Free radical scavenging enzymes and G protein mediated receptor signalling systems in ischaemically preconditioned porcine myocardium.

OBJECTIVE: Increased antioxidant defence and altered G protein mediated receptor signalling systems could be expected in myocardial preconditioning. The myocardial antioxidant defence and the integrity of the G protein mediated receptor signalling systems were therefore examined in normal and preconditioned myocardium. METHODS: Preconditioning in the porcine heart was induced by two occlusions of the mid left anterior descending coronary artery for 10 min, with a 30 min reperfusion interval. Left ventricular biopsies were obtained from control and preconditioned regions 30 min after the last occlusion. RESULTS: In biopsies from the preconditioning region, neither the activities of superoxide dismutase of glutathione peroxidase, nor the content of malondialdehyde were changed. There were no alterations in either the number of receptors (beta adrenergic, muscarinic and endothelin receptors) or the amount of G proteins. Furthermore, the activity of adenylyl cyclase remained unchanged. CONCLUSIONS: No change in the antioxidant defence was demonstrated in preconditioned myocardium. This finding does not support the hypothesis that increased antioxidant defence could contribute to the cardioprotection of preconditioning. Additionally, an intact G protein mediated receptor signalling system was found in preconditioned myocardium with regard to beta adrenergic, muscarinic, and endothelin receptors.

Activity of receptors coupled to guanine nucleotide binding regulatory protein in doxorubicin induced cardiomyopathy.

STUDY OBJECTIVE: The aim was to study the activity of receptors coupled to guanine nucleotide binding regulatory proteins (G proteins) in doxorubicin induced cardiomyopathy, with special attention to G proteins, beta adrenoceptors, muscarinic receptors, and adenylyl cyclase. DESIGN: Messenger RNA of G proteins, densities and high affinity agonist binding of beta adrenoceptors and muscarinic receptors, activity of adenylyl cyclase, calcium influx, and in vivo lipid peroxidation were determined before, in the early stage, and in the later stage of doxorubicin cardiomyopathic heart failure. SUBJECTS: Sprague-Dawley rats between 150-200 g were used. Doxorubicin was given intravenously at two doses of 4 mg.kg-1 and 6 mg.kg-1 every third week (1st, 4th, 7th week) for nine weeks. Doxorubicin treated rats plus corresponding controls were killed at 3 weeks (n = 7), 6 weeks (n = 7), and 9 weeks (n = 6), respectively. MEASUREMENTS AND MAIN RESULTS: Northern blot and dot blot hybridisations of the total RNA revealed that messenger RNA of both stimulatory and inhibitory G proteins were identical between doxorubicin treated rats and controls. No alterations in the densities of beta adrenoceptors and muscarinic receptors were observed, neither did the high affinity agonist binding of beta adrenoceptors and muscarinic receptors change. Furthermore, modulation of adenylyl cyclase was unimpaired. In contrast, Ca(2+)-ATPase and serum water soluble fluorescent substance, a product of in vivo lipid peroxidation, were shown to increase dramatically in doxorubicin treated rats (4 mg.kg-1 for 6 and 9 weeks, 6 mg.kg-1 for 3, 6 and 9 weeks) as compared with corresponding controls. CONCLUSIONS: The findings suggest that, despite increased calcium influx and lipid peroxidation in doxorubicin induced cardiomyopathy, the activity of receptors coupled to G proteins remained normal.

Reoxygenation-induced cell damage of isolated neonatal rat ventricular myocytes can be reduced by chain-breaking antioxidants.

To study the role of chain-breaking antioxidants on reperfusion injury in the ischemic heart, cultured ventricular heart cells (myocytes) were subjected to hypoxia and reoxygenation. The myocytes were prepared from neonatal rats and cultured in F10 medium that was supplemented with serum. As a marker for cell damage, lactate dehydrogenase was analyzed in the medium. Cells subjected to hypoxia for 5 h showed a 1.9 fold increase in lactate dehydrogenase (LD) leakage, while cells subjected to 1 h hypoxia followed by 4 h reoxygenation showed a 5-fold increase in LD intake. Alpha-tocopherol, beta-carotene, nordihydroguairetic acid (NDGA), butylated hydroxyltoluene (BHT), and ICI211965 were added to the cell medium every 24 h for 6 d prior to reoxygenation. All compounds protected against reoxygenation-induced cell damage. In the presence of the 5-lipoxygenase inhibitor ICI211965, protection against LD leakage was found only at high concentrations, which corresponded to the antioxidative effect of ICI211965, and not to inhibition of 5-lipoxygenase. We conclude that cultured ventricular myocytes can be used to evaluate the protective effect of antioxidants on reoxygenation-induced cell damage, and that chain-breaking antioxidants protected well against reoxygenation-induced cell damage.

