Regulation of Hepatic Metabolism : Intra- and Intercellular Compartmentation

The liver is an exceptionally complex and diverse organ that functions both as an exocrine and an endocrine gland. It secretes bile, which contains many con­ stituents in addition to bile salts, and it synthesizes and releases many substances in response to the body's demands, including prohormones, albumin, clotting factors, glucose, fatty acids, and various lipoproteins. It has a dual blood supply providing a rich mixture of nutrients and other absorbed substances via the portal vein and oxygen-rich blood via the hepatic artery. This functional heterogeneity is accompanied by cellular heterogeneity. The liver contains many cell types including hepatic parachymal cells, Kiipffer cells, Ito cells, and endothelial cells. The most abundant cell type, the parenchymal cells, are biochemically and structurally heterogeneous. The cells in the oxygen-rich areas of the portal triad appear more dependent on oxidative metabolism, whereas those around the central vein (pericentral, perivenous, or centrolobular areas) are more dependent upon an anaerobic mechanism. Throughout this volume the latter three terms are used synonymously by various authors to indicate the five to eight layers of cells radiating from the central vein. Structural and metabolic heterogeneity of hepatic parenchymal cells has been demonstrated by a variety of approaches, including histochemical, ultra­ structural, and ultramicrobiochemical studies. This microheterogeneity is linked to the physiological functions of the liver and its response to injurious substances.

I. Liver Structure.- 1 Liver Structure and Innervation.- 1. Introduction.- 2. Liver Structure.- 2.1. Development.- 2.2. General Histology.- 2.3. Ploidy.- 2.4. Ultrastructure.- 2.5. Sinusodal Cells.- 2.6. Architecture of Liver Parenchyma.- 3. Innervation of the Liver.- 3.1. Macroscopic Aspects.- 3.2. Microscopic Aspects.- 3.3. Electron-Microscopic Aspects.- 4. Summary.- References.- 2 Hepatic Hemodynamics and Microcirculation.- 1. Introduction.- 2. Hepatic Vascular Resistance: Control Values.- 3. Techniques.- 4. Hepatic Arterial Resistance.- 4.1. Intrinsic Regulation.- 4.2. BloodGases.- 4.3. Osmolarity.- 4.4. Nervous Control.- 4.5. Gastrointestinal and Pancreatic Hormones.- 4.6. Catecholamines and Related Drugs.- 4.7. Autacoids and Systemic Peptide Hormones.- 4.8. Summary.- 5. Portal Venous Resistance.- 5.1. Intrinsic Regulation.- 5.2. BloodGases.- 5.3. Osmolarity.- 5.4. Nervous Control.- 5.5. Gastrointestinal and Pancreatic Hormones.- 5.6. Catecholamines and Related Drugs.- 5.7. Autacoids and Systemic Peptide Hormones.- 5.8. Summary.- 6. Interrelationship Between the Hepatic Artery and the Portal Vein.- 6.1. Mutual Pressure-Flow Curves.- 6.2. “Transhepatic” Drug Effects.- 7. Capacitance and Exchange.- 7.1. Regulation of Liver Volume.- 7.2. Exchange in the Hepatic Microcirculation.- 8. Relationship of Liver Circulation to Liver Function.- 8.1. Postprandial Hyperemia.- 8.2. Metabolic Stimulation.- 9. Summary.- References.- II. Methods.- 3 Histology and Histochemistry.- 1. Introduction.- 2. Histology.