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OBJECTIVES
Identify the hormonal mechanisms involved in maintaining glucose homeostasis in the fasted and fed state.
Identify the specific roles of insulin, glucagon, glucocorticoids, catecholamines, growth hormone, and thyroid hormone in the regulation of energy substrate utilization, storage, and mobilization and maintenance of long-term energy balance.
Describe the hormonal regulation of energy substrate metabolism in response to stress including exercise and hypoglycemia.
Identify the processes involved in sodium homeostasis, including the roles of antidiuretic hormone, aldosterone, angiotensin, and atrial natriuretic hormone in the regulation of sodium balance.
Identify the processes involved in potassium homeostasis, including the hormonal regulation of plasma potassium concentration, distribution, and balance in the acute and chronic settings.
Identify the processes involved in calcium and phosphate homeostasis, including the hormonal regulation of plasma concentrations and the contributions of dietary intake, bone resorption, renal reabsorption, and intestinal absorption.
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In the first chapter, several of the key functions of the endocrine system that maintain homeostasis were outlined. Subsequent chapters described the specific physiologic effects of individual hormones, the mechanisms that regulate their production and release, and the consequences of their isolated excess or deficiency. The presentation of this material would not be complete without an attempt to integrate some of these actions into the overall regulation of specific functions. Although a complete description of the integrative control of physiologic function is beyond the scope of this book, this chapter integrates many of the concepts already presented. It describes how the different arms of the neuroendocrine system interact to regulate and maintain basic functions, which include energy substrate balance, blood volume and blood pressure, and preservation of bone mineral density (BMD). Finally, it presents an integrated discussion of the neuroendocrine mechanisms involved in mediating the stress response.
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NEUROENDOCRINE REGULATION OF ENERGY STORAGE, MOBILIZATION, AND UTILIZATION
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Two distinct phases directly related to the ingestion of a meal alternate throughout the day in the regulation of energy metabolism. The fed state reflects overall anabolic metabolism, during which energy is stored in the form of energy-rich compounds (adenosine triphosphate [ATP], phosphocreatine), glycogen, fat, and proteins. The fasted or catabolic phase is the period during which endogenous energy sources are utilized.
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The anabolic and catabolic phases alternate to preserve adequate glucose supply to the brain as well as sufficient energy to maintain body functions, such as thermoregulation (maintaining a constant core temperature), food digestion, and physical activity. The 2 hormones at the core of maintaining this balance are insulin and glucagon (see Chapter 7). In particular, their ratio plays a critical role in the dynamic regulation of substrate metabolism (summarized in Table 10–1). However, several other established and some newly discovered hormones participate in the regulation of energy metabolism to different extents, according to age, sex, nutritional state, and metabolic demands of the individual.
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