Alterations in myocardial lipid metabolism during lactation in the rat

Xin Wang1, David G. Hole1, Teresa H. M. Da Costa2,3, and Rhys D. Evans1,2

1 Nuffield Department of Anaesthetics and 2 Metabolic Research Laboratory, Nuffield Department of Clinical Medicine, University of Oxford, Oxfordshire OX2 6HE, United Kingdom; and 3 Universidade de Brasilia, Departamento de Nutrição, Campus Universitário, Asa Norte, CEP 70919-970, Brazil

    ABSTRACT
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

Metabolism of nonesterified fatty acid (palmitate, 1.1 mM) and triacylglycerol (TAG; triolein, 0.4 mM in the form of both rat chylomicrons and very low density lipoproteins) was studied in isolated perfused working hearts from fed nulliparous, lactating, and weaned rats. Hearts from virgin rats oxidized palmitate readily, but optimal cardiac mechanical performance occurred during perfusion with chylomicrons. In hearts from lactating dams, there was a significant increase in palmitate oxidation and a marked decrease in TAG oxidation from both chylomicrons and very low density lipoproteins compared with hearts from nulliparous animals. There was a concomitant decrease in lipoprotein lipase activity in hearts from lactating animals, and TAG in the absence of palmitate could not support optimal cardiac mechanical function. After litter removal, the changes in fatty acid and TAG metabolism observed in lactation returned to nulliparous values within 96 h. These results suggest that, during lactation, both exogenous and endogenous TAGs are directed away from heart and toward the lactating mammary gland; the heart, therefore, has to rely to a greater extent on nonesterified fatty acid for energy provision under these conditions.

heart; lipoprotein lipase; triacylglycerol

    INTRODUCTION
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

LACTATION IS A physiological state characterized by increased demand for maternal substrates. This is met partly by hyperphagia but also by highly regulated redirection of available circulating substrates away from nonessential depots to the lactating mammary gland for export as milk; in the case of lipids, triacylglycerols (TAGs) carried in plasma in the form of chylomicrons synthesized in the gastrointestinal tract and very low density lipoproteins (VLDLs) synthesized in the liver are directed away from storage depots such as white adipose tissue toward mammary gland. Tissue assimilation of TAG depends on the activity of the rate-limiting enzyme lipoprotein lipase (LPL; EC 3.1.1.34); during lactation, mammary gland LPL activity is high, whereas adipose tissue LPL activity is decreased (15). This reciprocal pattern of LPL activity and hence tissue TAG utilization is rapidly reversed on removal of the litter ("weaning") when adipose TAG stores are repleted (22). The physiological basis for this mechanism is still uncertain, but plasma insulin concentrations and relative tissue insulin responsiveness are probably important: during lactation, white adipose tissue and skeletal muscle (34) have marked insulin resistance, whereas mammary gland is exquisitely insulin sensitive (33).

Lactation is an energetically expensive process, rapidly inhibited after food deprivation, and the hypertrophied mammary gland has a high tissue blood flow requiring increased cardiac output (27). The heart is capable of utilizing a wide variety of substrates, including carbohydrates (glucose, lactate, and pyruvate), lipids [TAG, nonesterified fatty acids (NEFAs), and ketone bodies], and to a lesser extent amino acids, and contains small intracardiomyocyte stores of glycogen and TAG, although probably only sufficient for several minutes of sustained contraction (26). Cardiac substrate preference is a function of nutritional and physiological state, with NEFAs considered to be the preferred energy source (supplying ~70% of ATP requirements) under normal conditions (18), but the regulation of fuel selection in this tissue is still uncertain; its principal energy source during lactation is unknown. However, there is evidence that, as might be expected, the oxidation of dietary lipid in vivo is decreased in lactation (22). Substrate supply and mechanical activity are major determinants, but in muscle and heart, the basis for LPL regulation is less clear than in adipose tissue. Starvation and fat feeding both increase heart LPL activity in vivo, whereas high-carbohydrate diets decrease enzyme activity (8), an effect not directly due to insulin (4), although experimental diabetes mellitus with hypertriglyceridemia is associated with low cardiac LPL activity (3).

