Chronic Diabetic Hearts Research Articles

Improvement of cardiac ventricular function by magnesium treatment in chronic streptozotocin-induced diabetic rat heart.

Chronic diabetes mellitus is associated with detrimental cardiovascular complications and electrolyte imbalances such as hypomagnesaemia. We investigated the effect of magnesium (Mg2+) on cardiac function and the possible role of histological and electrical alterations in chronic, streptozotocin-induced diabetic rats. Wistar rats were treated once intraperitoneally with streptozotocin or citrate, and then daily with MgSO4 or saline for four weeks. Cardiac contractile and electrocardiographic parameters were measured on Langendorff-perfused hearts. Other hearts were histologically stained or immunoblotted for the mitochondrial ATP synthase (ATP5A). In diabetic hearts, Mg2+ prevented a diabetes-induced decrease in left ventricular developed pressure and improved contractility indices, as well as attenuated the reduction in heart rate and prolongation of QT interval, but not the QT interval corrected for heart rate (QTc). Histologically, there were neither differences in cardiomyocyte width nor interstitial collagen. The expression of ATP5A was not different among the treatment groups. Mg2+ supplementation improved cardiac contractile activity in chronic diabetic hearts via mechanisms unrelated to electrocardiographic or histologically detectable myocardial alterations.

Improvement in cardiac function of diabetic rats by bosentan is not associated with changes in the activation of PKC isoforms

We previously demonstrated that chronic treatment with the mixed endothelin A and B (ET(A) and ET(B)) receptor blocker bosentan improved isolated working heart function in streptozotocin (STZ) diabetic rats. Endothelin-1 (ET-1) peptide levels, ET-1 mRNA and ET(A) and ET(B) receptor mRNA were all increased in diabetic hearts, but were unaffected by bosentan treatment, indicating that the beneficial effects of bosentan on heart appear to be on downstream effectors of ET-1 and ET receptors rather than the ET-1 system itself. Stimulation of ET-1 receptors leads to increased activation of protein kinase C (PKC), which is associated with PKC translocation from the cytosol to the membrane. Persistent activation of specific PKC isoforms has been proposed to contribute to diabetic cardiomyopathy. The purpose of this study was to determine whether chronic treatment with bosentan influences the activation of PKC isoforms in hearts from diabetic rats. Male Wistar rats were divided into four groups: control, bosentan-treated control, diabetic, and bosentan-treated diabetic. Diabetes was induced by the intravenous injection of 60 mg/kg streptozotocin. One week later, treatment with bosentan (100 mg/kg/day) by oral gavage was begun and continued for 10 weeks. The heart was then removed, homogenized, separated into soluble (cytosolic) and particulate (membrane) fractions and PKC isoform content in each fraction was determined by Western blotting. PKC alpha, beta2, delta, epsilon and zeta were all detected in hearts from both control and diabetic rats. However, no change in the levels or distribution between the soluble and particulate fractions of any of these isoforms could be detected in chronic diabetic hearts compared to control, whether untreated or treated with bosentan. These observations indicate that bosentan does not improve cardiac performance in STZ diabetic rats by affecting the activation of PKC isoforms.

Remodelling of the sarcolemma in diabetic rat hearts: the role of membrane fluidity.

