New insight into the signalling pathways of cardioprotection in the non-diabetic and diabetic human myocardium
Background: The cardioprotection by IPC is a well-recognised phenomenon in healthy hearts, however the occurrence of IPC in diabetics is controversial. In this thesis I have investigated whether the diabetic myocardium can be preconditioned and the underlying mechanisms. Methods and results: Right atrial sections from 3 groups of patients: non-diabetics, IDDM, and NIDDM were randomised to receive one of the following protocols: aerobic perfusion, simulated ischaemia/reoxygenation, IPC, and pharmacological preconditioning with phenylephrine, adenosine, diazoxide (mitoKATP channel opener), PMA (protein kinase C agonist) or anisomycin (p38 MAPK activator). The assessment of CK leakage and MTT tissue viability at the end of the experiments demonstrated that diabetic myocardium cannot preconditioned by ischaemia or by the pharmacological activation of the signal transduction pathway upstream of mitoKATP channels but that protection can be obtained by the activation of the pathway beyond mitoKATP channels (e.g. PKC and P38 MAPK). In a second study, mitochondria isolated from the above groups of patients were used to assess for MMP after treatment with diazoxide by measuring the uptake of JC-1 dye. The partial depolarisation of MMP seen in non-diabetics was absent in the diabetic myocardium. The determination of ROS generation by the mitochondria of the non-diabetic and diabetic myocardium exposed to diazoxide showed an altered response in superoxide production in the diabetic myocardium. Finally, using specific PKC isoform inhibitors, I demonstrated that PKC a and PKC e are the two isoforms involved in IPC of human myocardium with PKCe begin upstream and PKCa being downstream of mitKATP channels. Conclusions: The failure to precondition the diabetic myocardium is caused by mitochondrial dysfunction. Since PKCa is the isoform beyond mitoKATP channels, this and P38 MAPK may represent potential clinical and therapeutic targets to protect the diabetic myocardium.