The present study was designed to compare utilization of lipids (NEFA and TAG as both chylomicrons and VLDL) in hearts isolated from lactating and weaned rats with that in hearts from nonlactating (virgin) rats.

    MATERIALS AND METHODS
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Abstract
Introduction
Materials & Methods
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The investigation was performed in accordance with the Home Office Guidance on the Operation of the Animals (Scientific Procedures) Act 1986 published by Her Majesty's Stationery Office, London, UK.

Animals. Female Wistar rats (250-350 g) were fed ad libitum a chow diet [composed of by weight ~52% carbohydrate, 21% protein, and 4% fat; the residue was nondigestible material (Special Diet Services, Witham, Essex, UK)] unless otherwise stated. Animals had free access to drinking water and were maintained at an ambient temperature of 20 ± 2°C with a 12:12-h light-dark cycle (light from 0730). Three groups were studied: 1) nulliparous rats ("virgin"); 2) lactating rats at 10-12 days postpartum, with a litter of 10-12 pups (total litter mass 285-325 g) ("lactating"); and 3) rats at 12-14 days postpartum having had their litters (10-12 pups, 240-310 g total mass) removed 72-96 h before experimentation ("weaned").

Chemicals. [1-14C]palmitic acid, [9,10(n)-3H]oleic acid, and glycerol tri[9,10(n)-3H]oleate were obtained from Amersham International (Bucks, UK). Waymouth's medium was purchased from GIBCO BRL (Life Technologies, Paisley, UK); other biochemicals were obtained from Sigma Chemical (Poole, Dorset, UK).

Preparation of lipid substrates. [14C]sodium palmitate (specific activity 485 mCi/mmol) was prebound to fatty acid-free BSA (30) and added to the perfusate to give a final concentration of 1.1 mM (palmitate groups).

[3H]triolein in the form of rat chylomicrons was prepared from rats with the use of a thoracic duct cannulation technique essentially as described by Bezman-Tarcher et al. (2). Briefly, anesthetized rats had a polyethylene catheter inserted into the lower thoracic duct via an extraperitoneal loin incision and externalized to continuously collect chyle; a gastrostomy was also performed. The animals were maintained in a restraining cage for 12 h with free access to food and water but were given additional intravenous fluid replacement (tail vein). After this initial recovery period, [3H]triolein (1.0 g, 22 mCi) was administered into the stomach, and chyle was collected for the subsequent 12 h. 3H-labeled chylomicrons were isolated by washing with BSA solution and centrifugation. Thin-layer chromatography of the 3H-labeled chylomicrons showed that >95% of the label was in the TAG band. Chylomicrons were suspended in fatty acid-free BSA, and TAG content was assayed with an enzymatic colorimetric test kit (GmbH; Boehringer Mannheim, Lewes, Sussex, UK); 3H-labeled chylomicrons were added to the perfusate reservoir to give a final concentration of 0.4 mM TAG (chylomicron groups).

[3H]triolein in the form of rat VLDL was prepared by an extended rat liver perfusion technique under aseptic conditions. Fasted rats were anesthetized with intraperitoneal pentobarbital sodium (60 mg/kg body wt), and the portal vein and thoracic inferior vena cava were rapidly cannulated; the abdominal inferior vena cava was ligated. Heparin was not used. The liver was perfused in situ with a recirculating solution composed of Waymouth's synthetic tissue culture medium supplemented with amino acids (glutamine, serine, alanine) and glucose. Washed red cells were added to give a final hematocrit of 10% (vol/vol), and the perfusate was gassed with 95% O2-5% CO2 (vol/vol) at 37°C; [3H]oleate prebound to fatty acid-free albumin was infused into the perfusate to maintain the circulating NEFA concentration at ~0.4 mM. After the perfusion, the perfusate was filtered through an ultrafilter with molecular mass cutoff at 30 kDa (Amicon, Stonehouse, Gloucestershire, UK) and then ultracentrifuged at 144,500 g to separate the layer at which density < 1.006 g/ml. Thin-layer chromatography of the 3H-labeled VLDLs showed that >95% of the label was in the TAG band. VLDLs were suspended in fatty acid-free BSA, and TAG content was assayed with an enzymatic colorimetric test kit (see above); they were added to the perfusate to give a final concentration of 0.4 mM TAG.

