Song-Yan Liu

Changes in sarcolemmal PLC isoenzymes in postinfarct congestive heart failure: partial correction by imidapril

We have examined the changes in quantity and activity of cardiac sarcolemmal (SL) phosphoinositide-phospholipase C (PLC)-beta(1), -gamma(1), and -delta(1) in a model of congestive heart failure (CHF) secondary to large transmural myocardial infarction (MI). We also instituted a late in vivo monotherapy with imidapril, an ANG-converting enzyme (ACE) inhibitor, to test the hypothesis that its therapeutic action is associated with the functional correction of PLC isoenzymes. SL membranes were purified from the surviving left ventricle of rats in a moderate stage of CHF at 8 wk after occlusion of the left anterior descending coronary artery. SL PLC isoenzymes were examined in terms of protein mass and hydrolytic activity. CHF resulted in a striking reduction (to 6-17% of controls) of the mass and activity of gamma(1)- and delta(1)-isoforms in combination with a significant increase of both PLC beta(1) parameters. In vivo treatment with imidapril (1 mg/kg body wt, daily, initiated 4 wk after coronary occlusion) improved the contractile function and induced a partial correction of PLCs. The mass of SL phosphatidylinositol 4,5-bisphosphate and the activities of the enzymes responsible for its synthesis were significantly reduced in post-MI CHF and partially corrected by imidapril. The results indicate that profound changes in the profile of heart SL PLC-beta(1), -gamma(1), and -delta(1) occur in CHF, which could alter the complex second messenger responses of these isoforms, whereas their partial correction by imidapril may be related to the mechanism of action of this ACE inhibitor.We have examined the changes in quantity and activity of cardiac sarcolemmal (SL) phosphoinositide-phospholipase C (PLC)-β1, -γ1, and -δ1 in a model of congestive heart failure (CHF) secondary to large transmural myocardial infarction (MI). We also instituted a late in vivo monotherapy with imidapril, an ANG-converting enzyme (ACE) inhibitor, to test the hypothesis that its therapeutic action is associated with the functional correction of PLC isoenzymes. SL membranes were purified from the surviving left ventricle of rats in a moderate stage of CHF at 8 wk after occlusion of the left anterior descending coronary artery. SL PLC isoenzymes were examined in terms of protein mass and hydrolytic activity. CHF resulted in a striking reduction (to 6-17% of controls) of the mass and activity of γ1- and δ1-isoforms in combination with a significant increase of both PLC β1 parameters. In vivo treatment with imidapril (1 mg/kg body wt, daily, initiated 4 wk after coronary occlusion) improved the contractile function and induced a partial correction of PLCs. The mass of SL phosphatidylinositol 4,5-bisphosphate and the activities of the enzymes responsible for its synthesis were significantly reduced in post-MI CHF and partially corrected by imidapril. The results indicate that profound changes in the profile of heart SL PLC-β1, -γ1, and -δ1 occur in CHF, which could alter the complex second messenger responses of these isoforms, whereas their partial correction by imidapril may be related to the mechanism of action of this ACE inhibitor.

Alterations of sarcolemmal phospholipase D and phosphatidate phosphohydrolase in congestive heart failure.

Phospholipase D 2 (PLD2) is the major PLD isozyme associated with the cardiac sarcolemmal (SL) membrane. Hydrolysis of SL phosphatidylcholine (PC) by PLD2 produces phosphatidic acid (PA), which is then converted to 1,2 diacylglycerol (DAG) by the action of phosphatidate phosphohydrolase type 2 (PAP2). In view of the role of both PA and DAG in the regulation of Ca(2+) movements and the association of abnormal Ca(2+) homeostasis with congestive heart failure (CHF), we examined the status of both PLD2 and PAP2 in SL membranes in the infarcted heart upon occluding the left coronary artery in rats for 1, 2, 4, 8 and 16 weeks. A time-dependent increase in both SL PLD2 and PAP2 activities was observed in the non-infarcted left ventricular tissue following myocardial infarction (MI); however, the increase in PAP2 activity was greater than that in PLD2 activity. Furthermore, the contents of both PA and PC were reduced, whereas that of DAG was increased in the failing heart SL membrane. Treatment of the CHF animals with imidapril, an angiotensin-converting enzyme (ACE) inhibitor, attenuated the observed changes in heart function, SL PLD2 and PAP2 activities, as well as SL PA, PC and DAG contents. The results suggest that heart failure is associated with increased activities of both PLD2 and PAP2 in the SL membrane and the beneficial effect of imidapril on heart function may be due to its ability to prevent these changes in the phospholipid signaling molecules in the cardiac SL membrane.

Reduction of phosphatidylinositol-4,5-bisphosphate mass in heart sarcolemma during diabetic cardiomyopathy.

