Thesis (MScMedSc (Biomedical Sciences. Medical Physiology))--Stellenbosch University, 2008. / Background and purpose. To limit ischaemic injury, rapid restoration of coronary blood flow is required, which will in turn reduce infarct size. However, reperfusion itself causes myocyte death – a phenomenon termed lethal reperfusion-induced injury, which limits protection of the ischaemic myocardium. Thus the reperfusion of irreversibly damaged myocytes may accelerate the process of cell necrosis. Additive protection of the ischaemic myocardium in the form of adjunct therapy remains a topic of intensive research. Levosimendan, a calcium sensitizing agent with positive inotropic effects has in several studies been found to alleviate the damaging effects of reperfusion injury. Levosimendan has been shown to be a KATP channel opener. These channels have been implicated to play an important role in ischaemic preconditioning (IPC). With this knowledge, the aim of this study was to determine whether levosimendan and IPC have certain cardioprotective mechanisms in common and whether protection with pharmacological preconditioning could be elicited with levosimendan. In this study, we investigated whether: 1) the isolated guinea pig heart could be protected by ischaemic preconditioning (IPC) and postconditioning (IPostC), 2) the heart could be pharmacologically pre- and postconditioned, using levosimendan (LPC & LPostC), 3) a combination of IPC & LPC had an additive protective effect on the heart, 4) the KATP (both mitochondrial and sarcolemmal) channels are involved in this protection and 5) the pro-survival kinases of the RISK (reperfusion injury salvage kinase) pathway are involved. Experimental approach. Isolated perfused guinea pig hearts were subjected to three different IPC protocols (1x5, 2x5 and 3x5 minutes of ischaemia) or levosimendan (0.1μM) preconditioning, before coronary artery occlusion (CAO – 40min@36.5ºC), followed by 30 minutes of reperfusion. Hearts were also subjected to a combination of IPC & LPC, to establish whether they had additive protective effects. In addition, hearts were pre-treated with levosimendan directly before induction of sustained ischaemia (without washout of the drug – levosimendan pre-treatment (LPT)) for 10min. With the postconditioning protocol, iii the hearts were subjected to 3x30second cycles of ischaemia/reperfusion or levosimendan/vehicle. In a separate series of experiments, hearts were treated with KATP channel blockers (for both sarcolemmal & mitochondrial), before LPC, LPT and LPostC. The endpoints that were measured were: cardiac reperfusion function, myocardial infarct size and RISK pathway expression and phosphorylation (PKB/Akt and extracellular signal-regulated kinase – ERK42/44). Results. IPC, IPostC, LPC & LPostC decreased myocardial infarct size significantly compared with their controls (21.9±2.2%, 21.4±2.2%, 20.6±3.1% and 20.6±1.8% respectively vs. 46.4±1.8% for controls, p<0.05). The combination of IPC & LPC had no additive protective effect. Pre-treating the hearts with levosimendan (without washout), before index ischaemia, proved to be the most effective method of cardioprotection (infarct size: 5.8±0.9% vs. 46.4±1.8% for controls, p<0.001). With LPT a significant increase (p < 0.05 vs. control) in phosphorylation of ER42/44 was also observed. An increase in the activity of one of the RISK pathway kinases, ERK42/44 seems to be one of the reasons for LPT’s efficacy. Treating the hearts with KATP channel blockers before subjecting them to LPC, LPT & LPostC abolished the protective effects induced by levosimendan, suggesting a role for the sarcolemmal and mitochondrial KATP channels in levosimendan-induced cardioprotection. Conclusions and implications. 1) Isolated guinea pig hearts could be pre- and postconditioned within the setting of ischaemia, 2) Hearts could be pharmacologically pre- and postconditioned with levosimendan, 3) levosimendan pre-treatment is the most effective way to reduce infarct size, possibly acting by increasing the phosphorylation of ERK42/44, 4) Myocardial protection was not increased by combining IPC & LPC (suggesting similar mechanisms of protection), 5) LPC, LPT and LPostC were abolished by both sarcolemmal and mitochondrial KATP channel blockers. .LPC and especially LPT, could be useful before elective cardiac surgery while LPostC may be considered after acute coronary artery events.

The KATP channels play an important role in the membrane excitability and vascular tone regulation. Previous studies indicate that the function of KATP channels is disrupted in oxidative stress seen in a variety of cardiovascular diseases, while the underlying mechanism remains unclear. Here, we demonstrate S-glutathionylation to be a modulation mechanism underlying the oxidant-mediated vascular KATP channel inhibition, the molecular basis for the channel inhibition and the alleviation of the channel inhibition by vasoactive intestinal peptide (VIP). We found that an exposure of isolated mesenteric rings to H2O2 impaired the KATP channel-mediated vascular dilation. In whole-cell recordings and inside-out patches, micromolar H2O2 or diamide caused a strong inhibition of the vascular KATP channel (Kir6.1/SUR2B) in the presence, but not in the absence, of glutathione (GSH), indicating S-glutathionylation. By co-expressions of Kir6.1 or Kir6.2 with SUR2B subunits, we found that the oxidant sensitivity of the KATP channel relied on the Kir6.1 subunit. Systematic mutational analysis revealed three cysteine residues (Cys43, Cys120 and Cys176) to be important. Among them, Cys176 was prominent, contributing to >80% oxidant sensitivity. Biochemical pull-down assay with biotinylated glutathione ethyl ester (BioGEE) showed that mutations of Cys176 impaired the oxidant-induced incorporation of GSH to the Kir6.1 subunit. Simulation modeling of Kir6.1 S-glutathionylation revealed that after incorporation to residue 176, the GSH moiety occupied a space between slide helix and two transmembrane helices. This prevented the necessary conformational change of the inner helix for channel gating, and retained the channel in its closed state. VIP is a potent vasodilator, and is shown to have protective role against oxidative stress. We found that the channel was strongly augmented by VIP and the channel activation relied on PKA phosphorylation. These results therefore indicate that 1) the vascular KATP channel is strongly inhibited in oxidative stress, 2) S-glutathionylation underlies the oxidant-mediated KATP channel inhibition, 3) Cys176 in the Kir6.1 subunit is the major site for S-glutathionylation, and 4) the Kir6.1/SUR2B channel is activated in a PKA-dependent manner by VIP that has been previously shown to alleviate oxidative stress.

