Investigating the Effects of Glycaemic Variability on Hypoxic and Ischaemic Preconditioning

Abstract

Cardiovascular diseases (CVDs) are the principal cause of death in people with diabetes. Initially, this was attributed to chronic hyperglycaemia and reduced insulin sensitivity. However, clinical trials have revealed that aggressive glycaemic control does not necessarily improve macrovascular outcomes in these individuals. One possible explanation for this is an increased occurrence of hypoglycaemia associated with intensive glucose lowering therapy. Hypoxic (HPC), ischaemic (IPC) and remote ischaemic preconditioning (R-IPC) are phenomena in which a degree of protection against a severe hypoxic or ischaemic insult is conferred by prior exposure to hypoxia or ischaemia. Evidence regarding the efficacy of IPC and R-IPC treatment in those with diabetes is somewhat contradictory although it seems that glycaemic events before preconditioning can be of influence. A project was designed to investigate how glycaemic variability can affect HPC, IPC and R-IPC. The primary objectives of this project were to develop in vitro and in vivo models of preconditioning after which acute hypoglycaemic and hyperglycaemic events were to be introduced within the timelines of any established models. The proposed in vivo R-IPC model consisted of repeated cycles of 5 minutes ischaemia/5 minutes reperfusion, induced using a blood pressure cuff around a murine hindlimb, for a total of 9 cycles over 3 days. Using laser doppler imaging (LDI) combined with phenylephrine (PE) and acetylcholine (ACh) iontophoresis, in vivo studies appeared to show that endothelial function paradoxically decreased following RIPC treatment. In contrast, it appears that when the injectable anaesthesia agents midazolam and Alfaxan were used in lieu of isoflurane, R-IPC led to an improvement in endothelial function. Taken together, these data provided evidence that isoflurane anaesthesia independently influences vascular function in vivo and thus, should not be used in models of preconditioning with assessment of peripheral vascular function. Validation of an in vitro IPC model in human umbilical vein endothelial cells (HUVECs) using cobalt chloride (CoCl2) was unsuccessful, as was development of both IPC and HPC models in HUVECs using a hypoxic incubator. However, a model of HPC was validated through the treatment of HUVECs for 24 hours with 200µM CoCl2. A screening study determined that single 3-hour episodes of hypoglycaemia and hyperglycaemia can potentially abolish HPC induced protection against a severe hypoxic insult. However, this disruption appears to depend upon when the glycaemic event occurs in relation to the HPC treatment 20 . In conclusion, the studies reported in this thesis suggest that the potential benefits of HPC are ameliorated in diabetes as a consequence of significant changes in glycaemia prior to the ischaemic insult.

Date of Award	2022
Original language	English
Awarding Institution	University of Dundee
Supervisor	Rory McCrimmon (Supervisor), Faisel Khan (Supervisor) & Alison McNeilly (Supervisor)