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,
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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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/1575 |
| Date | 12 1900 |
| Creators | Genis, Amanda |
| Contributors | Du Toit, E. F., Lochner, A., Stellenbosch University. Faculty of Health Sciences. Dept. of Biomedical Sciences. Medical Physiology. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | Stellenbosch University |
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