Alterations in myocardial lipid metabolism during lactation in the rat

Lactation Is a Risk Factor of Postpartum Heart Failure in Mice with Cardiomyocyte-specific Apelin Receptor (APJ) Overexpression

The G protein-coupled receptor APJ and its ligand apelin are highly expressed in cardiovascular tissues and are associated with the regulation of blood pressure and cardiac function. Although accumulating evidence suggests that APJ plays a crucial role in the heart, it remains unclear whether up-regulation of APJ affects cardiac function. Here we generated cardiomyocyte-specific APJ-overexpressing (APJ-TG) mice and investigated the cardiac phenotype in APJ-TG mice. Male and non-pregnant APJ-TG mice showed cardiac hypertrophy, contractile dysfunction, and elevation of B-type natriuretic peptide gene expression in the heart but not cardiac fibrosis and symptoms of heart failure, including breathing abnormality and pleural effusion. We further examined the influence of APJ overexpression in response to physiological stress induced by pregnancy and lactation in the heart. Interestingly, repeating pregnancy and lactation (pregnancy-lactation cycle) exacerbated cardiac hypertrophy and systolic dysfunction and induced cardiac fibrosis, lung congestion, pleural effusion, and abnormal breathing in APJ-TG mice. These data indicate that female APJ-TG mice develop postpartum cardiomyopathy. We showed that lactation, but not parturition, was critical for the onset of postpartum cardiomyopathy in APJ-TG mice. Furthermore, we found that lactating APJ-TG mice showed impaired myocardial angiogenesis and imbalance of pro- and antiangiogenic gene expression in the heart. These results demonstrate that overexpression of APJ in cardiomyocytes has adverse effects on cardiac function in male and non-pregnant mice and that lactation contributes to the development of postpartum cardiomyopathy in the heart with APJ overexpression.

Utilization of triacylglycerol-rich lipoproteins by the working rat heart: routes of uptake and metabolic fates

Very-low-density lipoprotein (VLDL) and chylomicrons (CMs) transport triacylglycerol (TAG) to peripheral tissues. Lipoprotein-TAG may gain access to target cells by lipoprotein lipase (LPL) hydrolysis or via receptor-mediated uptake; the principal routes of entry of VLDL and CM into heart are unknown, and different routes of entry may result in different metabolic fates. To examine this, isolated working rat hearts were perfused with rat VLDL and CMs, dual-labelled with [3H]TAG and [14C]cholesterol. Uptake and utilization of CM-TAG were significantly greater than VLDL-TAG, but both were decreased significantly (more than halved) by tetrahydrolipstatin (THL, an inhibitor of lipoprotein lipase). By contrast, uptake of VLDL-cholesterol was much higher than CM-cholesterol (P < 0.01), and suramin (a lipoprotein receptor antagonist) decreased cholesterol uptake of both forms. CM-TAG oxidation rate was more than 4-fold higher than VLDL-TAG oxidation. However, suramin decreased TAG oxidation from both VLDL and CM without affecting TAG uptake or total utilization, suggesting that the TAG gaining access through receptor-mediated pathways is preferentially 'channelled' towards oxidation. Most (79%) CM-TAG was oxidized whilst the proportion of VLDL-TAG oxidized was only about half (49%). In the presence of suramin, there was a significant increase in esterification (incorporation of assimilated [3H]TAG into myocardial tissue [3H]lipids, mainly TAG) of assimilated TAG from both VLDL and CMs, again suggesting that receptor-mediated TAG uptake is directed towards oxidation rather than esterification. The importance of this relatively small pool of TAG is indicated by the fact that cardiac mechanical function declined markedly when lipoprotein receptors were inhibited. These results suggest that CMs, most fatty acids of which gain access into cardiomyocytes through LPL-mediated hydrolysis, are the major supplier of TAG for hearts to oxidize; however, the metabolic fate of VLDL was split evenly between oxidation and deposition as myocardial tissue lipid. Most importantly, VLDL may play a regulatory role in heart lipid metabolism through a lipoprotein receptor-mediated mechanism.

