Changes in fatty acyl chain composition of rat heart phospholipids induced by noradrenaline

Imipramine as an alternative to formamide to detubulate rat ventricular cardiomyocytes

NEW FINDINGS

What is the central question of this study? Can imipramine, an antidepressant agent that is a cationic amphiphilic drug that interferes with the phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂ ) interactions with proteins maintaining the tubular system, be validated as a new detubulating tool? What is the main finding and its importance? Imipramine was validated as a more efficient and less toxic detubulating agent of cardiomyocytes than formamide. New insights are provided on how PI(4,5)P₂ is crucial to maintaining T-tubule attachment to the cell surface and on the cardiotoxic effects of imipramine overdoses.

ABSTRACT

Cardiac T-tubules are membrane invaginations essential for excitation-contraction coupling (ECC). Imipramine, like other cationic amphiphilic drugs, interferes with phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂ ) interactions with proteins maintaining the tubular system connected to the cell surface. Our main purpose was to validate imipramine as a new detubulating agent in cardiomyocytes. Staining adult rat ventricular myocytes (ARVMs) with di-4-ANEPPS, we showed that unlike formamide, imipramine induces a complete detubulation with no impact on cell viability. Using the patch-clamp technique, we observed a ∼40% decrease in cell capacitance after imipramine pretreatment and a reduction of I_Ca,L amplitude by ∼72%. These parameters were not affected in atrial cells, excluding direct side effects of imipramine. β-Adrenergic receptor (β-AR) stimulation of the remaining I_Ca,L with isoproterenol (Iso) was still effective. ECC was investigated in ARVMs loaded with Fura-2 and paced at 1 Hz, allowing simultaneous measurement of the Ca²⁺ transient (CaT) and sarcomere shortening (SS). Amplitude of both CaT and SS was decreased by imipramine and partially restored by Iso. Furthermore, detubulated cells exhibited Ca²⁺ homeostasis perturbations. Real-time cAMP variations induced by Iso using a Förster resonance energy transfer biosensor revealed ∼27% decreased cAMP elevation upon β-AR stimulation. To conclude, we validated a new cardiomyocyte detubulation method using imipramine, which is more efficient and less toxic than formamide. This antidepressant agent induces the hallmark effects of detubulation on ECC and its β-AR stimulation. Besides, we provide new insights on how an imipramine overdose may affect cardiac function and suggest that PI(4,5)P₂ is crucial for maintaining T-tubule structure.

Effect of dietary lipids and endocrine changes on polyunsaturated fatty acids in phospholipids of pancreas and brown adipose tissue of obese and lean rats

Obese (fa/fa) rats fed on control diet have lower proportions of linoleic acid (18:2n-6) and/or arachidonic acid (20:4n6) in IBAT and pancreas phospholipids compared with lean (Fa/-) rats. Lower stearic acid (18:0) to oleic acid (18:1n-9) mean ratios in fa/fa compared with Fa/- suggest enhanced delta 9-desaturase activity in the former. 18:2/20:4 mean ratios in pancreas, but not IBAT, are indicative of a reduced delta 6-desaturase activity in fa/fa rats. Absolute amounts of phospholipids (mg/unit tissue wt) were 2-fold greater in IBAT of fa/fa compared with Fa/- rats, irrespective of their diet. This was reflected in greater absolute amounts of 18:2n-6 and 20:4n-6 only in HSO and HTO fa/fa groups, but not in the control group. Adrenalectomy (Adx) or T3 treatment also modified phospholipid fatty acid composition of IBAT and pancreas phospholipids in animals fed on the control diet with fa/fa rats more sensitive to endocrine induced changes. In fa/fa rats T3 treatment increased docosahexaenoic acid (22:6n-3) in IBAT of both phenotypes compared with the control, but this effect was evident only in fa/fa and not Fa/- pancreas. T3 treatment also increased docosapentaenoic acid (22:5n-3) in IBAT from, both phenotypes, but no 22:5n-3 was evident in the pancreatic tissue of these animals. ADX also increased 22:6n-3 in the IBAT of fa/fa (Fa/- values were less than 1%) but not in the pancreas of fa/fa or Fa/-. ADX modified the relative proportions of 18:0 to 18:1 and 18:2 to 20:4 in IBAT and pancreas of fa/fa in a way that indicated decreased delta 9-desaturase and increased delta 6-desaturase activities; these effects tended toward normal again in pancreas of fa/fa rats on corticosterone replacement (CST).