Isolated perfused working heart preparation. All experiments were commenced between 1100 and 1200. Hearts from fed rats were perfused through the left atrium (anterograde) in "working" mode by the method of Taegtmeyer et al. (29). Rats were anesthetized with intraperitoneal pentobarbital sodium (60 mg/kg body wt). The heart was rapidly excised and briefly placed in ice-cold Krebs-Henseleit bicarbonate saline; it was then cannulated via the aorta (<2 min from excision) and perfused retrogradely through the coronary arteries in "Langendorff" mode while lung, mediastinal, and pericardiac brown adipose tissues were excised, right pulmonary arteriotomy was performed, and the left atrium was separately cannulated, after which point the apparatus was switched to working mode and cardiac perfusion was maintained through the left atrium. A recirculating Krebs-Henseleit bicarbonate buffer solution containing 1.3 mM CaCl2, 10 mM glucose, and 2.5% fatty acid-free BSA (wt/vol) was filtered through a 5-µm cellulose nitrate filter (Millipore, Bedford, MA) and gassed with 95% O2-5% CO2 at 37°C. The first 50 ml of coronary effluent were discarded to free the circuit of blood cells. Afterload was maintained at 100 cmH2O and preload (atrial filling pressure) at 15 cmH2O. After an initial 15-min stabilization period, lipid substrate was added slowly (2 min) to the reservoir ("time 0"). Peak systolic pressure and heart rate were measured by a calibrated pressure transducer (Druck, Groby, Leicestershire, UK) connected to a side arm of the aortic cannula. Aortic flow rate was measured by a timed collection of perfusate ejected through the aortic line, and coronary flow rate was measured by a timed collection of perfusate effluent dripping from the heart. Measurements were made at time 0 and at 10-min intervals for 60 min. Cardiac output was calculated as coronary flow rate plus aortic flow rate. Rate-pressure product was calculated as heart rate times peak systolic pressure. Hydraulic work was calculated as cardiac output times mean aortic pressure divided by heart weight. After the final measurements, the heart was rapidly excised, freeze-clamped in light alloy tongs cooled in liquid nitrogen, and weighed.

Measurement of lipid oxidation rate. NEFA oxidation rate was measured by collection of 14CO2 and H14CO-3 from [14C]palmitate as described (9, 26). Each item of the perfusion apparatus was made gas tight and interconnected in series with tubing; the O2-CO2 gas mixture was passed through each part and collected in a bubble trap containing Lumasorb (May & Baker, Dagenham, Essex, UK). At 10-min intervals, the Lumasorb was counted for radioactivity and replaced. An aliquot (1.0 ml) of the perfusate was also taken by a gas-tight glass syringe without atmospheric exposure and placed in the central well of a sealed Erlenmeyer flask containing 9 N sulfuric acid; the outer well contained Lumasorb. Flasks were agitated for 2 h, the Lumasorb was removed, and radioactivity was counted. [14C]palmitate oxidation was calculated from the sum of the 14CO2 absorbed in the flasks and bubble traps.

TAG oxidation rate was estimated by measuring 3H2O production in the perfusate from [3H]triolein as described (9, 26). Apparatus was identical to that above; [3H]triolein (as chylomicrons or VLDLs) was added, and, at similar intervals (10 min), aliquots of perfusate (1.0 ml) were removed and subjected to Folch lipid extraction with chloroform-methanol (2:1, vol/vol) and water (31). An aliquot of the water phase was removed and counted for radioactivity.

The two methods (based on 14C and 3H) have previously been shown to give good agreement (9). Oleate was selected in preference to palmitate for TAG synthesis because of its relative ease of solubilization; palmitate and oleate have similar metabolic handling in both humans in vivo and rodents in vitro (5, 14).

