Cardiac Metabolism and Contractile Function in Mice with Reduced Trans-Endothelial Fatty Acid Transport

Abstract

The heart is a metabolic omnivore that combusts a considerable amount of energy substrates, mainly long-chain fatty acids (FAs) and others such as glucose, lactate, ketone bodies, and amino acids. There is emerging evidence that muscle-type continuous capillaries comprise the rate-limiting barrier that regulates FA uptake into cardiomyocytes. The transport of FAs across the capillary endothelium is composed of three major steps-the lipolysis of triglyceride on the luminal side of the endothelium, FA uptake by the plasma membrane, and intracellular FA transport by cytosolic proteins. In the heart, impaired trans-endothelial FA (TEFA) transport causes reduced FA uptake, with a compensatory increase in glucose use. In most cases, mice with reduced FA uptake exhibit preserved cardiac function under unstressed conditions. When the workload is increased, however, the total energy supply relative to its demand (estimated with pool size in the tricarboxylic acid (TCA) cycle) is significantly diminished, resulting in contractile dysfunction. The supplementation of alternative fuels, such as medium-chain FAs and ketone bodies, at least partially restores contractile dysfunction, indicating that energy insufficiency due to reduced FA supply is the predominant cause of cardiac dysfunction. Based on recent in vivo findings, this review provides the following information related to TEFA transport: (1) the mechanisms of FA uptake by the heart, including TEFA transport; (2) the molecular mechanisms underlying the induction of genes associated with TEFA transport; (3) in vivo cardiac metabolism and contractile function in mice with reduced TEFA transport under unstressed conditions; and (4) in vivo contractile dysfunction in mice with reduced TEFA transport under diseased conditions, including an increased afterload and streptozotocin-induced diabetes.

Keywords: TCA cycle; capillary endothelium; cardiac metabolism; contractile function; fatty acid; glucose; pool size; trans-endothelial fatty acid transport.

Figure 1

Mechanisms of fatty acid uptake by the heart. (1) Lipolysis of TG contained in TGRLPs on the luminal side of the capillary endothelium; (2) FA uptake by the plasma membrane of the capillary endothelium; (3) intracellular FA transport through the capillary endothelium; (4) FA uptake by cardiomyocytes.

Figure 2

Catabolic pathways were suppressed and anabolic pathways were enhanced in pressure-overloaded hearts in mice with reduced FA uptake and compensatory glucose use. NA, nucleic acids; AA, amino acids; PPP, pentose phosphate pathway.

Figure 3

Putative bar graph regarding the pool size of the TCA cycle associated with the difference between energy supply (ES) and energy expenditure (EE). AU, arbitrary unit; WT, wild type; TAC, transverse aortic constriction; MCFA, medium-chain FA. + basal, ++ increased, → basal, ↓ reduced, ➘ mildly reduced.