Lipids and cardiac arrhythmia

In any discussion of lipids and heart disease it is beneficial from the outset to recognise that at least three different pathological processes may be involved. The first of these is atherosclerosis which involves the deposition of "fat" in the coronary vessels, another is thrombogenesis which describes the formation of blood clots in the coronary vessels, and the third is arrhythmia which refers to disorders in the beating of the heart which may become sufficiently serious to cause sudden cardiac death (SCD). Also it is this disturbance in the rhythmic beating of the heart which is responsible for much of the mortality from 'heart attacks' which occur 'outside-of-hospital' in societies like U.S.A., U.K. and Australia. It is this latter condition of cardiac arrhythmia which is the major concern of this review. Because it is often difficult to differentiate the role of lipids in 'heart disease' in man, it has frequently been assumed that all dietary fatty acids have similar effects on the different processes involved, and many unwarranted generalisations have been made which have led to conflicts of opinion amongst physicians and confusion in the lay public. From the animal studies discussed in this review, it is apparent that dietary fatty acids have an important role to play in determining the vulnerability of the myocardium to develop serious ventricular fibrillation (VF) and potentially lethal cardiac arrhythmia. In general, diets rich in saturated fatty acids promote a state of myocardial vulnerability, whilst diets rich in PUFA significantly diminish the probability of developing lethal disorders in cardiac rhythm when the heart is placed under pharmacological (or emotional) stress, or deprived of sufficient blood flow and supply of oxygen. Very recent experiments with the monounsaturated fatty acid (MUFA) oleic acid clearly demonstrate that, at least in rats subjected to ligation of their coronary artery, this acid is not 'neutral' as has been suggested by some for its role in atherosclerosis, but in fact is indistinguishable from saturated fatty acids in its effect in promoting arrhythmia during either regional ischaemia or reperfusion arrhythmia in this animal model of SCD.(ABSTRACT TRUNCATED AT 400 WORDS)

Polyunsaturated fatty acids in heart muscle and alpha 1-adrenoceptor binding properties

Modifications of membrane phospholipids and binding characteristics of adrenoceptors by hydrocortisone and epinephrine were examined in sarcolemmal preparation from rat heart muscle. The influence of hydrocortisone and epinephrine on the fatty acid composition of membrane phospholipids and the affinity (1/Kd) and number of binding sites (Bmax) of alpha 1- and beta-adrenoceptors was studied in male Wistar rats treated daily for 7 days with the hormones. The alpha 1- and beta-adrenoceptors were characterized by using the antagonist ligands [3H]prazosin and [3H]dihydroalprenolol, respectively. Administration of the hormones altered significantly the composition of fatty acids, decreased linoleic acid (18: 2(n-6)) level of both phosphatidylcholine and phosphatidylethanolamine, and increased arachidonic acid (20: 4(n-6)) level of phosphatidylcholine and docosahexaenoic acid (22: 6(n-3)) level in both phosphatidylcholine and phosphatidylethanolamine. The binding sites of alpha 1-adrenoceptors were of high affinity in the control group. Following administration of the hormones Kd of alpha 1-adrenoceptors increased markedly. The number of alpha 1-adrenoceptors binding sites did not change significantly due to the hormones. In contrast, while the hormone treatments did not alter the affinity of the beta-adrenoceptors the number of binding sites were significantly decreased by the hormones. The results indicate that the decrease in affinity of alpha 1-adrenoceptors and the down-regulation of beta-adrenoceptors is accompanied by alteration in percentage fatty acid compositions of phosphatidylethanolamine and phosphatidylcholine in cardiac muscle.