TAG utilization rate (disappearance from the perfusate) was measured by assay of TAG in the organic infranatant phase of the Folch extracts of the timed perfusate aliquots after evaporation of the chloroform and resolubilization with ethanol, using an enzymatic colorimetric assay test kit (see Preparation of lipid substrates).

Incorporation of exogenous lipid into myocardial lipid. Myocardial 14C- or 3H-labeled lipid content was estimated by grinding frozen myocardium to powder under liquid nitrogen and extracting the lipids from an aliquot with chloroform-methanol (Folch). After repeated washing, the lipids were resolubilized in chloroform and separated by thin-layer chromatography using a hexane-diethyl ether-acetic acid system (13). 14C or 3H radioactivity was measured in the various lipid bands.

LPL activity. Myocardial total LPL activity was estimated in duplicate in acetone/ether-dried tissue powders ground from the working hearts frozen in liquid nitrogen after lipid perfusion with the use of a [3H]triolein substrate emulsion containing starved rat serum as a source of apolipoprotein C-II (apo C-II) to maximize LPL detection (21); the serum was pretreated by being heated to 65°C to inactivate nonspecific plasma lipases. Radioactivity in evolved fatty acids was counted after extraction in methanol-chloroform-heptane (20). Results are expressed as nanomoles of fatty acid released per minute per milligram of acetone-dried powder.

In separate experiments, cardiac LPL activity in lactation was also examined in ex vivo hearts: virgin, lactating, and weaned rat groups were either 1) fed chow ad libitum, 2) starved for 24 h before experimentation, or 3) starved for 24 h and then refed chow for 5 h before experimentation. Hearts were excised and perfused retrogradely in Langendorff mode (as in Isolated perfused working heart preparation, but without atrial cannulation) with glucose- and calcium-supplemented Krebs-Henseleit solution for 8 min. After the second minute of perfusion, 5 IU/ml of heparin (Leo Laboratories, Princes Risborough, Bucks, UK) were added to the perfusate (17); perfusate was collected before and after heparin addition, and at 8 min of perfusion, the heart was freeze-clamped in liquid nitrogen (as in Isolated perfused working heart preparation). LPL activity ("heparin releasable") was determined immediately on the perfusate samples (pre- and postheparin) as above, but perfusate was directly added to the LPL substrate incubation medium (expressed as nmol fatty acid released · min-1 · total perfusate-1); frozen heart was ground to powder and assayed for LPL activity ("residual," expressed as nmol fatty acid released · min-1 · heart-1) as above. "Total" LPL activity in these experiments was heparin-releasable + residual LPL activity.

Statistics. Results are expressed as mean values ± SE. Statistical analysis was performed by one-way ANOVA for repeated measurements and Tukey's test or by Student's t-test with Bonferroni correction for multiple comparisons where appropriate. Statistical significance was set at P < 0.05.

    RESULTS
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Materials & Methods
Results
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References

Lactation was associated with a moderate but significant increase in heart wet weight (~20%; Table 1) (6); for this reason, lipid oxidation data were expressed both per gram wet weight and per total heart. Under conditions of moderate workload (29), hearts from chow-fed virgin rats oxidized NEFA (palmitate) at a greater rate than triolein, and VLDL TAG was oxidized at almost twice the rate of chylomicron TAG (Table 1). This effect was associated with a significantly greater total tissue LPL activity in VLDL-perfused hearts than in chylomicron-perfused hearts (Table 2). Hearts from lactating rats oxidized palmitate at a significantly greater rate than the corresponding hearts from virgin animals, but TAG oxidation either as chylomicrons or VLDLs was markedly impaired; after the litter was weaned, the palmitate oxidation rate decreased, and TAG oxidation rate increased to values observed in hearts from virgin rats (Table 1). Concomitant with the increased NEFA and decreased TAG oxidation seen during lactation was the proportionately decreased total tissue LPL activity in working hearts from lactating rats (Table 2). Furthermore, myocardial TAG utilization rate was significantly decreased during lactation but returned toward virgin values on weaning of the litter (Table 1).

