Effect of Systemic Insulin and Angiotensin II Receptor Subtype-1 Antagonist on Endothelin-1 Receptor Subtype(s) Regulation and Binding in Diabetic Rat Heart

This study reports on the regulation and remodeling role of endothelin-1 (ET-1) and its receptor subtypes, ETA-Rs/ETB-Rs, at the coronary endothelium (CE) and cardiomyocyte (CM) sites. It is carried out in normal and normotensive rats with streptozotocin-induced diabetes mellitus receiving different treatment modalities. Normal rats were divided into two groups, namely a placebo (N) and a losartan-treated (NL), and diabetic rats into four groups receiving placebo (D), insulin-treated (DI), losartan-treated (DL), and insulin/losartan-treated (DIL) respectively. Binding kinetics of ET-1 to ETA-Rs/ETB-Rs on CE and CMs were assessed in the above groups to try to explain the effect of therapeutic doses of an angiotensin II receptor subtype-1 blocker on the dynamics of this ligand and its receptor in insulin supplemented diabetic animals. Each group was divided into two subgroups: CHAPS-untreated and CHAPS-treated rat hearts perfused with [125I]ET-1 to respectively estimate ET-1 binding affinity (τ = 1/k−n) to its receptor subtype(s) on CE and CMs using mathematical modeling describing a 1:1 reversible binding stoichiometry. Heart perfusion results revealed that insulin treatment significantly decreased τ on CE but not on CMs in diabetic rats. In diabetics treated with losartan, an increase in τ value on CE but not on CMs was noted. Cotreatment of diabetic rats with insulin and losartan normalized τ on CE but decreased it on CMs. Western blot, using snap-frozen heart tissues, revealed increase in ETA-R density in all diabetic groups. However, significant decrease in ETB-R density was observed in all groups compared to the normal, and was reconfirmed by immunohistochemical analysis. In conclusion, coadministration of insulin and losartan in nonhypertensive animals suffering from diabetes type 1 may offer new cardiac protection benefits by improving coronary blood flow and cardiomyocyte contractility through modulating ET-1 receptor subtypes density and affinity at CE and CM sites.