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N S Dhalla

Publications and source records attributed to N S Dhalla.

At least 145 records · Page 8Linked to original sources
PubMed1990

Alteration of lipid methylation by oleic acid in rat heart sarcolemma.

Incubation of rat heart sarcolemma with the methyl donor S-adenosyl-L-[methyl-3H] methionine resulted in N-methylation of phosphatidylethanolamine and methylation of a heterogenous fraction of nonpolar lipids in the membrane. Oleic acid reduced the synthesis of N-methylated phospholipids and stimulated the methyl group incorporation into nonpolar lipids in a concentration-dependent manner. Both methylation reactions were not affected when oleic acid was substituted by methyl ester of oleic acid or by the detergents sodium deoxycholate or Triton X-100. This study suggests that the enzymatic biosynthesis of the N-methylated phospholipids may be altered by free fatty acids.

Animals↗
PubMed1990

Inhibition of cardiac phosphatidylethanolamine N-methylation by oxygen free radicals.

This study was undertaken to examine the effects of oxygen free radicals on phosphatidylethanolamine (PE) N-methylation in rat heart sarcolemmal (SL) and sarcoplasmic reticular (SR) membranes. Three catalytic sites involved in the sequential methyl transfer reaction were studied by assaying the incorporation of radiolabeled methyl groups from S-adenosyl-L-methionine (0.055, 10, and 150 microM) into SL or SR PE molecules under optimal conditions. In the presence of xanthine + xanthine oxidase (superoxide anion radicals generating system), PE N-methylation was inhibited at site I and III in the heavy SL fraction isolated by the hypotonic shock-LiBr treatment method. In the light SL fraction isolated by sucrose-density gradient, a significant inhibition of PE N-methylation was seen at all three sites. These inhibitory effects of xanthine + xanthine oxidase on PE N-methylation were prevented by the addition of superoxide dismutase. Hydrogen peroxide showed a significant inhibition of PE N-methylation at site I in the heavy SL fraction, and at site I and II in the light SL fraction. Catalase blocked the inhibitory effects of hydrogen peroxide. The effects of both xanthine + xanthine oxidase and hydrogen peroxide on the SR membranes were similar to those seen for the heavy SL fraction. These results suggest that, in addition to lipid peroxidation, the oxygen free radicals may affect the function of cardiac membranes by decreasing the phospholipid N-methylation activity.

Animals↗
PubMed1990

[35S]ATP gamma S binding sites in the purified heart sarcolemma membrane.

Purified heart sarcolemma membranes were found to bind a slowly hydrolyzable analogue of ATP [35S-labeled adenosine 5'-(gamma-thio)triphosphate [( 35S]ATP gamma S)] in a specific manner and exhibited two apparent affinity sites. The high-affinity site had a dissociation constant (KD) of 4.7-8.3 nM [maximum binding (Bmax) = 9.5-18.4 pmol/mg protein], whereas the low-affinity site had a KD of 655-1,257 nM (Bmax = 812-2,955 pmol/mg protein). Like ATP, other nucleotides such as GTP, UTP, ITP, and CTP were effective in displacing [35S]ATP gamma S binding. Although crude membrane preparations from different tissues also exhibited both high- and low-affinity sites for [35S]ATP gamma S, KD values for the high affinity sites were severalfold higher than that for the purified heart membranes. It is proposed that the high-affinity binding site for nucleotides may represent the ATP receptor in the heart cell membrane.

Adenosine Triphosphate↗
PubMed1990

Na(+)-H+ exchange in cardiac sarcolemmal vesicles isolated from diabetic rats.

