensitive potassium channel by isoflurane and halothane. Anesthesiology 97:50-56, 2002.
Fujimoto, K., Bosnjak, Z.J., Kwok, W.M. Isoflurane-induced facilitation of the cardiac sarcolemmal KATP channel. Anesthesiology 97:57-65, 2002.