Cardiolipins and biomembrane function

Evidence is discussed for roles of cardiolipins in oxidative phosphorylation mechanisms that regulate State 4 respiration by returning ejected protons across and over bacterial and mitochondrial membrane phospholipids, and that regulate State 3 respiration through the relative contributions of proteins that transport protons, electrons and/or metabolites. The barrier properties of phospholipid bilayers support and regulate the slow proton leak that is the basis for State 4 respiration. Proton permeability is in the range 10(-3)-10(-4) cm s-1 in mitochondria and in protein-free membranes formed from extracted mitochondrial phospholipids or from stable synthetic phosphatidylcholines or phosphatidylethanolamines. The roles of cardiolipins in proton conductance in model phospholipid membrane systems need to be assessed in view of new findings by Hübner et al. [313]: saturated cardiolipins form bilayers whilst natural highly unsaturated cardiolipins form nonlamellar phases. Mitochondrial cardiolipins apparently participate in bilayers formed by phosphatidylcholines and phosphatidylethanolamines. It is not yet clear if cardiolipins themselves conduct protons back across the membrane according to their degree of fatty acyl saturation, and/or modulate proton conductance by phosphatidylcholines and phosphatidylethanolamines. Mitochondrial cardiolipins, especially those with high 18:2 acyl contents, strongly bind many carrier and enzyme proteins that are involved in oxidative phosphorylation, some of which contribute to regulation of State 3 respiration. The role of cardiolipins in biomembrane protein function has been examined by measuring retained phospholipids and phospholipid binding in purified proteins, and by reconstituting delipidated proteins. The reconstitution criterion for the significance of cardiolipin-protein interactions has been catalytical activity; proton-pumping and multiprotein interactions have yet to be correlated. Some proteins, e.g., cytochrome c oxidase are catalytically active when dimyristoylphosphatidylcholine replaces retained cardiolipins. Cardiolipin-protein interactions orient membrane proteins, matrix proteins, and on the outerface receptors, enzymes, and some leader peptides for import; activate enzymes or keep them inactive unless the inner membrane is disrupted; and modulate formation of nonbilayer HII-phases. The capacity of the proton-exchanging uncoupling protein to accelerate thermogenic respiration in brown adipose tissue mitochondria of cold-adapted animals is not apparently affected by the increased cardiolipin unsaturation; this protein seems to take over the protonophoric role of cardiolipins in other mitochondria. Many in vivo influences that affect proton leakage and carrier rates selectively alter cardiolipins in amount per mitochondrial phospholipids, in fatty acyl composition and perhaps in sidedness; other mitochondrial membrane phospholipids respond less or not at all.(ABSTRACT TRUNCATED AT 400 WORDS)

Age-related changes of lipid fractions and total fatty acids in liver lipids and heart lipids of female and male rats aged 37-1200 days (liver) and 331-1200 days (heart)

1. Total lipids and the lipid fractions cholesterol ester, triacylglycerol, free cholesterol, free fatty acids and phospholipids, as well as the fatty acid patterns of total lipids, were measured in liver homogenates of female and male rats (Wistar SPF, strain Hannover) aged 37-1213 days. 2. The same parameters were measured in the apex of the heart in female and male rats aged 331-1213 days. 3. All parameters were monitored every 49th day. Five female and five male animals were used in each experiment. 4. The lipid fractions in liver showed a positive linear regression vs age, whereas all lipids in rat heart showed a negative regression vs age in both sexes. 5. The significance of regression vs age of fatty acids was much less than that in the lipid fractions of liver and heart of these animals.

Beta blockers and left ventricular hypertrophy in hypertension

It is now generally accepted that hypertension-induced left ventricular hypertrophy (LVH) represents a phenomenon of multifactorial origin. Antihypertensive therapy with beta-blocking drugs influences most of the factors involved in the control of left ventricular mass. Therefore, although initial animal experiments yielded conflicting results, it is not surprising that a great deal of evidence has been accumulated in clinical studies showing that successful long-term antihypertensive treatment with beta blockers induces regression of LVH in hypertensive subjects. Differences in molecular structure among various beta-blocking agents do not seem to influence this property. On the contrary, the question of whether reversal of LVH represents a beneficial or harmful byproduct of antihypertensive treatment with beta blockers is still unanswered. Animal and clinical studies suggest that left ventricular systolic function is unchanged or even improved after regression of LVH, whereas the ability of the heart to withstand recurrence of hypertension is slightly reduced. Furthermore, development of LVH in hypertensive subjects is associated with abnormalities in diastolic function which are not reduced by reversal of LVH induced by antihypertensive treatment with beta blockers.

