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Diabetes-induced alterations in isolated rat heart performance Vadlamudi, Rao Venkata Satya Veerabhadra


Chronic diabetic patients have a higher incidence of and mortality from cardiac disease. A wide spectrum of cardiac problems plague the chronic diabetic including coronary artery disease, congestive heart failure and diabetic cardiomyopathy. Cardiac disease in the diabetic is not simply due to accelerated atherosclerosis alone, but is also due to a combination of microangiopathy, autonomic neuropathy, and various other factors which produce biochemical, functional and structural alterations in the heart. Recently, cardiac function was studied in animals with experimentally-induced diabetes and cardiac-dysfunction was reported in acute as well as chronic phases of experimental diabetes. Since cardiac disease is a consequence of long-standing diabetes in diabetic patients, investigation of myocardial function at various time points after induction of experimental diabetes would yield information regarding the development and progression of cardiac dysfunction in diabetes. We. therefore investigated cardiac function and pharmacology in isolated perfused working hearts obtained from 7, 30, 100, 180, 240 and 360-day alloxan and streptozotocin (STZ) diabetic and age-matched control hearts. Diabetes was induced in the rat by injecting either alloxan (65 or 40 mg/kg) or STZ (50 or 60 mg/kg) into the tail vein. Diabetic and age-matched control rats were sacrificed at various time points after the induction of diabetes and hearts were isolated and perfused on a working heart apparatus. Cardiac function was studied at various left atrial filling pressures and was expressed in terms of left ventricular developed pressure (LVDP), rate of rise of left ventricular pressure (positive dP/dt) and rate of decline of left ventricular pressure (negative dP/dt). Dose-response curves to carbachol and isoproterenol were also performed. Blood samples were collected at the time of sacrifice, serum was separated and analyzed for insulin and glucose content. Both alloxan and STZ produced diabetes in the rat as shown by fasting hypoinsulinemia and hyperglycemia. Cardiac function was not altered in 7-day alloxan and STZ diabetic rats. Depressed function at various left atrial filling pressures was seen in hearts isolated from 30-day alloxan diabetic rats but not in 30-day STZ diabetic rats. Hearts isolated from 100-day alloxan and STZ diabetic rats, 180- and 360-day STZ diabetic rats and 240-day alloxan diabetic rats, all exhibited cardiac functional abnormalities. Cardiac functional abnormalities observed in d.iabetic rats were, depressed >LVOP and -positive«.and negative dP/dt at high left atrial filling pressures. Diabetic rat hearts exhibited no change in either sensitivity or responsiveness to the negative inotropic effect of carbachol at 7 and 30 days after induction of the disease. A sub-sensitivity to carbachol was observed in diabetic rat hearts at 100 days after induction of diabetes as compared to age-matched control rat hearts. However, 180- and 240-day diabetic rat hearts exhibited supersensitivity to the negative inotropic effect of carbachol. Isoproterenol produced an identical positive inotropic effect in control as well as diabetic rat hearts at all of the time points studied. However, the maximum changes produced by isoproterenol in negative dP/dt of diabetic rat hearts were depressed at various time points as compared to those in age-matched control rat hearts. We also studied the effect of isoproterenol on the cyclic AMP content and phosphorylase a activity in hearts obtained from 3 and 100 to 120 day control and diabetic rats. Basal cyclic AMP content and phosphorylase a activity were not altered in acute and chronic diabetic and age-matched control rat hearts. Isoproterenol produced similar time- and dose-dependent changes in cyclic AMP content and positive and negative dP/dt in isolated perfused working hearts obtained from 3 and 100 to 120 day control and diabetic rats. However, isoproterenol caused a significantly greater activation of phosphorylase enzyme in hearts isolated from 3 and 100 to 120 day diabetic rats as compared to age-matched controls. Diabetic rat hearts had a significantly higher total phosphorylase activity at 100 to 120 days as compared to age-matched controls. Prostaglandin E₁, a drug which increases cyclic AMP content without altering phosphorylase a activity in perfused rat hearts, increased phosphorylase a activity in acute as well as chronic diabetic rat hearts but not in control rat hearts. Cholinergic muscarinic receptors in the ventricles obtained from 180-day control and STZ diabetic rats were studied by performing radioligand binding studies. [³H]NMS was used as a radioligand to stereospecifically label all of the muscarinic receptor binding sites present in the ventricular membrane preparation. There was no change in either the receptor density or in the binding constants for antagonists and agonists at the muscarinic receptor site in 180-day diabetic rat hearts as compared to control. Ventricular noradrenaline content was estimated using an HPLC method, in 180-day alloxan and STZ diabetic and age-matched control rat hearts. There was no significant change in the noradrenaline content of diabetic rat hearts. Results obtained in the above studies demonstrate that various functional, pharmacological and biochemical alterations occur in the heart in experimental diabetes. Depressed cardiac performance was observed in isolated perfused diabetic rat hearts at various time points after the induction of diabetes and may represent the preclinical ventricular dysfunction phase of a developing diabetic cardiomyopathy. Changes noticed in the sensitivity of the.diabetic myocardium towards the negative inotropic effect of carbachol may represent various stages of a parasympathetic autonomic neuropathy of the heart in diabetes. The unaltered positive inotropic effect of Hsoproterenol and unchanged noradrenaline content in diabetic rat hearts indicate the absence of a sympathetic autonomic neuropathy. The depressed cardiac relaxant effect (maximum changes produced in negative dP/dt) of isoproterenol in diabetic rat hearts suggest defects in cardiac muscle relaxation, Ca²⁺ handling by the sarcoplasmic reticulum and perhaps ATP production and utilization. The enhanced sensitivity of the phosphorylase enzyme to agonists in diabetic rat hearts may be an outcome of alterations in Ca²⁺ homeostasis and other acute metabolic derangements in the heart caused by diabetes. All these changes could contribute to the pathogenesis of a diabetic cardiomyopathy.

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