Autoantibodies against the angiotensin receptor (AT1) in patients with hypertension.

Sera from patients with malignant essential hypertension (n = 14), malignant secondary hypertension mainly attributable to renovascular diseases (n = 12) and renovascular diseases without malignant hypertension (n = 11) and from normotensive healthy blood donors (n = 35) were studied for the presence of autoantibodies against G-protein-coupled cardiovascular receptors. Autoantibodies against the angiotensin II receptor (AT1) were detected in 14, 33, 18 and 14% of patients with malignant essential hypertension, malignant secondary hypertension, renovascular diseases and control patients, respectively. Sensitivity of the enzyme immunoassay was assessed as 5 microg/ml IgG. Patients did not show antibodies against bradykinin (B2) or angiotensin II subtype 2 (AT2) receptors. Autoantibodies affinity-purified from positive patients localized AT receptors in Chinese hamster ovary transfected cells, and displayed a positive chronotropic effect on cultured neonatal rat cardiomyocytes. These results demonstrate the existence of autoantibodies against a functional extracellular domain of human AT1 receptors in patients with malignant hypertension, and suggest that these autoantibodies might be involved in the pathogenesis of malignant hypertension.

Properties of G-protein modulated receptor-adenylyl cyclase system in myocardium of spontaneously hypertensive rats treated with adriamycin.

Properties of the receptor--G protein--adenylyl cyclase system were studied in spontaneously hypertensive rats (SHR) and age-matched normotensive Wistar-Kyoto rats (WKY) treated with adriamycin (ADR, 1 mg/kg per week) for 12 weeks. An identical dosing schedule caused a significantly greater decline in body weight gain and a marked elevation of plasma norepinephrine level in SHR than in WKY. A significant increase in the messenger RNA encoding Gi-alpha 2 was found in SHR+ADR group. The activity of the adenylyl cyclase stimulated by guanyliminodiphosphate [Gpp(NH)p] was decreased by 49% in SHR and 73% in SHR+ADR. However, stimulated activities of adenylyl cyclase by both sodium fluoride and forskolin remained unchanged. Functional level of stimulatory G-protein (Gs) as measured by reconstitution assay in sarcolemmal membrane was unaltered among different groups. Furthermore, the density of beta-adrenoceptor was significantly decreased without change of its affinity. Muscarinic receptors exhibited a three-site affinity distribution in SHR+ADR whereas other groups displayed only two-site affinity distribution. These results suggest that SHR exhibited a depressed myocardial adenylyl cyclase signaling system which may not be due to the functional uncoupling of beta-adrenoceptors from Gs but to the increased inhibitory G-protein (Gi) activity as demonstrated by the increased mRNA of Gi-alpha 2, increased inhibition of Gpp(NH)p-mediated adenylyl cyclase and the super high affinity for carbachol of the muscarinic receptors. Decreased beta-adrenoceptor density and functional alteration of Gi might be regarded as the predisposing factors for the increased susceptibility of myocardium of SHR to ADR.

The effect of diabetes on phosphatidylinositol turnover and calcium influx in myocardium.

