5.1 Hormones and their mechanisms of action
4 min read•august 14, 2024
Hormones are the body's chemical messengers, orchestrating various physiological processes. They come in different types - peptide, steroid, and amine - each with unique structures and functions. Understanding their classification helps grasp how they work in our bodies.
Hormones act through receptor-mediated and non-receptor-mediated pathways, triggering cellular responses. Their actions are finely tuned by feedback loops, ensuring balance in the body. This intricate system of hormone regulation is crucial for maintaining homeostasis and adapting to changes.
Chemical structure of hormones
Classification of hormones by chemical structure
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- Hormones can be classified into three main categories based on their chemical structure: , , and
- Peptide hormones consist of amino acids and include (regulates blood glucose), (stimulates growth and cell reproduction), and (involved in childbirth and bonding)
- Steroid hormones are derived from cholesterol and include (stress response), (regulates blood pressure and electrolyte balance), and sex hormones like and (development and regulation of reproductive system)
- Amine hormones are derived from amino acids and include and (fight-or-flight response) and (regulate metabolism and growth)
Synthesis and regulation of hormones
- The synthesis of hormones occurs in the endocrine glands and involves various enzymatic reactions and modifications specific to each hormone type
- For example, peptide hormones are synthesized as inactive precursor proteins that undergo post-translational modifications, while steroid hormones are synthesized from cholesterol through a series of enzymatic reactions
- The rate of hormone synthesis is often regulated by feedback loops, where the levels of the hormone itself or its effects on target tissues influence its production
- loops, such as the , help maintain hormone levels within a narrow range
- loops, such as the release of oxytocin during childbirth, amplify the hormone response until a critical point is reached
Mechanisms of hormone action
Receptor-mediated pathways
- Receptor-mediated pathways involve the binding of a hormone to a specific receptor protein, which can be located on the cell surface or inside the cell
- Cell surface receptors are typically associated with hydrophilic hormones, such as peptide hormones (insulin) and catecholamines (epinephrine)
- Intracellular receptors are usually associated with hydrophobic hormones, such as steroid (cortisol) and thyroid hormones (T3 and T4)
- The hormone-receptor complex triggers a cascade of intracellular signaling events that ultimately lead to changes in gene expression, protein synthesis, or cellular activity
- For example, the binding of epinephrine to its receptor activates the cAMP second messenger system, leading to the breakdown of glycogen and increased blood glucose levels
- The binding of steroid hormones to their intracellular receptors forms a complex that acts as a transcription factor, directly influencing gene expression in the nucleus
Non-receptor-mediated pathways
- Non-receptor-mediated pathways involve hormones that can directly influence cellular processes without binding to a specific receptor
- Examples include the action of insulin on glucose uptake by facilitating the translocation of glucose transporter proteins (GLUT4) to the cell membrane
- Thyroid hormones can directly influence mitochondrial function by uncoupling oxidative phosphorylation, leading to increased heat production and metabolic rate
Hydrophobic vs hydrophilic hormones
Hydrophobic hormones
- Hydrophobic hormones, such as steroid and thyroid hormones, are lipid-soluble and can easily pass through the cell membrane to bind to intracellular receptors
- The hormone-receptor complex acts as a transcription factor, directly influencing gene expression in the nucleus
- The effects of hydrophobic hormones are typically slower in onset but longer-lasting compared to hydrophilic hormones
- Examples of hydrophobic hormones include cortisol (regulates stress response and metabolism), estrogen (female reproductive development), and testosterone (male reproductive development)
Hydrophilic hormones
- Hydrophilic hormones, such as peptide hormones and catecholamines, are water-soluble and cannot pass through the cell membrane. They bind to cell surface receptors to exert their effects
- The binding of hydrophilic hormones to their receptors activates intracellular signaling cascades, often involving second messengers like cyclic AMP (cAMP) or calcium ions
- The effects of hydrophilic hormones are usually rapid in onset but shorter in duration compared to hydrophobic hormones
- Examples of hydrophilic hormones include insulin (lowers blood glucose), oxytocin (uterine contractions and milk letdown), and vasopressin (regulates water balance and blood pressure)
Feedback loops in hormone regulation
Negative feedback loops
- Negative feedback loops are the most common type, where the effects of a hormone inhibit its own synthesis or release, preventing excessive hormone action
- For example, high blood glucose levels stimulate insulin secretion, which in turn lowers blood glucose by promoting its uptake and storage. As blood glucose levels decrease, insulin secretion is inhibited
- The hypothalamic-pituitary-thyroid axis is another example of a negative feedback loop, where thyroid hormones inhibit the release of thyrotropin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH) when circulating levels are high
Positive feedback loops
- Positive feedback loops are less common but can be crucial in certain physiological processes, such as the release of oxytocin during childbirth or the LH surge during ovulation
- In these cases, the effects of the hormone stimulate its own production, leading to a rapid amplification of the response until a critical point is reached
- During childbirth, the pressure of the baby's head on the cervix stimulates oxytocin release, which causes uterine contractions. These contractions further stimulate oxytocin release, creating a positive feedback loop that continues until the baby is born
- Feedback loops involve the integration of signals from multiple endocrine glands and target tissues, ensuring a coordinated and adaptive response to changes in the internal or external environment
- For example, the integrates signals from the brain, pituitary gland, and adrenal glands to regulate the stress response
- The coordinates the actions of the brain, pituitary gland, and gonads to regulate reproductive function and development
Key Terms to Review (27)
Adrenal gland: The adrenal glands are small, triangular-shaped glands located on top of each kidney, responsible for producing a variety of hormones that regulate metabolism, immune response, blood pressure, and stress responses. They consist of two main parts: the adrenal cortex and the adrenal medulla, each producing different hormones that play vital roles in the body's physiological functions.
