Faculty Bibliography

K(ATP) channels are generally cardioprotective under conditions of metabolic impairment, consisting of pore-forming (Kir6.1 and/or Kir6.2) and sulphonylurea-binding, modulatory subunits [sulfonylurea receptor (SUR) 1, 2A, or 2B]. Cardiovascular K(ATP) channels are generally thought to consist of Kir6.2/SUR2A subunits (in the case of heart muscle) or Kir6.1/SUR2B subunits (smooth muscle), whereas SUR1-containing channels have well-documented roles in pancreatic insulin release. Recent data, however, demonstrated the presence of SUR1 subunits in mouse cardiac tissue (particularly in atria) and a surprising protection from myocardial ischemia/reperfusion in SUR1-null mice. Here, we review some of the extra-pancreatic roles assigned to SUR1 subunits and consider whether these might be involved in the sequelae of ischemia/reperfusion

BACKGROUND: Opening of cardiac ATP-sensitive potassium channels (K(ATP) channels) is a well-characterized protective mechanism against ischemia and reperfusion injury. Evidence exists for an involvement of both sarcolemmal and mitochondrial K(ATP) channels in such protection. Classically, cardiac sarcolemmal K(ATP) channels are thought to be composed of Kir6.2 (inward-rectifier potassium channel 6.2) and SUR2A (sulfonylurea receptor type 2A) subunits; however, the evidence is strong that SUR1 (sulfonylurea receptor type 1) subunits are also expressed in the heart and that they may have a functional role. The aim of this study, therefore, was to examine the role of SUR1 in myocardial infarction. METHODS AND RESULTS: We subjected mice lacking SUR1 subunits to in vivo myocardial ischemia/reperfusion injury. Interestingly, the SUR1-null mice were markedly protected against the ischemic insult, displaying a reduced infarct size and preservation of left ventricular function, which suggests a role for this K(ATP) channel subunit in cardiovascular function during conditions of stress. CONCLUSIONS: SUR1 subunits have a high sensitivity toward many sulfonylureas and certain K(ATP) channel-opening drugs. Their potential role during ischemic events should therefore be considered both in the interpretation of experimental data with pharmacological agents and in the clinical arena when the cardiovascular outcome of patients treated with antidiabetic sulfonylureas is being considered

The isoform-specific structure of the ATP-sensitive potassium (K(ATP)) channel endows it with differential fundamental properties, including physiological activation and pharmacology. Numerous studies have convincingly demonstrated that the pore-forming Kir6.2 (KCNJ11) and regulatory SUR2A (ABCC9) subunits are essential elements of the sarcolemmal K(ATP) channel in cardiac ventricular myocytes. Using a novel antibody directed against the COOH terminus of SUR1 (ABCC8), we show that this K(ATP) subunit is also expressed in mouse myocardium and is the dominant SUR isoform in the atrium. This suggests differential sarcolemmal K(ATP) composition in atria and ventricles, and, to test this, K(ATP) currents were measured in isolated atrial and ventricular myocytes from wild-type and SUR1(-/-) animals. K(ATP) conductance is essentially abolished in SUR1(-/-) atrial myocytes but is normal in SUR1(-/-) ventricular myocytes. Furthermore, pharmacological properties of wild-type atrial K(ATP) match closely the properties of heterologously expressed SUR1/Kir6.2 channels, whereas ventricular K(ATP) properties match those of heterologously expressed SUR2A/Kir6.2 channels. Collectively, the data demonstrate a previously unappreciated K(ATP) channel heterogeneity: SUR1 is an essential component of atrial, but not ventricular, K(ATP) channels. Differential molecular make-up of the 2 channels underlies differential pharmacology, with important implications when considering sulfonylurea therapy or dissecting the role of cardiac K(ATP) pharmacologically, as well as for understanding of the role of diazoxide in preconditioning

BACKGROUND: K+ channels are diverse; both in terms of their function and their molecular composition. Shal subunits were first described in Drosophila. There are three mammalian orthologs, which are members of the Kv4 subfamily. They are involved in neuronal firing patterns as well as control of the cardiac action potential duration. RESULTS: Here, we report the biophysical and pharmacological characterization of zShal3, which is the ortholog of the mammalian Kv4.3 subunit, which in mammals is involved in action potential repolarization and gives rise to neuronal A-type K+ currents involved in somatodendretic signal integration. CONCLUSION: We demonstrate that zShal has similar functional and pharmacological characteristics compared to Kv4.3 and it is similarly regulated by pharmacological agents and by the Kv4 accessory subunit, NCS-1

Gap junction redistribution and reduced expression, a phenomenon termed gap junction remodeling (GJR), is often seen in diseased hearts and may predispose towards arrhythmias. We have recently shown that short-term pacing in the mouse is associated with changes in connexin43 (Cx43) expression and localization, but not with increased inducibility into sustained arrhythmias. We hypothesized that short-term pacing, if imposed on murine hearts with decreased Cx43 abundance, could serve as a model for evaluating the electrophysiologic effects of GJR. We paced wildtype (normal Cx43 abundance) and heterozygous Cx43 knockout mice (Cx43(+/-), 66% mean reduction in Cx43) for six hours at 10-15% above their average sinus rate. We investigated the electrophysiologic effects of pacing on the whole animal using programmed electrical stimulation, and in isolated ventricular myocytes with patch clamp studies. Cx43(+/-) myocytes had significantly shorter action potential durations (APD) and increased steady state and inward rectifier potassium currents (Iss and IK1, respectively) compared to wildtype littermate cells. In Cx43(+/-) hearts, pacing resulted in significant prolongation of ventricular effective refractory period and action potential duration, and significant diminution of Iss compared to unpaced Cx43(+/-) hearts. However, these changes were not seen in paced wildtype mice. These data suggest that Cx43 abundance plays a critical role in regulating currents involved in myocardial repolarization and their response to pacing. Our study may aid in understanding how dyssynchronous activation of diseased, Cx43-deficient myocardial tissue can lead to electrophysiologic changes which may contribute to the worsened prognosis often associated with pacing in the failing heart. Key words: Connexin43, ventricular myocytes, mouse, gap junction