- 2.1. Tissue Pretreatment.- 2.2. Routine Staining Methods.- 2.3. Special Staining Methods.- 3. Histochemistry.- 3.1. Substances.- 3.2. Enzymes.- 4. Summary.- References.- 4 Immunohistochemistry.- 1. Introduction.- 2. Tissue Preparation.- 2.1. General Considerations.- 2.2. Fixation Procedures.- 2.3. Embedding Procedures.- 2.4. Unfixed, Frozen Sections.- 3. Immunoenzymatic Techniques.- 3.1. General Considerations.- 3.2. Direct and Indirect Peroxidase Methods.- 3.3. Unlabeled Antibody Peroxidase-Antiperoxidase Method.- 3.4. Avidin-Biotin-Peroxidase Method.- 4. Immunofluorescence Techniques.- 4.1. General Considerations.- 4.2. Direct Immunofluorescence.- 4.3. Indirect Immunofluorescence.- 5. Dual-Labeling Techniques.- 5.1. General Considerations.- 5.2. Dual Immunoenzymatic Labeling Methods.- 5.3. Dual Immunofluorescence Labeling Methods.- 5.4. Combined Immunoenzymatic and Immunofluorescence Labeling Methods.- 6. Quantitative Immunohistochemical Techniques.- 6.1. General Considerations.- 6.2. Microdensitometry.- 6.3. Microfluorometry.- 7. Summary.- References.- 5 Quantitative Histochemical Measurements within Sublobular Zones of the Liver Lobule.- 1. Introduction.- 2. Preparation of Tissue Samples for Analyses.- 2.1. Sampling and Sectioning of Tissues.- 2.2. Drying and Storage of Samples.- 2.3. Identification of Sublobular Zones and Histological Control.- 2.4. Dissection and Weighing of Samples.- 3. Analytical Procedures.- 3.1. General Considerations.- 3.2. Examples of Quantitative Histochemical Measurements in Liver.- 4. Advantages and Limitations of the Quantitative Histochemical Approach.- References.- 6 Separation of Functionally Different Liver Cell Types.- 1. Introduction.- 2. Dispersion of Liver Tissue into Isolated Cells.- 2.1. Mechanical and Chemical Methods.- 2.2. Enzymatic Methods.- 3. Separation of Hepatocytes and Nonhepatocytes.- 3.1. Centrifugation Methods.- 3.2. Enzymatic Methods.- 4. Separation of Various Types of Nonhepatocytes.- 4.1. Kupffer Cells and Endothelial Cells.- 4.2. Fat-Storing Cells and Pit Cells.- 4.3. Cells from the Vascular Trees.- 5. Separation of Periportal and Perivenous Hepatocytes.- 5.1. Background.- 5.2. Separation Based on Physicochemical Differences.- 5.3. Separation of Cells from Either Subacinar Zone.- 6. Conclusions.- 7. Summary.- References.- 7 New Micromethods for Studying Sublobular Structure and Function in the Isolated, Perfused Rat Liver.- 1. Introduction.- 1.1. Criteria for Direct, Dynamic Measurements of Sublobular Events with Miniature Probes.- 2. Miniature Probes.- 2.1. Micro-Light Guide.- 2.2. Miniature Oxygen Electrode.- 3. Localization of Sublobular Regions on the Surface of Perfused Liver.- 3.1. Models of Liver Lobular Structure.- 3.2. India Ink Injections and Vascular Casts.- 3.3. Microcirculation at the Liver Surface.- 3.4. Hepatic Lobular Continuum.- 4. Lobular Oxygen Gradients.- 4.1. Surface Measurements of NADH Fluorescence.- 4.2. Surface Oxygen Tension.- 4.3. Precautions and Other Considerations.- 5. Other Applications of Miniature Probes.- 5.1. NAD + -linked Cellular Metabolism.- 5.2. Mixed-Function Oxidations.- 6. Assessment of Liver Injury After Toxic or Metabolic Insult.