Fuel Utilization and Lipid Transport in Cardiac Muscle

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Fuel Utilization in Cardiac Muscle

Overview of Cardiac Energy Metabolism

The cardiac muscle requires a continuous and high supply of ATP due to its constant activity and minimal energy storage. The heart primarily relies on the oxidative metabolism of fatty acids, but the fuel source can shift depending on physiological or pathological conditions.

Continuous, high ATP demand with minimal energy storage.
Oxidative metabolism of fatty acids is the main energy source.
Fuel sources shift in response to physiological (rest, exercise) and pathological (ischemia) states.

Primary Fuels Used by the Heart (Normal Conditions)

Fatty acids: Provide 60–80% of ATP at rest.
Glucose/lactate: Contribute 20–30% of ATP at rest.
Ketone bodies: Minor contributors under normal conditions, but their use increases during fasting or diabetes.
High mitochondrial density in cardiac muscle supports sustained oxidative phosphorylation.

Fatty Acid Oxidation in Cardiac Muscle

Transport and Oxidation Pathway

Fatty acids enter cardiomyocytes via FAT/CD36 and FABPs (fatty acid binding proteins).
Transport into mitochondria is mediated by the carnitine shuttle (CPT I and CPT II).
β-oxidation: Fatty acids are converted to acetyl-CoA, which enters the TCA cycle, then electrons are transferred to the electron transport chain (ETC) to generate ATP.
Oxygen requirement: Fatty acid oxidation produces more ATP per molecule but requires more oxygen per ATP than glucose oxidation.

Ketone Body Utilization

Role and Metabolism

Ketone bodies (β-hydroxybutyrate, acetoacetate) enter cardiomyocytes freely.
Converted to acetyl-CoA in mitochondria for entry into the TCA cycle.
Utilized during fasting, prolonged exercise, diabetes, and heart failure.
Efficient fuel source when glucose is limited.

Cardiac Fuel Use in Ischemia

Metabolic Shifts During Ischemia

Ischemia: Reduced blood flow leads to insufficient oxygen supply.
Oxygen limitation inhibits fatty acid oxidation.
Shift toward glucose utilization (glycolysis), which can proceed anaerobically.
Results in lower ATP yield, increased lactate, and intracellular acidosis.
Impaired calcium handling and contractility.

Lipolysis During Fasting and Exercise

Mobilization of Stored Triacylglycerols

Stored triacylglycerols in adipocytes are mobilized during fasting and exercise.
Key enzymes: Adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL).
Ester linkages in triacylglycerols are hydrolyzed to release free fatty acids and glycerol.

Hormonal Regulation of Lipolysis

Glucagon (fasting): Increases cAMP, activates PKA, which phosphorylates and activates HSL.
Epinephrine (fasting & exercise): β-adrenergic receptor activation increases cAMP, activates PKA, and stimulates HSL.
Net effect: Release of free fatty acids and glycerol into the bloodstream.

Fate of Lipolysis Products

Fatty acids: Bind to albumin in blood, delivered to muscle (including heart) and liver for oxidation.
Glycerol: Transported to the liver for gluconeogenesis or glycolysis.

Lipoprotein Structure and Function

Lipoprotein Structure

Surface: Composed of phospholipids, free cholesterol, and apoproteins (e.g., Apo E, Apo CII, Apo B100).
Amphipathic design enables lipid transport in plasma.

Lipid Delivery in the Fed State

Dietary (exogenous) lipids are packaged into chylomicrons (first enter lymphatic circulation).
Endogenous lipids are packaged into VLDLs (very low-density lipoproteins).
Both primarily carry triacylglycerides to tissues.

Classes of Lipoproteins

Chylomicrons

Function: Transport dietary (exogenous) triacylglycerols (TG).
Apo B-48 is unique to chylomicrons.
Nascent chylomicrons acquire apo C-II and apo E in circulation to become mature.
Lipoprotein lipase (LPL) releases fatty acids from chylomicrons for tissue uptake.
Primary apolipoproteins: Apo B-48, Apo C-II, Apo E.

VLDL (Very Low-Density Lipoprotein)

Synthesized in the liver.
Function: Transport endogenous triacylglycerol (TG) synthesized from excess carbohydrate.
LPL releases fatty acids to tissues.
VLDL is converted to IDL and then LDL.
Primary apolipoproteins: Apo B100, Apo E, Apo C-II.

IDL (Intermediate-Density Lipoprotein)

Transitional lipoprotein formed during VLDL metabolism.
Intermediate in density and size between VLDL and LDL.
Primary apolipoproteins: Apo B-100, Apo E.
Two fates: hepatic uptake (via apo E and B100) or further metabolism to LDL.

LDL (Low-Density Lipoprotein)

Derived from IDL after loss of triacylglycerol.
Primary function: Transport cholesterol to peripheral tissues.
Uptake by cells via LDL receptors (binds apo B-100).
Cholesterol used for membrane synthesis, steroid hormone synthesis, and bile acid synthesis.

Regulation of LDL Receptor Expression

Low intracellular cholesterol: SCAP escorts SREBP from ER to Golgi, where it is cleaved to release an active transcription factor.
The transcription factor binds SRE, increasing LDL receptor gene expression and LDL uptake, lowering plasma LDL cholesterol.

Oxidized LDL and Atherosclerosis

Oxidation of LDL alters Apo B-100, preventing receptor-mediated uptake.
Oxidized LDL is taken up by macrophage scavenger receptors, leading to foam cell formation and atherosclerotic plaque development.
Persistent elevation of plasma LDL cholesterol increases cardiovascular risk.

Endothelial Dysfunction and Foam Cell Formation

Endothelial dysfunction increases permeability, triggered by oxidized LDL, smoking, hyperglycemia, hypertension, shear stress, inflammation, and ROS.
Macrophages ingest oxidized LDL via scavenger receptors (not downregulated by cholesterol), accumulate cholesterol esters, and become foam cells.
Foam cells secrete inflammatory cytokines and form fatty streaks, the earliest atherosclerotic lesions.

HDL (High-Density Lipoprotein)

Role in Cholesterol Transport

Primary role: Reverse cholesterol transport from tissues to liver.
Considered "protective" against atherosclerosis.
Synthesized in liver and intestine.
Primary apolipoprotein: Apo A-I, which activates LCAT (lecithin:cholesterol acyltransferase).

LCAT and Cholesterol Esterification

LCAT converts free cholesterol into cholesteryl esters, which move into the hydrophobic core of HDL.
Prevents cholesterol from diffusing back out of HDL.
Promotes efficient reverse cholesterol transport to the liver, reducing foam cell formation and atherosclerosis risk.

Summary Table: Major Lipoproteins

Lipoprotein	Main Function	Primary Apolipoproteins	Origin
Chylomicron	Transport dietary TG	Apo B-48, Apo C-II, Apo E	Intestine
VLDL	Transport endogenous TG	Apo B-100, Apo C-II, Apo E	Liver
IDL	Transitional (VLDL to LDL)	Apo B-100, Apo E	Circulation
LDL	Transport cholesterol	Apo B-100	Circulation (from VLDL/IDL)
HDL	Reverse cholesterol transport	Apo A-I	Liver, Intestine

Additional info: This summary integrates key biochemistry concepts on lipid metabolism, fuel utilization, and lipoprotein function relevant to cardiovascular health and disease.