The hyperglycaemia and oxidative stress, that occur in diabetes mellitus, cause impairment of membrane functions in cardiomyocytes. Also reduced sensitivity to Ca-overload was reported in diabetic hearts (D). This enhanced calcium resistance is based on remodelling of the sarcolemmal membranes (SL) with down-regulated, but from the point of view of kinetics relatively well preserved Na,K-ATPase and abnormal Mg- and Ca-ATPase (Mg/Ca-ATPase) activities. It was hypothesised that in these changes may also participate the non-enzymatic glycation of proteins (NEG) and the related free radical formation (FRF), that decrease the membrane fluidity (SLMF), which is in reversal relationship to the fluorescence anisotropy (D 0.235 +/- 0.022; controls (C) 0.185 +/- 0.009; p < 0.001). In order to check the true role of SLMF in hearts of the diabetic rats (streptozotocin, single dose, 45 mg/kg i.v.) animals were treated in a special regimen with resorcylidene aminoguanidine (RAG 4 mg/kg i.m.). The treatment with RAG eliminated completely the diabetes-induced decrease in the SLMF (C 0.185 +/- 0.009; D + RAG 0.167 +/- 0.013; p < 0.001) as well as in NEG (fructosamine microg x mg(-1) of protein: C 2.68 +/- 0.14; D 4.48 +/- 0.85; D + RAG 2.57 +/- 0.14; p < 0.001), and FRF in the SL (malondialdehyde: C 5.3 +/- 0.3; D 8.63 +/- 0.2; D + RAG 5.61 +/- 0.53 micromol x g(-1); p < 0.05). Nevertheless, the SL ATPase activity in diabetic animals was not considerably influenced by RAG (increase in D + RAG vs. D 3.3%, p > 0.05). On the other hand, RAG increased considerably the vulnerability of the diabetic heart to overload with external Ca2+ (C 100% of hearts failed, D 83.3%, D + RAG 46.7% of hearts survived). So we may conclude, that: (i) The NEG and FRF caused alterations in SLMF, that accompanied the diabetes-induced remodelling of SL, also seem to participate in the protection of diabetic heart against Ca2+-overload; (ii) Although, the changes in SLMF were shown to influence considerably the ATPase activities in cells of diverse tissues, they seem to be little responsible for changes in ATPases-mediated processes in the SL of chronic diabetic hearts.

Diabetic cardiomyopathy in rats: biochemical mechanisms of increased tolerance to calcium overload

Abundant information is now available about changes in subcellular organelles that are responsible for the impaired intracellular calcium homeostasis in diabetic cardiomyopathy. Some of these changes concern heart sarcolemma and include decrease in the following variables: calcium binding, influx of calcium through the L-type calcium channels, (Na,K)-ATPase activity and its affinity to sodium, Na +-Ca 2+ exchange, H +-Na 2+ exchange, etc. Diabetic hearts also exhibited increased tolerance to calcium, but none of the above membrane perturbations were clearly identified as the source of this effect. The present study was undertaken in order to identify those alterations appearing in diabetes which are specific for the diabetic heart only. Our interest was focused on changes in sarcolemmal ATPase activities, particularly those of the (Na,K)-ATPase and its activation by increasing concentrations of sodium and potassium. Studies were performed in the acute (8 days) and chronic (63 days) phase of development of insulin-dependent diabetic cardiomyopathy. Wistar rats were made diabetic by administration of streptozotocin. To test the effect of excess calcium, the well-established model of calcium paradox was used. From the results obtained the following conclusions have been made: (a) diabetic hearts exceed normal hearts in their tolerance to calcium overload. In this respect the effect of chronic diabetes is more pronounced than the effect of acute diabetes; (b) the activities of sarcolemmal ATPases in diabetic hearts remain relatively well preserved. For this reason and with respect to modulation of calcium tolerance, the changes in specific properties of the ATPases, particularly those in the (Na,K)-ATPase, outweigh the importance of perturbations in their activities; (c) the enormous decrease in affinity of the (Na,K)-ATPase to sodium (increased K m value) monitored in calcium paradox in acute diabetic hearts was absolutely missing in “calcium-tolerant” chronic diabetic hearts. This observation pointed to a possible relation that may exist between the specific properties (Na,K)-ATPase adapted to work in chronic diabetic hearts and the enhanced calcium tolerance of those hearts; (d) the specific mechanism responsible for improved activation of the (Na,K)-ATPase by sodium and also partially responsible for potassium ions, which is clearly manifested in chronic diabetic hearts upon calcium paradox, still remains to be elucidated. Nevertheless, it could be assumed that the same mechanism may be also co-responsible for the enhanced tolerance of diabetic hearts to calcium.

Influence of arachidonic acid metabolites on cardiac alpha 1-adrenoceptor responses in diabetic rats.