- 6.1. Light and Electron Microscopy.- 6.2. Enzyme Release.- 7. Conclusion.- References.- 8 Redox Scanning in the Study of Metabolic Zonation of Liver.- 1. Introduction.- 2. Indicators of the Intracellular Redox State of Pyridine Nucleotides.- 2.1. Indicator Metabolite Method.- 2.2. Tissue Fluorescence.- 2.3. Different Pyridine Nucleotide Pools.- 2.4. Flavoprotein/Pyridine Nucleotide Ratio Method.- 3. Low-Temperature Scanning Technique.- 3.1. General Description.- 3.2. Details of the Instrument.- 4. Spatial Resolution of the Instrument.- 5. Freeze-Quenching of Tissue.- 6. Examples of Scanning Results.- 6.1. Histogram Distribution of Redox States.- 6.2. Two- and Three-Dimensional Redox-Ratio Images.- 7. Conclusions.- 8. Summary.- References.- III. Distribution of Metabolic Functions.- 9 Metabolism of Carbohydrates.- 1. Introduction.- 2. Role of the Liver in the Metabolism of Carbohydrates: Interorgan Relationships.- 2.1. Glucose Uptake.- 2.2. Glucose Release.- 3. Role of Substrates, Hormones, and Nerves in the Regulation of Carbohydrate Metabolism.- 3.1. Glycogen Metabolism.- 3.2. Glycolysis and Gluconeogenesis.- 4. Role of Zonal Hepatocyte Heterogeneity in the Regulation of Carbohydrate Metabolism.- 4.1. Zonal Distribution of Enzymes.- 4.2. Zonal Heterogeneity of Signals and Signal Transmission.- 4.3. Functional Significance of Zonal Heterogeneity: Metabolic Zonation.- 4.4. Comparison of Non-zonation and Zonation Models for the Regulation of Carbohydrate Metabolism.- 5. Summary.- References.- 10 Metabolism of Lipids.- 1. Introduction.- 2. Role of the Liver in the Metabolism of Lipids: Interorgan Relationships.- 2.1. Fatty Acid Synthesis.- 2.2. Lipolysis and ?-Oxidation.- 2.3. Ketogenesis.- 2.4. Esterification.- 2.5. Lipoprotein Formation and Processing.- 3. Role of Metabolites and Hormones in the Regulation of Lipid Metabolism.- 3.1. Liponeogenesis.- 3.2. Lipolysis and ?-Oxidation.- 3.3. Ketogenesis.- 3.4. Esterification.- 3.5. Lipoprotein Metabolism.- 4. Zonal Hepatocyte Heterogeneity of Lipid Metabolism.- 4.1. Liponeogenesis.- 4.2. ?-Oxidation.- 4.3. Ketogenesis.- 4.4. Lipoprotein Metabolism.- 5. Summary.- References.- 11 Metabolism of Amino Acids and Ammonia.- 1. Introduction.- 2. Role of the Liver in the Metabolism of Amino Acids and Ammonia.- 2.1. Interorgan Relationships in Amino Acid Metabolism.- 2.2. Interorgan Relationships in Ammonia Metabolism.- 3. Regulation of Hepatic Amino Acid and Ammonia Metabolism: Role of Transport, Substrates, and Hormones.- 3.1. Amino Acid Metabolism.- 3.2. Ammonia Metabolism.- 4. Role of Zonal Hepatocyte Heterogeneity in the Regulation of Amino Acid and Ammonia Metabolism in the Liver.- 4.1. Zonal Distribution of Enzymes.- 4.2. Dynamics of Zonal Hepatocyte Heterogeneity in Amino Acid and Ammonia Metabolism.- 4.3. Functional Significance of Zonal Hepatocyte Heterogeneity in Amino Acid and Ammonia Metabolism.- 5. Summary.- References.- 12 Lobular Oxygen Gradients: Possible Role in Alcohol-Induced Hepatotoxicity.- 1. Introduction.- 2. Oxygen Uptake.- 3. Ethanol Metabolism.- 3.1. Alcohol Dehydrogenase.- 3.2. Catalase.- 3.3. Cytochrome P-450.- 3.4. Swift Increase in Alcohol Metabolism.