The influence of fish oil diet and norepinephrine treatment on fatty acid composition of rat heart phospholipids and the positional fatty acid distribution in phosphatidylethanolamine

The effect of chronic norepinephrine (NE) administration with increasing dosage from 1-4 mg/kg over a period of 2 weeks was studied on cardiac phospholipids and their fatty acid distribution in rats. Animals were fed a control diet or a 10% cod liver oil (CLO)-enriched diet. The relative distribution of various polyunsaturated fatty acids esterified to the 1- and 2-position of the phosphatidylethanolamine fraction was estimated. NE stress during control feeding significantly reduced the total phospholipid content in rat heart. No differences in the phospholipid class distribution were found. However, CLO feeding as well as chronic NE administration resulted in a decrease of omega 6 fatty acids, mainly C 18:2 omega 6 and C 20:4 omega 6, which was compensated with an increase in omega 3 fatty acids, mainly C 20:5 omega 3 and C 22:6 omega 3. The changes in fatty acid composition qualitatively agree with those reported by Gudbjarnason et al. (23), except that the mortality in our NE-treated control or CLO-fed groups was considerably lower. It can probably be attributed to a different mode of NE administration. On the other hand, at the end of the CLO feeding period in rats treated with NE or not, comparing with control fed rats without NE treatment, the incidence rate of ST segment elevation in electrocardiogram (ECG) recorded under light diethylether-induced anesthesia was higher. Independent of whether the fatty acid composition of myocardial phospholipids was dietary or pharmacologically manipulated, most of the polyunsaturated fatty acids were found at the 2-position of the phosphatidylethanolamine molecules. The polyunsaturated fatty acids account for 45-50% of the fatty acyl residues and preferentially occupy the 2-position, where they can exchange for each other.

Assay of lipids in dog myocardium using capillary gas chromatography and derivatization with boron trifluoride and methanol

The fatty acids of three lipid classes (free fatty acids, triglycerides, and cholesteryl esters) from dog heart were analysed by gas chromatography. Samples of the left ventricle were homogenized and total lipids were extracted. After separation by thin-layer chromatography, the bands of the lipid classes studied were scraped off, transmethylated according to the boron trifluoride-methanol procedure, and the fatty acid methyl esters were extracted and analysed. The problems related to the quantitation of fatty acids were investigated, namely transmethylation procedure, thin-layer chromatography, and gas chromatographic conditions. Fatty acid methyl esters were separated on capillary columns coated in the laboratory with SP 2340 stationary phase. The high performance of the separation ensured the reliability and the precision of the analysis.

Regulation of liver lipase. II. Involvement of the alpha 1-receptor

The effects of different adrenergic agents on high density lipoprotein (HDL) cholesterol concentration and on the neutral NaCl-resistant triacylglycerol hydrolase (liver lipase) activity of the liver were studied in rats. Treatment of rats with the beta-blockers metoprolol, atenolol or propranolol led to a lowering of the HDL-cholesterol (esterified and non-esterified) content. The alpha 1-antagonist prazosin had no effect. Administration of norepinephrine for 10 days resulted in an increase of HDL non-esterified cholesterol. This effect of norepinephrine was largely abolished by prazosin, but not by propranolol. In normal rats the liver lipase activity was not influenced by alpha- or beta-blockade. Adrenergic stimulation, either short-term (by diethyl ether stress) or long-term (by norepinephrine treatment), led to a lowered liver lipase activity. The lipase activity was restored by prazosin but not by propranolol. The apparent involvement of the alpha 1-receptor in the regulation of liver lipase activity was further studied in vitro. Blockade of alpha- or beta-receptors with prazosin or propranolol did not affect the secretion of the liver lipase activity by isolated parenchymal liver cells. Stimulation of alpha- or beta-receptors by epinephrine led to a lower secreted lipase activity. Selective stimulation by isoprenaline had no effect. The effect of epinephrine could be abolished by prazosin but not by propranolol. Vasopressin and the calcium ionophore A23187 also lowered the secretion of liver lipase activity in vitro. Glucagon and/or the phosphodiesterase inhibitor Ro 20-1724 had no effect. These results indicate an involvement of the alpha 1-receptor in the regulation of liver lipase activity at the level of synthesis or secretion of the lipase. The effect of the alpha 1-receptor is presumably mediated through changes in the intracellular free calcium concentration. The effect of adrenergic modulation on HDL-cholesterol concentrations can partly be explained through modification of the liver lipase activity.