Protein kinase A (PKA)-independent actions of adenosine 3',5'-cyclic monophosphate (cAMP) are mediated by Epac, a cAMP sensor expressed in pancreatic beta-cells. Evidence that Epac might mediate the cAMP-dependent inhibition of beta-cell ATP-sensitive K(+) channels (K(ATP)) was provided by one prior study of human beta-cells and a rat insulin-secreting cell line (INS-1 cells) in which it was demonstrated that an Epac-selective cAMP analogue (ESCA) inhibited a sulphonylurea-sensitive K(+) current measured under conditions of whole-cell recording. Using excised patches of plasma membrane derived from human beta-cells and rat INS-1 cells, we now report that 2'-O-Me-cAMP, an ESCA that activates Epac but not PKA, sensitizes single K(ATP) channels to the inhibitory effect of ATP, thereby reducing channel activity. In the presence of 2'-O-Me-cAMP (50 microM), the dose-response relationship describing ATP-dependent inhibition of K(ATP) channel activity (NP(o)) is left-shifted such that the concentration of ATP producing 50% inhibition (IC(50)) is reduced from 22 microM to 1 microM for human beta-cells, and from 14 microM to 4 microM for rat INS-1 cells. Conversely, when patches are exposed to a fixed concentration of ATP (10 microM), the administration of 2'-O-Me-cAMP inhibits channel activity in a dose-dependent and reversible manner (IC(50) 12 microM for both cell types). A cyclic nucleotide phosphodiesterase-resistant ESCA (Sp-8-pCPT-2'-O-Me-cAMPS) also inhibits K(ATP) channel activity, thereby demonstrating that the inhibitory actions of ESCAs reported here are unlikely to arise as a consequence of their hydrolysis to bioactive derivatives of adenosine. On the basis of such findings it is concluded that there exists in human beta-cells and rat INS-1 cells a novel form of ion channel modulation in which the ATP sensitivity of K(ATP) channels is regulated by Epac

The immature and mature heart differ from each other in terms of excitability, action potential properties, contractility, and relaxation. This includes upregulation of repolarizing K(+) currents, an enhanced inward rectifier K(+) (Kir) current, and changes in Ca(2+), Na(+), and Cl(-) currents. At the molecular level, the developmental regulation of ion channels is scantily described. Using a large-scale real-time quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) assay, we performed a comprehensive analysis of ion channel transcript expression during perinatal development in the embryonic (embryonic day 17.5), neonatal (postnatal days 1-2), and adult Swiss-Webster mouse hearts. These data are compared with publicly available microarray data sets (Cardiogenomics project). Developmental mRNA expression for several transcripts was consistent with the published literature. For example, transcripts such as Kir2.1, Kir3.1, Nav1.5, Cav1.2, etc. were upregulated after birth, whereas others [e.g., Ca(2+)-activated K(+) (KCa)2.3 and minK] were downregulated. Cl(-) channel transcripts were expressed at higher levels in immature heart, particularly those that are activated by intracellular Ca(2+). Defining alterations in the ion channel transcriptome during perinatal development will lead to a much improved understanding of the electrophysiological alterations occurring in the heart after birth. Our study may have important repercussions in understanding the mechanisms and consequences of electrophysiological alterations in infants and may pave the way for better understanding of clinically relevant events such as congenital abnormalities, cardiomyopathies, heart failure, arrhythmias, cardiac drug therapy, and the sudden infant death syndrome

Phospholemman (PLM) is a small sarcolemmal protein that modulates the activities of Na(+)/K(+)-ATPase and the Na(+)/Ca(2+) exchanger (NCX), thus contributing to the maintenance of intracellular Na(+) and Ca(2+) homeostasis. We characterized the expression and subcellular localization of PLM, NCX, and the Na(+)/K(+)-ATPase alpha1-subunit during perinatal development. Western blotting demonstrates that PLM (15kDa), NCX (120kDa), and Na(+)/K(+)-ATPase alpha-1 (approximately 100kDa) proteins are all more than 2-fold higher in ventricular membrane fractions from newborn rabbit hearts (1-4-day old) compared to adult hearts. Our immunocytochemistry data demonstrate that PLM, NCX, and Na(+)/K(+)-ATPase are all expressed at the sarcolemma of newborn ventricular myocytes. Taken together, our data indicate that PLM, NCX, and Na(+)/K(+)-ATPase alpha-1 proteins have similar developmental expression patterns in rabbit ventricular myocardium. Thus, PLM may have an important regulatory role in maintaining cardiac Na(+) and Ca(2+) homeostasis during perinatal maturation