- 3.5. Physiological Trigger for the Increase in Oxygen Uptake Caused by Ethanol Administration.- 3.6. Mechanism of Increase in Hepatic Oxygen Uptake Caused by Treatment with Ethanol.- 3.7. Role of Glycolysis in the Mechanism of Increased Oxygen Uptake Caused by Treatment with Ethanol.- 4. Role of Hypoxia in Liver Damage.- 4.1. Hepatic Oxygen Supply.- 4.2. Is Hypoxia Involved in the Mechanism of Alcohol-Induced Liver Damage?.- References.- 13 Biotransformation and Zonal Toxicity.- 1. ZonalToxicity.- 1.1. General Considerations.- 1.2. Agents That Cause Damage in Periportal Regions of the Liver Lobule.- 1.3. Agents That Cause Damage in Pericentral Regions of the Liver Lobule.- 2. Mixed-Function Oxidation.- 2.1. General Considerations.- 2.2. Intralobular Distributions of Xenobiotic-Metabolizing Enzymes.- 2.3. Sources of Reducing Equivalents in Specific Regions of the Liver Lobule.- 2.4. Distribution of Function in Intact Periportal and Pericentral Regions of the Liver.- 3. Conjugation Reactions.- 3.1. General Considerations.- 3.2. Glucoronidation.- 3.3. Sulfation.- 3.4. Mercapturic Acid Synthesis.- 4. Conclusions.- References.- 14 Protein Synthesis and Secretion.- 1. Introduction.- 2. Interorgan Relationships.- 3. Regulation of Hepatic Protein Synthesis by Extrahepatic and Hepatic Factors.- 3.1. Photoperiod.- 3.2. Nutrition.- 3.3. Osmotic Pressure.- 3.4. Hormones.- 3.5. Age.- 3.6. Polyploidy.- 3.7. Stress.- 3.8. Genetic Diseases.- 3.9. BloodFlow.- 3.10. Turnover.- 4. Regulation of Hepatic Protein Synthesis at the Subcellular Level.- 4.1. Transcription.- 4.2. Competition for Ribosomes.- 4.3. Translation.- 4.4. Transport.- 4.5. Secretion.- 5. Intraacinar Localization.- 5.1. Radioautography.- 5.2. Immunohistology.- 6. Intracellular Localization.- 7. Summary.- References.- 15 Bile Acid Metabolism.- 1. Introduction.- 2. Functional Units.- 2.1. Mature Liver.- 2.2. Developing Liver.- 3. Bile Acid Transport.- 3.1. Enterohepatic Circulation.- 3.2. Unidirectional Perfusion of Bile Acids.- 3.3. Mechanisms of Bile Acid Uptake.- 3.4. Zonal Uptake of Bile Acids.- 4. Intracellular Metabolism of Bile Acids in the Mature Liver.- 4.1. Bile Acid Synthesis.- 4.2. Bile Acid Conjugation.- 4.3. Intracellular Transport.- 4.4. Biliary Secretion.- 5. Hepatocellular Metabolism and Transport of Bile Acids during Liver Development.- 5.1. Concept of “Physiologic Cholestasis” in Infancy.- 5.2. Bile Acid Metabolism.- 5.3. Bile Acid Transport.- 6. Summary.- References.- IV. Induction of Liver Cell Heterogeneity.- 16 Zonal Signal Heterogeneity and Induction of Hepatocyte Heterogeneity.- 1. Introduction.- 2. Zonal Heterogeneity of Signals and Signal Transmission.- 2.1. Oxygen.- 2.2. Carbon Substrates.- 2.3. Hormones.- 2.4. Nerves.- 2.5. Hormone Receptors.- 3. Induction of Zonal Hepatocellular Heterogeneity.- 3.1. Modulation of Enzyme Induction by Oxygen.- 3.2. Insulin-Glucagon Antagonism in Enzyme Induction.- 3.3. Role of the Hepatic Nerves in Enzyme Induction.- 3.4. Dynamics of Zonal Hepatocyte Heterogeneity.- 3.5. Conclusion.- 4. Summary.- References.- V. Speculations and Directions for the Future.- Challenges for the Future.