Phosphatidylinositol 4,5-bisphosphate (Ptdlns 4,5-P2) is an important signaling factor as it is a membrane attachment site for proteins containing PH domains and/or an essential requirement for several proteins, which are associated with the cardiac cell plasma membrane (sarcolemma, SL) for normal cardiac function (1,2). Ptdlns 4,5-P2 is synthesized in the SL membrane by the coordinated and successive action of Ptdlns 4-kinase (which catalyses the phosphorylation of Ptdlns to Ptdlns 4-phosphate) and Ptdlns 4-phosphate 5-kinase (which catalyses the phosphorylation of Ptdlns 4-phosphate to Ptdlns 4,5-P2) (3). Further, Ptdlns 4,5-P2 can be phosphorylated by phosphatidylinositol 3-kinase to another membrane-delimited messenger, phosphatidylinositol 3,4,5, trisphosphate (Ptdlns 3,4,5-P3) (4). Changes in the membrane phosphoinositide levels due to the activation of the phosphoinositide signaling pathway (5) may be taken into consideration as being part of the receptor-mediated signaling events via this pathway in the heart (3).

Effects of selenium supplement on the de novo biosynthesis of glycerolipids in the isolated rat heart

The effect of selenium supplement on glycerolipid biosynthesis in the isolated rat heart was investigated. Selenium was administered to the rat by intraperitoneal injection of 4.33 mumol/kg per day for 3 consecutive days. Animals administered with an equal volume of saline were used as controls. Hearts from both animal groups were perfused in Krebs-Henseleit buffer containing labelled glycerol. Subsequent to perfusion, the radioactivity associated with each glycerolipid group was determined. Selenium supplement caused elevations in the labelling of phosphatidic acid and phosphatidylcholine but not in other phospholipids, diacylglycerol or triacylglycerol. The mechanisms for the enhancement of labelling into phosphatidic acid and phosphatidylcholine were examined. The activity of the enzymes responsible for the synthesis of phosphatidic acid in the rat heart was not changed by selenium supplement. However, a 51% increase in the acyl-CoA level was detected which might account for the elevated labelling of phosphatidic acid in the selenium supplemented animal. The 2-fold increase in the activity of CDPcholine:diacylglycerol cholinephosphotransferase might also account for the increase in the labelling of phosphatidylcholine in the heart of the selenium-supplemented rat. It is clear from this study that selenium plays a regulatory role in the control of cellular lipid metabolism.

KINETICS OF MYOCARDIAL PHOSPHOLIPASE D

Myocardial phospholipase D (PLD) is located in different subcellular membranes, including sarcolemma (SL) and sarcoplasmic reticulum (SR). In this study, the kinetics of PLD-dependent hydrolytic and transphosphatidylation activities were examined in SL and SR fractions isolated from rat heart by measuring the formation of phosphatidic acid and phosphatidylethanol, respectively. The results showed that, compared to SR PLD, SL PLD had a higher Vmax, i.e. 373 vs. 70 nmol/mg protein/h for the hydrolytic activity and 415 vs. 60 nmol/mg protein/h for the transphosphatidylation activity. In comparison with the SR enzyme, SL PLD had a lower Km value for the hydrolytic activity (0.46 vs. 0.65 mM), but a higher Km for the transphosphatidylation activity (225 vs. 179 mM). These distinctive kinetic parameters suggest that SL PLD and SR PLD may be isoforms of the enzyme and/or have different membrane domain. Therefore, SL- and SR-localized PLD activities may be under independent control mechanism(s) and play distinct roles in normal conditions and pathological processes.

Effects of selenium and α-tocopherol on liver damage induced by feeding grains from an endemic area of Keshan disease in rats

Previous studies have shown the pathogenic effects of grains cultivated in the endemic areas of Keshan disease and selenium is effective in the prevention of this disease. In this study, liver damages induced by feeding grains from an endemic area (endemic diet), and the effects of selenium and α-tocopherol supplement were examined. After 3 months on the endemic diet, the amounts of serum enzymes were significantly increased when compared to controls (animals receiving diet from a non-endemic area). Liver enzymes (alkaline phosphatase and choline esterase) were also found to be altered in the serum, further suggesting liver damages in animals on an endemic diet. Supplement of the endemic diet with selenium or α-tocopherol reversed the changes in serum enzymes. Increase in lipid peroxidation in the liver of animals on the endemic diet was observed when compared to that in control animals. Selenium and α-tocopherol supplements prevented the increase in lipid peroxidation in the liver by the endemic diet. Semi-quantitative histochemical analysis of glutamate dehydrogenase and succinate dehydrogenase in liver tissue showed that the livers of animals on an endemic diet were more sensitive to ischemic damagesin vitro. Supplementation of the endemic diet with either selenium or α-tocopherol reduced the sensitivity to ischemic damages. The results suggest that increased lipid peroxidation in the liver of rats on an endemic diet may be responsible for liver damages and elevation of serum enzymes. Restoration of glutathione peroxidase activity by selenium supplement or an increase in the content of α-tocopherol in the liver can prevent lipid peroxidation in animals on an endemic diet and thus provide the protective effects against liver damages.