Torpor and Hibernation in Mammals: Metabolic, Physiological, and Biochemical Adaptations

The sections in this article are: 1 Basic Patterns of Torpor and Hibernation 2 Physiological Manifestations in a Bout of Torpor 2.1 Entry into Torpor 2.2 The Torpor State 2.3 Arousal from Torpor 3 Energetics of Natural Hibernation 3.1 Energetics of Mammals Exhibiting Hibernation and Torpor 3.2 Current Theories on Why Animals Arouse Periodically 4 Physiological and Biochemical Adaptations: Organ Systems, Organs, and Tissues 4.1 Regulation of Heat Production in a Torpor Cycle 4.1.1 Entry into Hibernation and during Deep Hibernation 4.1.2 During Arousal 4.2 Cardiovascular Functions 4.2.1 Excitation-Contraction Coupling of the Cardiac Muscle 4.2.2 Blood Pressure and Blood Flow Regulation 4.3 Respiratory Functions 4.3.1 Ventilatory Modification 4.3.2 Acid Base Balance and Metabolic Implications 4.4 Reproductive Functions 4.4.1 Changes in Gonadal Activity during Hibernation Cycle 4.4.2 Sex Hormones and Hibernation Cycle 4.5 Photoperiodism and Rhythmicity 4.5.1 Pineal Gland and Melatonin 5 Biochemical and Cellular Adaptations 5.1 Substrate Utilization in Hibernation 5.1.1 Lipid Metabolism 5.1.2 Glucose Metabolism 5.2 Ionic Regulation 5.2.1 Sodium and Potassium Ions 5.2.2 Calcium Ions 5.3 Membrane Aspects 5.3.1 Membrane Enzyme Activities 5.3.2 Membrane Fluidity 5.3.3 Possible Mechanism(s) Responsible for Maintaining Membrane Functions at Low Temperature 6 The “Hibernation Induction Trigger”: Fact or Fiction? 6.1 Historical Background and Recent Advances 6.2 An Evaluation 6.3 The Opioids 7 Summary and Perspective