MECHANISMS THAT MAY BE INVOLVED IN CALCIUM TOLERANCE OF THE DIABETIC HEART

In diabetes the hearts exhibit impaired membrane functions, but also i ncreased tolerance to Ca2+ (iCaT) However, neither the true meaning no r the molecular mechanisms of these changes are fully understood. The present study is devoted to elucidation of molecular alterations. part icularly those induced by non-enzymatic glycation of proteins, that ma y be responsible for iCaT of the rat hearts in the stage of fully deve loped, but still compensated diabetic cardiomyopathy (DH). Insulin-dep endent diabetes (DIA) was induced by a single i.v. dose of streptozoto cin (45 mg.kg(-1)). Beginning with the subsequent day, animals obtaine d 6 U insulin daily. Glucose, triglycerides, cholesterol and glycohemo globin were investigated in blood. ATPase activities, the kinetics of activation of (Na,K)-ATPase by Na+ and K+, further the fluorescence an isotropy of diphenyl-hexatriene as well as the order parameters of mem branes in isolated heart sarcolemma (SL) were also investigated. In ad dition, the degree of glycation and glycation-related potency for radi cal generation in SL proteins were determined by investigating their f ructosamine content. In order to study calcium tolerance of DH in a 't ransparent' model, hearts were subjected to calcium paradox (Ca-Pa, 3 min of Ca2+ depletion; 10 min of Ca2+ repletion). In this model of Ca2 +-overload, Ca2+ ions enter the cardiac cells in a way that is not med iated by receptors. Results revealed that more than 83% of the isolate d perfused DH recovered, while the non-DIA control hearts all failed a fter Ca-Pa. DH exhibited well preserved SL ATPase activities and kinet ics of (Na,K)-ATPase activation by Na+, even after the Ca-Pa, This was considered as a reason for their iCaT. Pretreatment and administratio n of resorcylidene aminoguanidine (RAG 4 or 8 mg.kg(-1)) during the di sease prevented partially the pathobiochemical effects of DIA-induced glycation of SL proteins. DLA induced perturbations in anisotropy and order parameters of SL were completely prevented by administration of RAG (4 mg.kg(-1)). Although, the latter treatment exerted little influ ence on the (Na,K)-ATPase activity, it decreased the calcium tolerance of the DH. Results are supporting our hypothesis that the glycation-i nduced enhancement in free radical formation and protein crosslinking in SL may participate in adaptive mechanisms that may be also consider ed as 'positive' and are responsible for iCaT of the DH.