Intracellular pH and [Na+] in the heart are regulated by the sarcolemmal membrane Na(+)-H+ exchange pathway. No data are currently available regarding the adaptation of this system to pathological conditions in the heart. Because ionic interactions with the heart are altered in cardiomyopathy during chronic experimental diabetes, it was hypothesized that Na(+)-H+ exchange may become abnormal. In addition, the effects of treating diabetic rats with daily injection of L-propionylcarnitine were investigated to determine whether alterations in lipid metabolism may be involved in any potential changes in ion transport. Rats were injected with streptozotocin (65 mg/kg) and killed 8-10 wk later, and sarcolemmal membrane vesicles were isolated from pooled ventricles. Significant depressions in Na(+)-K(+)-adenosinetriphosphatase (ATPase) activity and Na(+)-Ca2+ exchange were observed in the diabetic preparations in comparison to control. L-Propionylcarnitine treatment of the diabetic rats partially normalized these activities. A striking depression in cardiac sarcolemmal Na(+)-H+ exchange was observed in the diabetic animals in comparison to control, and this was not a result of a nonspecific increase in membrane permeability. L-Propionylcarnitine treatment of the diabetic rats did not improve sarcolemmal Na(+)-H+ exchange.

Amiloride↗
PubMed1990

Transient inotropic effects of low extracellular sodium in perfused rat heart.

The inotropic effects of low concentrations of extracellular Na+ (35-110 mM) were studied using Langendorff-perfused rat hearts. Low Na+ induced an initial positive inotropic response proportional to the decrease of transsarcolemmal Na+ gradient. At 35 mM Na+, this effect was followed by a secondary fall in contractility and rise of resting force (RF) and then by a delayed positive inotropic effect and recovery of RF. The magnitude of these low Na(+)-induced transient changes was dependent on the extracellular Ca2+ concentration and was altered by amiloride (6 x 10(-4) and 2.5 x 10(-3) M), ouabain (5 x 10(-5) and 5 x 10(-4) M), ryanodine (2 x 10(-8), 1 x 10(-7) and 1 x 10(-6) M), and sodium azide (1 x 10(-3) and 5 x 10(-3) M) but not by verapamil (2 x 10(-8) and 1 x 10(-7) M) or vanadate (4 x 10(-6) M). The data indicate the initial positive inotropic response of the rat heart to low Na+ may be due to rapid loading of myocytes with Ca2+ through the Na(+)-Ca2+ exchange mechanism. The secondary depression of contractility and the rise of RF appear to be the consequence of the short-lived intracellular Ca2+ overload. Furthermore, the recovery of contractions and the delayed positive inotropic response may be the result of the intracellular redistribution of excessive Ca2+ into the sarcoplasmic reticulum with mitochondria and increased transsarcolemmal Ca2+ efflux apparently playing a more minor role.

Amiloride↗
PubMed1990

Stimulation of phospholipid N-methylation by isoproterenol in rat hearts.

Phosphatidylethanolamine (PtdEtn) N-methyltransferase activities were studied in rat heart sarcolemmal and sarcoplasmic reticular fractions after a single intraperitoneal injection of isoproterenol (0.5-5.0 mg/kg). Three active sites (I, II, and III) for PtdEtn N-methylation were assayed by measurement of [3H]methyl group incorporation from 0.055, 10, and 150 microM S-adenosyl-L-[methyl-3H]methionine into membrane PtdEtn molecules. Total methylation activity for catalytic site I of both sarcolemma and sarcoplasmic reticulum was stimulated within 2 minutes by isoproterenol in a dose-dependent manner. Although the increased methyltransferase activity in sarcoplasmic reticulum was normalized at 10 minutes, the enzyme activity in sarcolemma was normalized at 5 minutes but was again increased at 10-30 minutes after isoproterenol injection. No changes in response to isoproterenol were seen for site II and III N-methylation activities in either membrane. Individual N-methylated phospholipids (phosphatidyl-N-monomethylethanolamine, phosphatidyl-N,N-dimethylethanolamine, and phosphatidylcholine), which specifically formed at each site, showed similar behavior. Pretreatment of the animals with a beta-blocking drug, atenolol, for 2 days prevented the isoproterenol-induced changes in hemodynamic parameters and sarcolemmal methylation without affecting the enhanced methylation activities in sarcoplasmic reticulum. In vitro addition of cyclic AMP-dependent protein kinase (catalytic subunit) plus Mg-ATP enhanced methyltransferase activities in sarcolemma and sarcoplasmic reticulum from control hearts by 2.7- and 2.3-fold, respectively; however, under the same in vitro conditions, only about 20% activation was seen in both subcellular membranes isolated from the heart of isoproterenol-injected animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗
PubMed1990

Nitrendipine binding in congestive heart failure due to myocardial infarction.

