5.4 Endocrine regulation of metabolism and growth

Hormones play a crucial role in regulating metabolism and growth. Insulin and glucagon maintain glucose balance, while thyroid hormones control metabolic rate. These processes are essential for energy production and utilization in the body.

Growth hormone and insulin-like growth factors drive physical development and cell growth. Meanwhile, leptin and ghrelin regulate appetite and energy balance. Understanding these hormones helps explain how the body manages energy and grows.

Pancreatic Hormones and Glucose Regulation

Insulin and Glucagon in Glucose Homeostasis

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• Insulin is a peptide hormone secreted by the beta cells of the pancreas
  • Lowers blood glucose levels by promoting glucose uptake into cells (muscle, adipose tissue)
  • Stimulates the liver to convert glucose into glycogen for storage
  • Enhances lipogenesis and protein synthesis
• Glucagon is a peptide hormone secreted by the alpha cells of the pancreas
  • Raises blood glucose levels by promoting glycogenolysis (breakdown of glycogen) in the liver
  • Stimulates gluconeogenesis (production of glucose from non-carbohydrate sources) in the liver
  • Promotes lipolysis (breakdown of lipids) in adipose tissue
• Glucose homeostasis is the maintenance of stable blood glucose levels within a narrow range
  • Involves a balance between glucose production (liver) and glucose utilization (peripheral tissues)
  • Regulated by the opposing actions of insulin and glucagon
  • Disruption of this balance can lead to conditions like diabetes mellitus (hyperglycemia) or hypoglycemia

Metabolic Rate Regulation

• Metabolic rate refers to the amount of energy expended by the body per unit time
  • Basal metabolic rate (BMR) is the energy required to maintain basic physiological functions at rest
• Thyroid hormones (thyroxine and triiodothyronine) play a key role in regulating metabolic rate
  • Increase BMR by stimulating cellular respiration and heat production
  • Enhance the effects of other hormones like catecholamines (epinephrine and norepinephrine)
• Insulin also influences metabolic rate by promoting energy storage and anabolism
  • Increases glucose uptake and utilization in tissues
  • Stimulates lipogenesis and protein synthesis, which require energy expenditure

Growth and Development Hormones

Growth Hormone and Insulin-like Growth Factors

• Growth hormone (GH) is a peptide hormone secreted by the anterior pituitary gland
  • Stimulates linear growth and cell proliferation in skeletal muscle, bone, and cartilage
  • Promotes lipolysis and protein synthesis, leading to increased lean body mass
  • Induces insulin resistance, leading to elevated blood glucose levels
• Insulin-like growth factors (IGFs) are peptides produced primarily in the liver in response to GH
  • IGF-1 mediates many of the growth-promoting effects of GH
  • Stimulates cell proliferation, differentiation, and survival in various tissues (bone, muscle, cartilage)
  • Enhances glucose and amino acid uptake in peripheral tissues

Thyroid Hormones and Cortisol in Growth and Development

• Thyroid hormones (thyroxine and triiodothyronine) are essential for normal growth and development
  • Stimulate bone growth and maturation, particularly during childhood and adolescence
  • Promote neuronal differentiation and myelination in the developing brain
  • Enhance the effects of GH and IGF-1 on linear growth
• Cortisol is a glucocorticoid hormone secreted by the adrenal cortex
  • Plays a permissive role in growth and development
  • Necessary for the normal functioning of GH and thyroid hormones
  • Excess cortisol (Cushing's syndrome) can lead to growth retardation and muscle wasting
  • Insufficient cortisol (Addison's disease) can impair growth and development

Appetite and Energy Balance Hormones

Leptin and Ghrelin in Appetite Regulation

• Leptin is a peptide hormone secreted primarily by adipose tissue
  • Acts as a satiety signal, reducing appetite and food intake
  • Communicates the body's energy status to the hypothalamus, regulating long-term energy balance
  • Deficiency or resistance to leptin can lead to obesity and increased appetite (hyperphagia)
• Ghrelin is a peptide hormone secreted mainly by the stomach
  • Stimulates appetite and food intake, acting as a hunger signal
  • Levels increase before meals and decrease after eating
  • Plays a role in short-term appetite regulation and meal initiation
• Leptin and ghrelin work in opposition to maintain energy balance
  • Leptin decreases appetite and increases energy expenditure
  • Ghrelin increases appetite and promotes food intake
  • Dysregulation of these hormones can contribute to obesity or eating disorders (anorexia nervosa)