Neonatal changes in fatty acid profile of phospholipids in rat heart muscle

The fatty acid composition of rat heart phospholipids was examined during the neonatal and postnatal period. The rats were killed on days 1, 7, 14 and 21 after birth and at the ages of 2 and 6 months. The fatty acyl chain composition of the two major phospholipids, phosphatidylcholine (PC) and phosphatidylethanolamine (PE) changed significantly during the first 2 months. In PC there was a marked and immediate increase in stearic acid, a significant but transient increase in arachidonic acid and late increase in linoleic acid content. In PE there was an immediate increase in stearic acid and docosahexaenoic acid, followed by a late increase in linoleic acid content. The observed alterations in fatty acid composition of heart muscle phospholipids resemble changes induced by repeated administration of norepinephrine and subsequent recovery. Neonatal stress and increased cardiac function play an important role in the modification of the fatty acid composition of rat heart muscle phospholipids during early development.

Reversible alterations in fatty acid profile of glycerophospholipids in rat heart muscle induced by repeated norepinephrine administration

Rats were injected subcutaneously for 2 weeks with increasing amounts of norepinephrine. The lipid composition of the heart muscle was examined for nearly 2 months. The treatment caused major changes in fatty acyl chain composition of myocardial phosphatidylethanolamine and phosphatidylcholine. In these phospholipids, linoleic acid was decreased to about half of the control value but docosahexaenoic acid increased about 50% in phoshatidylethanolamine and more than doubled in phosphatidylcholine. Arachidonic acid content rose about 50% in phosphatidylcholine but was lowered in phosphatidylethanolamine. The cardiolipin fraction retained its high amount of linoleic acid and the fatty acid composition of the triacylglycerol was not altered, although the amount was significantly decreased. These changes reverted to control levels in 4-8 days after the final injection, although rebound behaviour was observed. An inverse relationship between arachidonic acid content of phosphatidylcholine and phosphatidylethanolamine was observed.

Differences in the fatty acid composition of atrial and ventricular phospholipids of rat heart following standard and lipid-supplemented diets

1. The major saturated fatty acids of the phospholipids of rat heart atria and ventricles are similar and are not greatly altered by supplementing the diet with widely different types of lipid. 2. There are important differences in the relative proportions of the major unsaturated fatty acids of the phospholipids of these anatomically and functionally distinct regions of the heart. 3. The proportions of linoleic (C18:2, eta-6) and docosahexaenoic (C22:6, eta-3) acid are significantly higher in the ventricles than in the atria; the proportions of oleic (C18:1, eta-9) arachidonic (C20:4, eta-6) and docosatetraenoic acids (22:4, eta-6) are higher in atria. 4. The differences in unsaturated fatty acid profiles persist even after twelve months of feeding lipid supplements of sunflower seed oil (SSO) or sheep kidney (perirenal) fat (SKF). 5. However, the ratios of arachidonic to docosahexaenoic acid in both tissues are changed by decreasing the intake of linoleic acid, which apparently favours the conversion of dietary linolenic (C18:3, eta-3) to docosahexaenoic acid. The level of docosahexaenoic acid is greater in the ventricles than in the atria, and greatest when the animals were fed SKF diet. 6. The physiological and pharmacological differences in ventricles and atria may arise from differences as fundamental as the phospholipid fatty acid composition of cardiac membranes.