Depressed cardiac pump function is the hallmark of congestive heart failure, and it is suspected that decreased influx of Ca2+ into the cardiac cell is responsible for depressed contractile function. Since Ca2+ channels in the sarcolemmal membrane are considered to be an important route for the entry of Ca2+, we examined the status of Ca2+ receptors/channels in failing rat hearts after myocardial infarction of the left ventricular free wall. For this purpose, the left coronary artery was ligated and hearts were examined 4, 8, and 16 weeks later; sham-operated animals served as controls. Hemodynamic assessment revealed decreased total mechanical energy (left ventricular systolic pressure x heart rate), increased left ventricular diastolic pressure, and decreased positive and negative dP/dt in experimental animals at 4, 8, and 16 weeks. Although accumulation of ascites in the abdominal cavity was evident at 4 weeks, other clinical signs of congestive heart failure in experimental rats were evident from the presence of lung congestion and cardiac dilatation at 8 and 16 weeks after induction of myocardial infarction. The density of Ca2+ receptors/channels in crude membranes, as assessed by [3H]nitrendipine binding assay, was found to be decreased in the uninfarcted experimental left ventricle at 8 and 16 weeks; however, no change in the affinity of nitrendipine was evident. A similar depression in the specific binding of another dihydropyridine compound, [3H]PN200-110, was also evident in failing hearts. Brain and skeletal muscle crude membrane preparations, unlike those of the right ventricle and liver, revealed a decrease in Ca2+ receptors/channels density in experimental animals at 16 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Alterations in phospholipid N-methylation of cardiac subcellular membranes due to experimentally induced diabetes in rats.

Phosphatidylethanolamine N-methylation was examined in cardiac subcellular membranes after inducing chronic experimental diabetes in rats (65 mg streptozotocin/kg, i.v.). The incorporation of radiolabeled methyl groups from S-adenosyl-L-methionine in diabetic sarcolemma was significantly depressed at all three catalytic sites (I, II, and III) of the methyltransferase system. An increase in methyl group incorporation was evident at site I without any changes at sites II and III in diabetic sarcoplasmic reticulum and mitochondria. Similar changes were also seen for the individual N-methylated lipids (monomethyl-, dimethylphosphatidylethanolamine, and phosphatidylcholine) specifically formed at each catalytic site in all cardiac membranes from diabetic animals. These alterations in N-methylation were reversible by a 14-d insulin therapy to the diabetic animals. In the presence of 10 microM ATP and 0.1 microM Ca2+, N-methylation was maximally activated at site I in both control and diabetic sarcolemma and sarcoplasmic reticulum, but not in mitochondria. Incubation of cardiac membranes with of S-adenosyl-L-methionine showed that Ca2(+)-stimulated ATPase activities in both sarcolemma and sarcoplasmic reticulum were augmented; however, the activation of diabetic sarcolemma was lesser and that of diabetic sarcoplasmic reticulum was greater in comparison with the control preparations. These results identify alterations in phosphatidylethanolamine N-methylation in subcellular membranes from diabetic heart, and it is suggested that these defects may be crucial in the development of cardiac dysfunction in chronic diabetes.

Animals↗
PubMed1990

Depression in cardiac contractile force induced by cholesterol.

Cholesterol, combined with albumin, induced rapid functional changes in the isolated perfused rat heart. Under constant perfusion pressure, the decline in contractile force and the rate of development of force due to cholesterol was accompanied by an increase in resting tension and a decrease in coronary flow. Such effects were not produced by albumin alone. Depression in contractile force and increase in resting tension were also evident when coronary flow was kept constant; these effect of cholesterol were dependent on time and dose. The findings support the view that cholesterol may affect cardiac function independently of the atherosclerotic lesion in the hypercholesterolemic state.