                              
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Table 1.   Lipid oxidation and TAG utilization rates in perfused working hearts from fed virgin, lactating, and weaned rats

                              
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Table 2.   LPL activity in perfused working hearts from fed virgin, lactating, and weaned rats

Myocardial LPL activity was further investigated in hearts from lactating rats in a variety of nutritional states by brief perfusion ex vivo with heparin (Fig. 1). In the fed state, lactation was associated with decreased heparin-releasable (endothelial bound) myocardial LPL activity; weaning restored heparin-releasable LPL activity to values seen in hearts from virgin rats (Fig. 1A). Food withdrawal for 24 h increased heart LPL activity in virgin, lactating, and weaned rats, an effect mainly due to an increase in the heparin-releasable fraction (>75% of total tissue activity in all groups; Fig. 1B). Refeeding chow to 24 h-starved animals (Fig. 1C) decreased LPL activity to values observed in corresponding fed groups (Fig. 1A), with significantly decreased heparin-releasable activity in lactation compared with nonlactating states.


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Fig. 1.   Lipoprotein lipase (LPL) activity in hearts from fed, starved, and refed virgin, lactating, and weaned rats. Hearts were excised and briefly (8 min) perfused through aorta (retrograde) with heparin; LPL activity was measured in perfusate (heparin releasable) and frozen heart tissue (residual); total activity = heparin-releasable + residual activity. Numerical values within bars indicate proportion of total LPL activity that is heparin releasable (as % of total). A: fed chow ad libitum. B: 24 h starved. C: 24 h starved, 5 h refed chow. FA, fatty acid. Results are means ± SE for n = 5-9 experiments. * Significant differences between groups, P < 0.05.

Working hearts perfused with palmitate generally had greater incorporation of exogenous lipid into all myocardial tissue lipid classes examined compared with triolein-perfused hearts, with most of the nonoxidized, assimilated label appearing as tissue TAG and a minority esterified as diacylglycerol and cholesterol ester; however, incorporation of both exogenous palmitate and triolein was significantly decreased during lactation, with some recovery on weaning (Table 3). In hearts from virgin rats, perfusion with chylomicron TAG caused optimal cardiac mechanical performance (cardiac output, hydraulic work); rate-pressure product, an index of cardiac work commonly used in vivo, was unchanged between groups in this in vitro model. By contrast, hearts from lactating animals showed significantly increased hydraulic work when perfused with palmitate, these hearts having decreased cardiac output and hydraulic work with chylomicrons or VLDLs as sole lipid source, compared with values in hearts from virgin rats (Fig. 2). On weaning, chylomicron TAG again became the preferred substrate in terms of cardiac mechanical function (Fig. 2).

                              
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Table 3.   Myocardial tissue lipid incorporation in perfused working hearts from fed virgin, lactating, and weaned rats


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Fig. 2.   Cardiac mechanical function in perfused working hearts from fed virgin, lactating, and weaned rats. For details see text. Results are means ± SE for 60 min of perfusion (n = 5-7 experiments). TAG, triacylglycerol; VLDL, very low density lipoprotein. Significant differences between virgin and lactating groups: * P < 0.05, ** P < 0.01, and *** P < 0.001. Significant differences between lactating and weaned groups: # P < 0.05 and ## P < 0.01.

