Proceedings of The Physiological Society

University of Oxford (2011) Proc Physiol Soc 23, C78

Oral Communications

Acute elevations in extracellular glucose confer a mild cardioprotective effect to isolated ventricular myocytes

M. W. Sims1, C. E. Poile1, I. B. Squire1, R. D. Rainbow1

1. Cardiovascular Sciences, University of Leicester, Leicester, United Kingdom.

Hyperglycaemia in patients suffering an acute myocardial infarction is a marker of poor prognosis. This worsened outcome is independent of the patient being diagnosed as diabetic intimating that the hyperglycaemia rather than diabetes per se is the cause of the poor prognosis. Paradoxically, infarct size is often smaller in diabetic subjects compared to non-diabetics suggesting some form of cardioprotection may exist. To investigate whether glucose itself imparted a cardioprotection to ventricular myocytes, cells were isolated from adult male Wistar rats (animals were humanely sacrificed in accordance with Home Office regulations). Isolated cells were then subjected to a chemically mimicked ischaemia/reperfusion injury (I/R) model. Briefly, cells were paced at 1 Hz with electric field stimulation at 32±2°C and perfused with normal Tyrode (NT) solution for 2 minutes, followed by 7 minutes with substrate-free metabolic inhibition Tyrode (SFT) containing 2 mM cyanide and 1 mM iodoacetic acid. Cells were then ‘reperfused’ with NT for 10 minutes and the contractile recovery assessed as a measure of cardioprotection. In NT with 5 mM and 10 mM glucose, 32±4% and 35±4% (n=112 and 160) respectively, recovered their contractile activity. In 15 mM glucose this number was increased to 49±5% (n=79) (P<0.05 compared to 5 mM glucose)) and was further increased to 59±5% (n=117) (P<0.001 compared to 5 mM glucose, ANOVA with Bonferroni post-hoc test) in 20 mM glucose. To determine whether the effect of glucose on contractile recovery was an osmotic effect the solutions were osmotically balanced with mannitol (a non-metabolised sugar) yielding data that was not significantly different to the ‘unbalanced’ solutions (30±4% (n=134), 33±5% (n=119), 49±5% (n=79) and 62±6% (n=46) for 5, 10, 15 and 20 mM glucose respectively, all P>0.05 (paired t-test)). Interestingly, pretreatment of cardiomyocytes with 20 mM NT for 5 minutes prior to the I/R protocol with 5 mM NT also conferred a marked improvement to the survival of cells (59±4%, n=127). Fructose, replacing glucose, also imparted a cardioprotected phenotype at elevated concentrations (5mM, 45±4% (n=120), 20mM, 59±4%, (n=125) and 20 mM pretreatment, 58±4%, (n=123). Cardiomyocytes use fatty acids as their predominant metabolic substrate, however in ischaemic conditions switch to anaerobic glycolytic metabolism. We hypothesise that the cardiomyocytes show improved survival after I/R as they are able to “glucose-load” during a brief period of elevated glucose. Cells are, therefore, able to maintain their ATP concentrations for longer and so limit the calcium loading during metabolic inhibition. This reduced calcium loading therefore limits the hypercontracture on reperfusion and so improves cellular recovery.

Where applicable, experiments conform with Society ethical requirements