The effect of losartan treatment on the response of diabetic cardiomyocytes to ATP depletion

The present work aimed to investigate the effect of losartan treatment of healthy and diabetic rats on cardiomyocyte response to ATP depletion.Cells were isolated from normoglycemic (N) and streptozotocin-injected (55 mg/kg) rats (D) treated or not treated with losartan (20 mg/kg/day in the drinking water; NL and DL, respectively) for 3 weeks. In each group of cells, enzyme activities such as glucose-6-phosphate (G6PDH) and glycerol-3-phosphate dehydrogenases (G3PDH), lactate/pyruvate, glycogen levels and citrate synthase were measured as an index of glycolytic dysregulation and mitochondrial mass, respectively.Cells were then challenged with NaCN (2 mM) in glucose-free Tyrode solution (metabolic intoxication, MI), a protocol to study ischemia at cell level. Under these conditions, the time to contractile failure up to rigor-type hyper-contracture in field-stimulated cells and KATP current activation by patch-clamp recordings were measured.In comparison with N and NL, D cells presented higher G6PDH and cytoplasmic G3PDH activities, lactate/pyruvate, glycogen content but similar levels of citrate synthase, and decay time of contraction. When subjected to MI, D cells showed delayed activation of the KATP current (25.7 ± 7.1 min; p < 0.001 vs. N and NL), increased time to contractile failure and rigor-type hyper-contracture (p < 0.001 vs. N and NL). In cells from DL rats both functional (time to rigor and to KATP current activation) and metabolic parameters, approached values similar to those measured in N and NL cells.These results demonstrate that diabetic cardiomyocytes from rats treated with losartan, maintain the capacity to respond promptly to ATP depletion reaching contractile failure, rigor-type hypercontracture and KATP opening with a similar timing of N cells.

Simplified schematic representation of metabolic–electric interactions in the cardiomyocyte in “hypoxic-like” conditions. Cardiac energy production changes from physiological to pathological conditions. While in the healthy heart, >80% of ATP generated comes from mitochondrial oxidative phosphorylation and the remainder from glycolysis, in the diabetic heart, fatty acids represent the unique substrate, a condition less oxygen-efficient in producing ATP. Moreover, overactivation of fatty acid oxidation uncouples the mitochondrial system and retro-inhibits glucose metabolism too. This reduced ATP efficiency contributes detrimentally to cardiac response to hypoxic and ischemic stresses. Our insulin-resistant cells have indeed a retro-inhibition of glycolysis, with accumulation of substrates including lactate and present activation of other glucose metabolism pathways, including the pentose phosphate shunt and “de novo” fatty acid synthesis and mobilization from glycogen stores. Those alternative ATP sources, ensure energy for contraction even at reduced ATP production despite un-favourable conditions such as limited oxygen supply. AT1 receptor activation is detrimentally related to the onset of insulin resistance and glucose dysmetabolism and the block of this receptor counteracts glucose dysmetabolism and restores KATP sensor activity to cell ATP depletion.Figure optionsDownload as PowerPoint slide