Adenosine Triphosphatases↗
PubMed1990

Cardiac sarcolemma as a possible site of action of caffeine in rat heart.

Caffeine (0.1-10 mM) produced a biphasic effect on Na(+)-K+ ATPase activity in the rat heart sarcolemmal preparations. The Na(+)-K+ ATPase activity was stimulated by about 25% at low concentrations (0.1-1 mM), whereas the enzyme was inhibited by about 25% at higher concentrations (10 mM) of caffeine. The stimulatory effect of 1 mM caffeine was associated with about 30% increase in the Vmax value for Na(+)-K+ ATPase, whereas the depressant action of 10 mM caffeine was associated with an increase of the Km value from 1.4 to 2.1 mM ATP. The Na(+)-induced Ca++ release from the sarcolemmal vesicles was stimulated with caffeine in a concentration-dependent manner; about 80% increase in the activity was observed at 0.1 mM caffeine. The apparent Ka (millimolar Na+) values for the Na(+)-induced Ca++ release were about 17 and 6 in the absence and presence of 1 mM caffeine, respectively. However, the sarcolemmal Na(+)-dependent Ca++ uptake and ATP-independent Ca++ binding were not affected, whereas the ATP-dependent Ca++ accumulation and Ca+(+)-stimulated ATPase activities were depressed by 1 to 10 mM caffeine. This agent at concentrations of 0.1 to 10 mM produced a biphasic effect on the contractile activity of the isolated perfused rat heart. The initial transient positive inotropic (30-60%) effect was followed by a sustained negative inotropic (50-80%) response of the drug; the delayed decrease in contractile force was associated with a significant increase (35-50%) in the resting tension. The initial positive inotropic effect of caffeine was dependent on the concentration of Ca++ (0.2-3 mM) in the perfusion medium; however, this response was attenuated either by lowering the concentration of Na+ from 140 to 35 mM or by different concentrations (0.5-1 mM) of amiloride in the medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphatases↗
PubMed1989

Diabetes-like action of intermittent fasting on sarcoplasmic reticulum Ca2+-pump ATPase and myosin isoenzymes can be prevented by sucrose.

Experimental diabetes results in a reduction of the sarcoplasmic reticulum (SR) Ca2+-stimulated ATPase activity and a redirection of myosin isoenzymes from V1 to V3. Similar, but less pronounced, changes were induced by subjecting rats to intermittent fasting for 6 weeks. Low amounts of sucrose (0.8%) in the drinking water prevented the subcellular changes in fasted rats; however, sucrose neither affected the levels of plasma thyroid hormones nor normalized the reduced body weight. Plasma glucose was lowered without any changes in plasma insulin in the fasted rats receiving sucrose; this suggested an enhanced peripheral glucose utilization. Thus, the signals in the diabetic heart leading to changes in SR and myosin can be mimicked by intermittent fasting and seem to be linked to a shift in fuel utilization by the myocytes.

Animals↗
PubMed1989

Measurement of adrenolutin as an oxidation product of catecholamines in plasma.

Using the reverse phase high-performance liquid chromatography (HPLC) with mobile phases composed of simple acids, we have developed an assay technique for the measurement of adrenolutin, one of the oxidation products of catecholamines, in rat plasma. Ion-pairing chromatography permits the separation and quantitation of plasma adrenolutin (microM) in a linear manner. Sample preparation involved the precipitation of plasma proteins with perchloric acid and it is easier to handle a large number of samples at a time. However, we were unable to demonstrate the presence of adrenochrome, another oxidation product of catecholamines, in plasma since adrenochrome was rapidly destroyed in acid as well as in blood and was quickly changed into adrenolutin. Adrenolutin peak in HPLC was confirmed by 1) the retention time; 2) co-injection of adrenolutin and; 3) the appearance of 3H-adrenolutin after injection of 3H-norepinephrine. Administration of different catecholamines as well as adrenochrome and adrenolutin in rats also increased the level of adrenolutin in plasma. Adrenolutin was found to be present in plasma in other species including dog, rabbit and pig. High level of adrenolutin, which may represent total concentration of aminolutin in plasma, suggests the presence of an efficient mechanism for the oxidation of catecholamines under in vivo conditions.