SUR2A-mediated regulation of KATP channels is affected by residues belonging to the C terminus of the first nucleotide binding domain (NBD1). We studied the C-terminal region of NBD1 by comparing experiments using NBD1 S615-D914 and NBD1 S615-K972 constructs to studies of NBD1 S615-L933 also performed in our laboratory. Our NMR data suggests that the C-terminal region of NBD1 from residues Q915 to L933 is disordered and transiently contacts the NBD1 core, which may affect NBD1 phosphorylation. Tryptophan quenching fluorescence experiments corroborate that the Q915-L933 C-terminal tail contacts the NBD1 core. Fluorescence thermal denaturation experiments suggest that NBD1 S615-D914 has a higher affinity for MgATP compared with NBD1 S615-L933, implying that the C-terminal tail varies MgATP binding. Additional experiments were performed to identify soluble constructs of hSUR1 NBD1 and rSUR2A NBD2 that would allow detailed biophysical studies of these domains. Some of the constructs studied showed improved solubility and stability.

It is known that muscles that lack KATP channel activity generate much greater unstimulated [Ca2+]i and force than normal muscles during fatigue. The increase in unstimulated force in KATP channel deficient muscles is abolished by a partial inhibition of L-type Ca2+ channels, suggesting that it is due to a Ca2+ influx through L-type Ca2+ channels and a subsequent increased myoplasmic Ca2+. However, there is also evidence that the increase in resting force is abolished by NAC, a ROS scavenger. The objective of this study was to reconcile these observations by studying the hypothesis that “the increase in resting [Ca2+]i during fatigue in KATP channel deficient muscles starts with an excess Ca2+ influx through L-type Ca2+ channels, followed by an excess ROS production that causes a further increase in resting [Ca2+]i”. To test the hypothesis, single FDB fibres were fatigued with one tetanic contraction/sec for 180 sec. KATP channel deficient fibres were obtained i) by exposing wild type muscle fibers to glibenclamide, a KATP channel blocker and ii) by using fibres from Kir6.2-/- mice, which are null mice for the Kir6.2 gene that encodes for the protein forming the channel pore. Verapamil, a L-type Ca2+ channel blocker, applied at 1 μM, significantly reduced resting [Ca2+]i during fatigue in glibenclamide-exposed wild type fibres. NAC (1 mM) also reduced resting [Ca2+]i in glibenclamide-exposed muscles. The results suggest that the increase in resting [Ca2+]i during fatigue in KATP channel deficient FDB fibres is due to an influx through L-type Ca2+ channels, and an excess ROS production.

SUR2A-mediated regulation of KATP channels is affected by residues belonging to the C terminus of the first nucleotide binding domain (NBD1). We studied the C-terminal region of NBD1 by comparing experiments using NBD1 S615-D914 and NBD1 S615-K972 constructs to studies of NBD1 S615-L933 also performed in our laboratory. Our NMR data suggests that the C-terminal region of NBD1 from residues Q915 to L933 is disordered and transiently contacts the NBD1 core, which may affect NBD1 phosphorylation. Tryptophan quenching fluorescence experiments corroborate that the Q915-L933 C-terminal tail contacts the NBD1 core. Fluorescence thermal denaturation experiments suggest that NBD1 S615-D914 has a higher affinity for MgATP compared with NBD1 S615-L933, implying that the C-terminal tail varies MgATP binding. Additional experiments were performed to identify soluble constructs of hSUR1 NBD1 and rSUR2A NBD2 that would allow detailed biophysical studies of these domains. Some of the constructs studied showed improved solubility and stability.

It is known that muscles that lack KATP channel activity generate much greater unstimulated [Ca2+]i and force than normal muscles during fatigue. The increase in unstimulated force in KATP channel deficient muscles is abolished by a partial inhibition of L-type Ca2+ channels, suggesting that it is due to a Ca2+ influx through L-type Ca2+ channels and a subsequent increased myoplasmic Ca2+. However, there is also evidence that the increase in resting force is abolished by NAC, a ROS scavenger. The objective of this study was to reconcile these observations by studying the hypothesis that “the increase in resting [Ca2+]i during fatigue in KATP channel deficient muscles starts with an excess Ca2+ influx through L-type Ca2+ channels, followed by an excess ROS production that causes a further increase in resting [Ca2+]i”. To test the hypothesis, single FDB fibres were fatigued with one tetanic contraction/sec for 180 sec. KATP channel deficient fibres were obtained i) by exposing wild type muscle fibers to glibenclamide, a KATP channel blocker and ii) by using fibres from Kir6.2-/- mice, which are null mice for the Kir6.2 gene that encodes for the protein forming the channel pore. Verapamil, a L-type Ca2+ channel blocker, applied at 1 μM, significantly reduced resting [Ca2+]i during fatigue in glibenclamide-exposed wild type fibres. NAC (1 mM) also reduced resting [Ca2+]i in glibenclamide-exposed muscles. The results suggest that the increase in resting [Ca2+]i during fatigue in KATP channel deficient FDB fibres is due to an influx through L-type Ca2+ channels, and an excess ROS production.