Diabetes was induced in rats by administration of streptozotocin. Diabetes occurred within 24 h after treatment. Two forms of diabetes were studied, an acute form (4 days) and a chronic form (2 months). In a separate experiment the effect of insulin and an aldose reductase inhibitor on acute diabetes was studied. Phosphoinositide labelling was done in biopsies of heart with [3H] myo-inositol. It was shown that the incorporation of myo-inositol amounted to about 65% in acute diabetes and 80% in chronic diabetes compared to age-matched controls. The incorporation both in atria and ventricles was affected in a similar way. Muscarinic receptor-mediated phosphatidylinositol breakdown and release of myo-Ins-1 P (myo-inositol 1-phosphate) was unaffected in diabetic hearts in the chronic model. In hearts of diabetic ketotic animals uncoupling of the muscarinic receptor from the phosphoinositide metabolism was apparent. Calcium net influx was significantly reduced in both acute and chronic diabetes compared to age-matched controls. Insulin supplementation to acute diabetic animals significantly improved phosphoinositide labelling with [3H] myo-inositol. No improvement was seen in calcium transport. An aldose reductase inhibitor also facilitated phosphoinositide labelling without improving calcium transport. It is suggested that phosphoinositide metabolism and calcium entry through the slow inward current are independent of one another and the former is sensitive to insulin. It is suggested that insulin by regulating the pool of phosphoinositides and release of endogenous calcium may modulate cardiac function.

Effect of metoprolol on activity of beta-adrenoceptor coupled to guanine nucleotide binding regulatory proteins in adriamycin-induced cardiotoxicity.

Prevention of cardiotoxicity without interfering with the therapeutic efficacy of adriamycin is a very crucial question. We have investigated the activity of beta-adrenoceptor coupled to guanine nucleotide binding regulatory proteins (G-proteins) and Ca(2+)-ATPase activity in experimental adriamycin-induced cardiotoxicity and the influence of metoprolol treatment on these variables. Adriamycin was administered to rats intravenously as a single dose of 6 mg/kg, and metoprol was continuously given by means of implanted osmotic pumps. beta-Adrenoceptor characteristics were measured by radioligand-binding experiments and by basal and stimulated adenylyl cyclase activity. Northern blot and dot blot analysis was used to quantify G-protein mRNA. It was shown that adriamycin did not induce any change in the total beta-adrenoceptor density, nor did the high affinity agonist binding to beta-adrenoceptor change. Adriamycin did not induce any alteration in the amount of mRNA encoding for stimulatory (Gs) or inhibitory (Gi) G-proteins. Also, basal and stimulated adenylyl cyclase activities were identical in the different experimental groups. In contrast, the Ca(2+)-ATPase was shown to increase in adriamycin-treated rats compared to control rats (45 +/- 3.8 versus 23 +/- 1.2 mumol Pi/mg/h, P less than .01). Metoprolol was shown to normalize this increase (29 +/- 2.1 mumol Pi/mg/h). Thus, it may be concluded that in experimental adriamycin-induced cardiotoxicity, despite Ca(2+)-overloading, the beta-adrenoceptor-G protein-adenylyl cyclase system remains intact. Metoprolol seems to prevent Ca(2+)-overloading independently of the beta-adrenoceptors studied here.

Diabetes-induced changes in the Gi-modulated muscarinic receptor-adenylyl cyclase system in rat myocardium.

The inhibitory guanine nucleotide binding regulatory protein (Gi)-mediated muscarinic receptor-adenylyl cyclase system was studied in myocardium from adult male Wistar rats with 10 weeks of diabetes induced by a single intravenous injection of streptozotocin (60 mg/kg). Neither the messenger ribonucleic acid level nor the amount of Gi was changed in the streptozotocin diabetic group as compared to the control group. The activity of the adenylyl cyclase stimulated by guanyliminodiphosphate was decreased by 48% in the streptozotocin diabetic group whereas stimulated activities of adenylyl cyclase by sodium fluoride and forskolin remained unchanged. The inhibition of forskolin-stimulated adenylyl cyclase activity by carbachol was more potent in membranes from the streptozotocin diabetic group than that in membranes from the control group. The competition binding curve between (3H)- quinuclidinyl benzilate and carbachol obtained from the streptozotocin diabetic group was shifted to the left as compared to the control group. These results suggest that the myocardium of streptozotocin-induced diabetic rats exhibited an increase in Gi function as demonstrated by the increased inhibition of guanyliminodiphosphate-mediated adenylyl cyclase and the superhigh affinity for carbachol of the muscarinic receptors. As there were signs, similar to those seen in clinical heart failure, in the streptozotocin diabetic group, these results demonstrate that functional alteration of Gi might underlie, at least in part, the cardiac dysfunction that is associated with diabetes.