Aldosterone: Aldosterone is a steroid hormone produced by the adrenal glands that plays a key role in regulating sodium and potassium balance, as well as blood pressure. It primarily acts on the kidneys to promote sodium reabsorption and potassium excretion, which helps maintain fluid balance and electrolyte homeostasis in the body.
Amine Hormones: Amine hormones are a class of hormones derived from amino acids, particularly tyrosine and tryptophan. They play crucial roles in various physiological processes, including metabolism, growth, and the body's response to stress. Their structure allows them to act on target cells through specific receptors, leading to various biological effects depending on the type of amine hormone involved.
Cortisol: Cortisol is a steroid hormone produced by the adrenal glands, playing a critical role in the body's response to stress and metabolism regulation. It helps control blood sugar levels, manage how the body uses fats, proteins, and carbohydrates, and has anti-inflammatory effects. Additionally, cortisol is influenced by circadian rhythms, impacting its release and effects on sleep and lactation processes during post-partum changes.
Epinephrine: Epinephrine, also known as adrenaline, is a hormone and neurotransmitter produced by the adrenal glands that plays a crucial role in the body's 'fight or flight' response. It is released during stressful situations, preparing the body to respond quickly to perceived threats by increasing heart rate, blood flow to muscles, and energy availability. This powerful chemical is key in regulating various physiological responses and is vital for maintaining homeostasis under stress.
Estrogen: Estrogen is a group of steroid hormones that play a crucial role in the development and regulation of the female reproductive system and secondary sexual characteristics. These hormones are key players in various physiological processes, including the menstrual cycle, reproductive health, and overall female physiology, influencing everything from fertility to bone density.
G protein-coupled receptors: G protein-coupled receptors (GPCRs) are a large family of cell surface receptors that play a critical role in transmitting signals from outside the cell to the inside. These receptors interact with G proteins, which then activate various intracellular signaling pathways, ultimately leading to a cellular response. GPCRs are key players in the action of many hormones and neurotransmitters, influencing a wide range of physiological processes.
Growth hormone: Growth hormone (GH) is a peptide hormone produced by the anterior pituitary gland that stimulates growth, cell reproduction, and regeneration in humans and other animals. It plays a crucial role in regulating metabolism and influencing muscle and bone growth, making it essential for overall development and physical health.
Growth Stimulation: Growth stimulation refers to the biological processes that promote growth and development in organisms, primarily influenced by hormones. These hormones act as chemical messengers that trigger specific cellular responses, leading to increases in size, mass, and overall organismal growth. Understanding growth stimulation is essential for comprehending how hormonal signals orchestrate developmental processes in various tissues and systems within the body.
Hypothalamic-pituitary-adrenal axis: The hypothalamic-pituitary-adrenal axis is a complex set of interactions among the hypothalamus, pituitary gland, and adrenal glands that regulates stress response, metabolism, and immune function. This axis plays a crucial role in maintaining homeostasis and responding to stressors by releasing hormones like corticotropin-releasing hormone (CRH), adrenocorticotropic hormone (ACTH), and cortisol, connecting the endocrine and nervous systems in a coordinated manner.
Hypothalamic-pituitary-gonadal axis: The hypothalamic-pituitary-gonadal (HPG) axis is a complex set of interactions between the hypothalamus, the pituitary gland, and the gonads (ovaries in females and testes in males) that regulates reproduction and sexual development through hormonal signaling. This axis controls the release of hormones such as gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH), and follicle-stimulating hormone (FSH), which in turn influence the production of sex hormones like estrogen, progesterone, and testosterone, essential for various reproductive processes.
Hypothalamic-pituitary-thyroid axis: The hypothalamic-pituitary-thyroid axis is a complex feedback system that regulates thyroid hormone production and secretion in the body. It involves the hypothalamus releasing thyrotropin-releasing hormone (TRH), which stimulates the pituitary gland to secrete thyroid-stimulating hormone (TSH). TSH then prompts the thyroid gland to produce and release thyroid hormones, primarily thyroxine (T4) and triiodothyronine (T3), crucial for metabolism, growth, and development.