Adrenochrome↗
PubMed1989

Altered sympathetic system and adrenoceptors during the development of cardiac hypertrophy.

Increasing experimental evidence suggests that the development of cardiac hypertrophy may involve the sympathetic system and associated receptor mechanisms. However, very little work has been done so far to understand changes in the sympathetic system and cardiac adrenoceptors soon after an increased work load is imposed on the heart. Accordingly rat hearts subjected to aortic banding-induced pressure overload were assessed 3, 7, and 14 days postoperatively. Sham-operated rats without aortic banding were used as a control group. Rats with aortic constriction had increases in heart rate, left ventricular systolic pressure, and total mechanical energy during the entire study period. The cardiac RNA level was increased without a significant increase in left ventricular mass on days 3 and 7 in aortic-banded animals; these results were associated with a decrease in the cardiac norepinephrine (NE) store and an increase in the plasma level of NE and dopamine beta-hydroxylase (DBH) activity. By day 14 a significant increase in left ventricular mass and the NE store were found; both plasma NE and DBH remained elevated. Catecholamines in other tissues such as the spleen and kidney were depleted in the banded group, whereas the dopamine level, particularly in the brain, was significantly higher during the entire study. Furthermore, the density of alpha-adrenoceptors was higher on day 3 of aortic banding, and a reciprocal correlation was evident between cardiac alpha- and beta-adrenoceptors on day 14; the density of beta-adrenoceptors was increased, whereas that of alpha-adrenoceptors was decreased in the banded group.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1989

Specific stimulation of heart sarcolemmal Ca2+/Mg2+ ATPase by concanavalin A.

The effects of concanavalin A (Con A) on membrane Ca2+/Mg2+ ATPase activities as well as the characteristics of Con A binding were examined by employing rat heart sarcolemmal preparations. Con A stimulated the Ca2+ ATPase and Mg2+ ATPase activities in sarcolemma; maximal stimulation in these parameters was seen at a concentration of 10 micrograms/ml. The observed effects of Con A were blocked by alpha-methylmannoside. Sarcolemmal Na+-K+ ATPase and Ca2+-stimulated ATPase were not affected by Con A. Likewise, Con A did not alter the mitochondrial, sarcoplasmic reticular, and myofibrillar ATPase activities. Con A was found to bind to sarcolemma; alpha-methylmannoside prevented this binding. The Scatchard plot analysis of the data on specific Con A binding showed a straight line with a Kd of about 530 nM and a Bmax of 235 pmol/mg protein, thus indicating that there was only one kind of binding site for Con A in sarcolemma. These results suggest that Con A is a specific activator of the low affinity Ca2+/Mg2+ ATPase system in the heart sarcolemmal membrane.

Animals↗
PubMed1989

Reduction of calcium channel antagonist binding sites by oxygen free radicals in rat heart.

In view of the importance of Ca2+-channels in controlling the entry of Ca2+ into the myocardium, this study was undertaken to examine the effects of oxygen free radicals on the binding of Ca2+-channel antagonists in rat heart by employing [3H]-nitrendipine as a ligand. Isolated heart membranes were incubated with xanthine + xanthine oxidase (a superoxide anion radicals generating system), hydrogen peroxide (an activated species of oxygen), or hydrogen peroxide + Fe2+ (a hydroxyl radicals generating system). The assay of the [3H]-nitrendipine binding activity revealed that the maximal number of binding sites (Bmax) were reduced in a time-dependent manner by superoxide radicals without any changes in the binding constant (Kd); a significant reduction of Bmax was seen after incubating membranes with xanthine + xanthine oxidase for a 10-min-period. Superoxide dismutase showed a protective effect on the superoxide radicals induced reduction in Bmax. Both hydrogen peroxide and hydroxyl radicals also depressed the Bmax for [3H]-nitrendipine binding without any significant change in Kd; catalase and mannitol showed protective effects on hydrogen peroxide or hydroxyl radicals induced depression in Bmax, respectively. These results indicate that oxygen free radicals may reduce the number of Ca2+-channels in the cell membrane and this change may contribute towards decreasing the voltage-dependent Ca2+ influx in the cardiac cell.