Routes of FA delivery to cardiac muscle: modulation of lipoprotein lipolysis alters uptake of TG-derived FA

Long-chain fatty acids (FA) supply 70-80% of the energy needs for normal cardiac muscle. To determine the sources of FA that supply the heart, [(14)C]palmitate complexed to bovine serum albumin and [(3)H]triolein [triglyceride (TG)] incorporated into Intralipid were simultaneously injected into fasted male C57BL/6 mice. The ratio of TG to FA uptake was much greater for hearts than livers. Using double-labeled Intralipid with [(3)H]cholesteryl oleoyl ether (CE) and [(14)C]TG, we observed that hearts also internalize intact core lipid. Inhibition of lipoprotein lipase (LPL) with tetrahydrolipstatin or dissociation of LPL from the heart with heparin reduced cardiac uptake of TG by 82 and 64%, respectively (P < 0.01). Palmitate uptake by the heart was not changed by either treatment. Uptake of TG was 88% less in hearts from LPL knockout mice that were rescued via LPL expression in the liver. Our data suggest that the heart is especially effective in removal of circulating TG and core lipids and that this is due to LPL hydrolysis and not its bridging function.

Chylomicron and palmitate metabolism by perfused hearts from diabetic mice

Hydrolysis of triacylglycerols (TG) in circulating chylomicrons by endothelium-bound lipoprotein lipase (LPL) provides a source of fatty acids (FA) for cardiac metabolism. The effect of diabetes on the metabolism of chylomicrons by perfused mouse hearts was investigated with db/db (type 2) and streptozotocin (STZ)-treated (type 1) diabetic mice. Endothelium-bound heparin-releasable LPL activity was unchanged in both type 1 and type 2 diabetic hearts. The metabolism of LPL-derived FA was examined by perfusing hearts with chylomicrons containing radiolabeled TG and by measuring (3)H(2)O accumulation in the perfusate (oxidation) and incorporation of radioactivity into tissue TG (esterification). Rates of LPL-derived FA oxidation and esterification were increased 2.3-fold and 1.7-fold in db/db hearts. Similarly, LPL-derived FA oxidation and esterification were increased 3.4-fold and 2.5-fold, respectively, in perfused hearts from STZ-treated mice. The oxidation and esterification of [(3)H]palmitate complexed to albumin were also increased in type 1 and type 2 diabetic hearts. Therefore, diabetes may not influence the supply of LPL-derived FA, but total FA utilization (oxidation and esterification) was enhanced.

Chylomicron metabolism by the isolated perfused mouse heart

The catabolism of rat chylomicrons, labeled in their triacylglycerol (TG) component, was investigated using perfused working mouse hearts. Perfusion of mouse hearts with heparin increased lipoprotein lipase (LPL) activity in the perfusate. This heparin-releasable LPL pool remained constant over a variety of experimental conditions, including workload and fatty acid concentrations, making the mouse heart a suitable model to study chylomicron catabolism. Endothelium-bound LPL hydrolyzed radiolabeled (3)H-labeled chylomicrons (0.4 mM TG); the fate of LPL-derived (3)H-labeled fatty acids was split evenly between oxidation (production of (3)H(2)O) and esterification (incorporation into tissue lipids, mainly TG). In comparison, the oxidation of 0.4 mM [(3)H]palmitate complexed to albumin was fourfold greater than esterification into tissue lipids. Surprisingly, the addition of unlabeled palmitate (0.4 or 1.2 mM) to perfusions with (3)H-chylomicrons did not affect the fate (either oxidation or esterification) of LPL-derived (3)H-fatty acids. These results suggest that fatty acids produced from lipoprotein hydrolysis by the action of LPL and fatty acids from a fatty acid-albumin complex do not enter a common metabolic pool in the heart.

Utilization of very low density lipoprotein by rat heart: the effect of endotoxin

The effect of endotoxin on myocardial utilization of very low density lipoprotein (VLDL) triacylglycerol (TAG) was studied. VLDL was prepared by rat liver perfusion and tested as substrate in the isolated working rat heart. Both liver and heart donor rats were pretreated in vivo with endotoxin or vehicle (control). VLDL-TAG synthesized by endotoxin-pretreated livers was assimilated and oxidized at an increased rate by hearts compared with control VLDL-TAG, regardless of the cardiac endotoxic status, with increased cardiac mechanical performance (cardiac output, hydraulic work). There was no change in incorporation of labeled VLDL lipids into myocardial tissue lipids. Lipoprotein lipase (LPL) activity was increased in endotoxin-pretreated hearts, and after perfusion with "endotoxic" VLDL, there was a tendency for translocation of LPL from tissue-residual to heparin-releasable compartments, but these changes were modest. Analysis of the VLDL composition showed that endotoxin-pretreated livers produced apolipoprotein (apo)-B48 VLDL with decreased particle size (and hence TAG content), but apo-B100 VLDL was unchanged. Oleate content of VLDL was increased, but there was no difference in apo-C or apo-E content. These results suggest that VLDL-TAG produced during sepsis/endotoxinemia may be destined for utilization by the heart as energy substrate. However, the mechanism for its increased efficacy is uncertain.