There is evidence that one or more metabolites of arachidonic acid can produce positive inotropic effects and may also be implicated in the enhanced alpha 1-adrenoceptor responses in hearts from diabetic rats. We therefore carried out a study to investigate the possibility that arachidonic acid metabolites could be involved in the altered cardiac alpha 1-effect of norepinephrine (in the presence of propranolol) in chronic streptozotocin-diabetic rats. Our results have shown that in the presence of the cyclooxygenase inhibitor indomethacin or the thromboxane synthetase inhibitor imidazole, the norepinephrine-stimulated positive inotropic effect and the formation of inositol 1,4,5-trisphosphate were significantly increased in control hearts but were unaltered in hearts from diabetic rats. The addition of the prostacyclin synthetase inhibitor tranylcypromine reduced the norepinephrine-stimulated positive inotropic effect and inositol 1,4,5-trisphosphate formation only in diabetic hearts and had no effect in the controls. The nature and physiological significance of the enhanced positive inotropic effect and inositol 1,4,5-trisphosphate formation in the control heart with the addition of indomethacin and imidazole are still unclear. The effect of tranylcypromine may indicate the participation of prostacyclin in mediating the enhanced alpha 1-inotropic effect of norepinephrine in the chronic diabetic heart.

Arachidonic acid causes postischemic dysfunction in control but not diabetic hearts

Isovolumically perfused control and chronic diabetic rat hearts were subjected to 20 min of global ischemia plus 30 min of reperfusion at preischemic flow rates. Recoveries of contractile function during reperfusion were similar in both groups. Addition of arachidonic acid produced profound postischemic dysfunction in nondiabetic hearts (isovolumic minute work = 19 +/- 8 vs. 86 +/- 10% of preischemic levels after 30 min), whereas arachidonic acid had no detrimental effect in diabetic hearts. Arachidonic acid also augmented endogenous prostacyclin release in control hearts (untreated 2.28 +/- 0.23 ng/ml; arachidonic acid 4.07 +/- 0.22 ng/ml) but failed to alter postischemic prostacyclin release in diabetic hearts. The arachidonic acid-induced postischemic dysfunction was significantly attenuated by coadministration of the oxygen free radical scavengers, superoxide dismutase plus catalase, but not by indomethacin. Thus arachidonic acid-induced dysfunction in normal hearts appears to be related, in part, to free radical production. The intrinsic capacity of the heart to synthesize prostacyclin as a result of ischemia and reperfusion does not appear to be impaired by diabetes. In contrast, the arachidonic acid-induced increase in prostacyclin following ischemia is blunted in the diabetic heart. Although chronic diabetic hearts showed increased tolerance to arachidonic acid-induced dysfunction during reperfusion, a defect in prostacyclin stimulation may place the diabetic at greater risk of complications of ischemic reperfusion in vivo by reducing the capacity to adequately respond to the aggregatory and vasospastic actions of increased circulating thromboxane consequent to myocardial ischemia and reperfusion.

Diabetes alters postischemic response to a prostacyclin mimetic

Eicosanoid metabolism is altered by diabetes. However, postischemic responses of diabetic hearts (DH) to eicosanoids such as prostacyclin are unknown. a prostacyclin analogue iloprost (Ilo) was given to isovolumically beating rat hearts during 20 min of total global ischemia and 30 min of reperfusion. Acute DH (48 h) but not chronic DH (2 mo) had pronounced postischemic dysfunction (developed pressure = 22 +/- 11%), which was completely reversed by 3 X 10(-8) M Ilo (developed pressure = 113 +/- 15%). Ilo also stimulated endogenous prostacyclin release in postischemic control hearts (CH) but not acute or chronic DH. Ilo significantly decreased postischemic recovery in CH (from 67 +/- 11 to 15 +/- 4%), which was partially blocked by the coadministration of the calcium-entry blocker diltiazem or almost completely reversed by the free radical scavengers superoxide dismutase plus catalase (100 U/ml). These data suggest that Ilo may promote functional recovery in DH at concentrations that produce dysfunction in CH. Furthermore, Ilo may induce dysfunction in CH by a calcium ionophoretic action, which appears depressed in diabetes, and by concomitant free radical production (presumably via prostaglandin hydroperoxidases) during Ilo-stimulated endogenous prostacyclin synthesis.