    DISCUSSION
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Although a physiological process, lactation in iteroparous mammals incurs a substantial drainage of substrates from the mother to the young: in the rat, lactation lasts for 21 days, during which time sufficient nutrients pass through the mammary gland to increase the mass of the litter by ~400 g, more than the weight of the lactating dam. Strategies are therefore required to ensure maintenance of an adequate nutrient supply to the lactating mammary gland as well as to protect the maternal body in the event of inadequate substrate availability. Hyperphagia occurs, providing increased dietary substrate supply for lactogenesis; exogenous lipids are carried as TAGs in chylomicrons, and exogenous glucose can be used by the liver to synthesize fatty acids (lipogenesis) and hence VLDL TAG. In the event of food limitation (starvation), lactation is rapidly inhibited to protect the lactating mother from substrate depletion (23). Various mechanisms also occur to direct circulating substrate away from nonessential maternal tissues, notably white adipose tissue, which is prevented from assimilating circulating TAG by inhibition of its LPL activity, whereas mammary gland LPL activity is high to permit TAG uptake by this tissue (10, 15). The heart in nonlactating animals is known to possess significant LPL activity and utilizes TAG (19, 28); however, the regulation of cardiac LPL is not as fully understood as that of the adipose tissue or even the mammary gland enzyme. Although cardiac LPL activity does vary with certain physiological and pathological states, changes observed are modest compared with the profound alteration observed in the adipose and mammary activity during lactation and after litter weaning, and it has proved difficult to define a metabolic rationale for such changes seen in the heart LPL activity or to implicate a particular endocrine, paracrine, metabolic, or mechanical mechanism for the change (8). Furthermore, although the heart has been shown to utilize chylomicrons (obtained radiolabeled from thoracic duct cannulas and with or without serum supplementation to provide the apo C-II in which such particles are relatively deficient and which is required for LPL activation), it has previously been difficult to isolate and radiolabel VLDLs in sufficient quantities to test in a perfused heart system. The heart is certainly an "essential" tissue in lactation; indeed, cardiac work is increased as the mammary gland undergoes hypertrophy with greatly increased tissue blood flow (27), but cardiac substrate utilization at this time is unknown. This is the first report to examine this aspect of intermediary metabolism in this state.

Hearts were perfused with lipid substrates at concentrations chosen to approximate physiological plasma concentrations in vivo [plasma NEFA concentration varies from ~0.1 mM (fed state) to 1.8 mM (starvation, exercise), and plasma TAG concentration is typically 0.3-1.3 mM, again depending on nutritional state]. Hearts preferentially utilized NEFA compared with TAG in all groups studied. Even with correction for the difference in expression of fatty acid and TAG oxidation rates (i.e., multiplying the latter by 3), palmitate was oxidized at over twice the rate of triolein in virgin and weaned rats, and this effect was accompanied by increased tissue labeled lipid accumulation. This finding is in good agreement with previously published data (32) and indicates facilitated NEFA access to the cardiomyocyte compared with the complex and highly regulated mechanism of TAG access involving initial endothelial LPL hydrolysis. VLDL TAG was shown for the first time to be assimilated by the heart at a similar rate to chylomicrons and was oxidized readily at a rate of only one-half that of palmitate but significantly greater than that of chylomicron TAG (P < 0.05; Table 1). Interestingly, tissue LPL activity was high in VLDL-perfused hearts; a relationship between exposure of myocardial tissue [to both NEFA (1, 24) and TAG (12, 25)] and cardiac LPL activity has been previously reported, and it now seems likely that this is another example of substrate regulation of metabolic pathway activity independent of hormone control. However, some caution is needed in the interpretation of this data, since the chylomicrons used in the current study were not supplemented with fasted rat serum (and hence were relatively apo C-II deficient) to keep the perfusate medium strictly defined (32); despite this, chylomicron-perfused hearts from virgin rats had significantly greater cardiac output and hydraulic work compared with palmitate- and VLDL-perfused hearts, and chylomicron TAG was a better substrate for myocardial TAG synthesis than VLDL TAG (P < 0.05; Table 3). This surprising finding was not explored further but may be related to the apparently different metabolic handling of VLDL TAG and chylomicron TAG whereby fatty acids from the former tend to be oxidized and those from the latter tend to be esterified. The data imply that chylomicrons, even in the absence of apo C-II, are more "efficiently" utilized than VLDL, an effect possibly related to obligatory intracellular TAG cycling (26). Glucose was present as a cosubstrate, although the rate of glucose oxidation was not measured in the present study.