Insulin: Insulin is a peptide hormone produced by the beta cells of the pancreas that plays a crucial role in regulating glucose metabolism and maintaining blood sugar levels within a normal range. By facilitating the uptake of glucose into cells, insulin supports energy production and storage, thereby contributing to homeostasis within the body.
Metabolism regulation: Metabolism regulation refers to the processes that control the rates of metabolic reactions in the body, ensuring that energy production and consumption are balanced. This regulation is crucial for maintaining homeostasis and is influenced by various factors, including hormones, which act as signaling molecules to coordinate metabolic activities. Different glands release these hormones, responding to the body's needs and environmental changes, making metabolism regulation a complex interplay of multiple systems.
Negative feedback: Negative feedback is a regulatory mechanism in biological systems that counteracts changes from a set point to maintain homeostasis. This process involves detecting deviations from a normal range and initiating responses that restore conditions back to their optimal state, ensuring stability within various physiological systems.
Norepinephrine: Norepinephrine is a neurotransmitter and hormone produced primarily by the adrenal glands and sympathetic nerve endings, playing a critical role in the body’s 'fight or flight' response. It acts on various receptors throughout the body to increase heart rate, blood pressure, and energy availability while modulating other physiological functions such as digestion and muscle contraction.
Nuclear Receptors: Nuclear receptors are a class of proteins found within cells that play a key role in regulating gene expression in response to hormonal signals. They act as transcription factors that bind to specific hormones, such as steroid hormones, thyroid hormones, and retinoic acid, allowing them to directly influence DNA transcription and the subsequent production of proteins. This mechanism is crucial for maintaining homeostasis and mediating the physiological effects of hormones on target tissues.
Oxytocin: Oxytocin is a hormone produced in the hypothalamus and secreted by the posterior pituitary gland, primarily known for its role in social bonding, reproduction, and childbirth. This hormone influences various physiological processes, such as uterine contractions during labor and milk ejection during breastfeeding, making it crucial in reproductive health and maternal behaviors.
Peptide Hormones: Peptide hormones are a class of hormones made up of chains of amino acids that play crucial roles in regulating various physiological processes in the body. These hormones are synthesized in the endoplasmic reticulum and Golgi apparatus of cells and are released into the bloodstream, where they exert their effects on target cells by binding to specific receptors on their surfaces, initiating signaling pathways. Their relatively short half-life allows for rapid responses to physiological changes.
Positive Feedback: Positive feedback is a physiological mechanism that amplifies changes or responses in the body, leading to an enhancement of the initial stimulus. Unlike negative feedback, which works to reverse changes and maintain stability, positive feedback intensifies processes and can lead to a specific outcome until a certain goal is achieved. This mechanism plays crucial roles in various biological processes and is particularly significant during critical events in human physiology.
Receptor Binding: Receptor binding refers to the interaction between a hormone or signaling molecule and its specific receptor on a target cell, which initiates a biological response. This process is crucial for the mechanism of action of hormones, as it determines how effectively a hormone can influence the activity of target cells. The specificity and affinity of this binding determine the strength and duration of the signal transmitted within the cell.
Renin-Angiotensin System: The renin-angiotensin system (RAS) is a hormonal cascade that plays a critical role in regulating blood pressure and fluid balance in the body. It begins with the release of renin from the kidneys in response to low blood pressure or low sodium levels, leading to the production of angiotensin II, a potent vasoconstrictor that increases blood pressure. This system highlights the complex interplay between hormones and physiological mechanisms that maintain homeostasis, particularly in blood pressure regulation.
Signal Transduction: Signal transduction is the process by which cells respond to external signals, converting these signals into a functional response. This intricate communication system involves various molecular pathways, allowing cells to interpret stimuli from their environment, which is crucial for maintaining homeostasis and coordinating physiological functions.
Steroid hormones: Steroid hormones are a class of hormones that are derived from cholesterol and are characterized by their lipid-soluble nature, which allows them to easily pass through cell membranes. These hormones play critical roles in various physiological processes, including metabolism, immune response, and the regulation of inflammation, by binding to specific receptors inside target cells and influencing gene expression.
Testosterone: Testosterone is a steroid hormone primarily produced in the testes in males, playing a key role in the development of male reproductive tissues and secondary sexual characteristics. This hormone influences many physiological processes, including muscle and bone mass, fat distribution, and the production of sperm. It also acts as a signaling molecule that communicates with various target tissues to regulate bodily functions.
Thyroid Gland: The thyroid gland is an endocrine gland located in the neck that produces hormones crucial for regulating metabolism, growth, and development. It plays a significant role in maintaining the body’s metabolic rate and influences various bodily functions through the release of hormones such as thyroxine (T4) and triiodothyronine (T3). These hormones affect nearly every cell in the body, making the thyroid gland essential for overall health.
Thyroid Hormones: Thyroid hormones, primarily thyroxine (T4) and triiodothyronine (T3), are essential hormones produced by the thyroid gland that regulate metabolism, growth, and development in the body. These hormones play a crucial role in maintaining energy balance, influencing the metabolic rate, and impacting various physiological functions such as heart rate and temperature regulation.