Animals↗
PubMed1989

Alterations of phosphatidylethanolamine N-methylation in rat heart by quinidine.

To elucidate the molecular mechanism underlying the adverse depression of myocardial contractility observed during antiarrhythmic therapy of quinidine, we investigated its action on the phosphatidylethanolamine N-methyltransferase (EC 2.1.1.17) activities of cardiac subcellular membranes. Rat heart sarcolemma, mitochondria, and microsomes (sarcoplasmic reticular fragments) were isolated, and the three catalytic sites for N-methylation activities were examined with 0.055 (site I), 10 (site II), and 150 (site III) microM concentrations of S-adenosyl-L-[methyl-3H]methionine as a methyl donor. Total methyl group incorporation into sarcolemmal phosphatidylethanolamine was depressed by 10(-6)-10(-3) M quinidine at sites II and III. The activity of site I was stimulated at low (10(-9) M) concentrations and inhibited at high concentrations of the drug. A similar behaviour was observed with procainamide, although the inhibitory effect was less pronounced and was not additive with quinidine. Quinidine-induced inhibition was associated with a depression of Vmax, while the apparent affinity for S-adenosyl-L-methionine was unaltered. Analysis of individual methylated phospholipids confirmed inhibition by quinidine at sites II and III in sarcolemma. Microsomal phosphatidylethanolamine N-methylation was affected by 10(-6) M quinidine only at site II, whereas no changes were noted in mitochondria. Quinidine also inhibited both the positive inotropic response and concomitant increase in tissue N-methylated phospholipids observed upon L-methionine perfusion of rat heart. These results suggest that quinidine alters the intramembranal level of N-methylated phospholipids, and this may serve as a biochemical mechanism contributing to its negative inotropic effect.

Animals↗
PubMed1989

Influence of gramicidin S on cardiac membrane Ca2+/Mg2+ ATPase activities and contractile force development.

The effects of gramicidin S (GS), an antibiotic, on the rat heart membrane ATPases and contractile activity of the right ventricle strips were investigated. GS inhibited sarcolemmal Ca2+-stimulated ATPase (IC50 = 3 microM), Ca2+/Mg2+ ATPase which is activated by millimolar Ca2+ or Mg2+ (IC50 = 3.4 microM), and sarcoplasmic reticulum Ca2+-stimulated ATPase (IC50 = 6 microM). The type of inhibition for the sarcolemmal Ca2+/Mg2+ ATPase by GS was apparently uncompetitive, while that for Ca2+-stimulated ATPases in sarcolemma or sarcoplasmic reticulum was of mixed type. Other ATPases, including mitochondrial ATPase, sarcolemmal Na+-K+ ATPase, and myofibrillar ATPase, were not inhibited by this agent. GS also decreased the rat right ventricle maximum force development (half-maximal inhibitory concentration was 2-4 microM), maximum velocity of contraction, and maximum velocity of relaxation. The resting tension was increased by GS to over 200%. The contractile actions of GS were mostly irreversible upon washing the muscle 3 times over a 10-min period. Decreased Ca2+, Mg2+, Na+, K+ concentrations in the perfusate increased the effects of GS. These findings showed that GS was a potent inhibitor of divalent cation ATPases of heart sarcolemma and sarcoplasmic reticulum and it is suggested that these membrane effects may explain the cardiodepressant action of this agent.

Animals↗