In hearts from rats at midlactation, a clear shift in lipid utilization has been demonstrated. Palmitate oxidation increased, whereas less lipid was incorporated into tissue lipid stores; TAG utilization, however, was markedly impaired, regardless of expression per gram wet weight or per heart to correct for cardiac hypertrophy. Calculation of expression of TAG utilization as fatty acid utilization indicates ~50-fold greater NEFA oxidation rate compared with TAG-fatty acid oxidation during lactation in contrast to the two- to threefold greater NEFA oxidation rate compared with TAG-fatty acid oxidation rate in hearts from virgin rats on a gram wet weight basis. This finding of increased myocardial capacity for palmitate oxidation is supported by recent evidence of significantly increased plasma NEFA levels during lactation in the fed rat, indicating increased NEFA substrate availability in this state, with a rapid (24 h) decrease in circulating NEFA after litter weaning (7). Cardiac LPL activity was decreased in the presence of all substrates during lactation (although the ability of VLDLs to increase cardiac LPL activity was retained), supporting previous suggestions of such an effect (11), and hence both oxidation and tissue incorporation of TAG from both chylomicrons and VLDLs were impaired. As a result, TAG was not able to maintain optimum cardiac performance in contrast to palmitate. Confirmation of lactation-induced decrease in cardiac LPL activity was provided by ex vivo experiments; the heparin-releasable enzyme fraction (the endothelial-bound and hence physiologically active portion) was decreased in lactation, but rapid restoration of enzyme activity (principally heparin-releasable fraction) occurred after inhibition of lactation by either litter weaning or starvation. The rapid decrease in the activity of heart LPL in lactating rats on refeeding may be associated with the restoration of the activity of mammary gland LPL after 2 h of chow refeeding (23).

This is the first demonstration of changes in myocardial lipid metabolism occurring in vitro during lactation, and the results are consistent with the finding of redirection of TAG toward the mammary gland and away from other tissues. The surprising finding is that the heart, which continues to function well with NEFA during lactation, is able to inhibit TAG utilization to such a profound extent, greater than in any previously reported physiological metabolic perturbation.

These results also confirm that, under nonlactating (normal, nulliparous) conditions, chylomicron TAG is an efficient substrate for the heart. By contrast, VLDL TAG is readily utilized, but associated mechanical function is less than in chylomicron-perfused hearts and is comparable with that seen during perfusion with NEFA.

Hearts were studied at 72-96 h after the litter was weaned, by which time the changes in cardiac lipid metabolism had reverted to prelactation status; the reciprocal changes in lipid metabolism observed in adipose tissue and mammary gland revert within 24 h (23), and the time course observed here lends support to the concept of cardiac TAG metabolism being central to the maternal strategy of substrate redirection and energy conservation. Cardiac lipid metabolism in the late partum period and 24 h postweaning was not examined but deserves study.

The mechanism of the regulation of cardiac LPL, both physiological and pathological, has proved to be difficult to elucidate, but the magnitude of the effect seen in the present study suggests an endocrine mechanism; the heart is subjected to increased workload during lactation, yet LPL activity is low, suggesting that local paracrine or mechanical mechanisms are not involved. Candidate endocrine hormones include insulin, prolactin, and/or thyroxine; the increased prolactin level of lactation is associated with a functional hypothyroid state, indicating a reciprocal relationship between prolactin and thyroxine at this time (7). The effect of prolactin on cardiac lipid metabolism is unknown, but experimental hypothyroidism in the rat is associated with increased cardiac LPL activity (16). The roles of these hormones in the changes observed in cardiac lipid metabolism during lactation and weaning remain to be fully elucidated.

    ACKNOWLEDGEMENTS

We are grateful to Dr. Dermot Williamson and Reg Hems, Metabolic Research Laboratory, Univ. of Oxford, for helpful discussions and technical assistance.

    FOOTNOTES

We thank the Wellcome Trust and the British Journal of Anaesthesia for financial support.

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Address for reprint requests: R. D. Evans, Nuffield Dept. of Anaesthetics, Univ. of Oxford, Radcliffe Infirmary, Woodstock Road, Oxford, Oxfordshire OX2 6HE, UK.

Received 13 January 1998; accepted in final form 29 April 1998.

    REFERENCES
Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References

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Am J Physiol Endocrinol Metab 275(2):E265-E271
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