37.2 How Hormones Work
3 min read•june 14, 2024
signaling is a crucial aspect of cellular communication. Hormones, secreted by , travel through the bloodstream to with specific . This binding initiates signaling cascades that amplify the hormone's message and lead to cellular responses.
Hormones can interact with intracellular or cell surface receptors, depending on their chemical nature. This distinction affects how the signal is transmitted and the resulting cellular changes. play a key role in amplifying hormone signals and producing specific responses within target cells.
Hormone Signaling Mechanisms
Hormone binding and cellular responses
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- Hormones are signaling molecules secreted by endocrine glands that travel through the bloodstream to target cells (, )
- Hormones bind to specific receptors on or within target cells
- Receptor specificity ensures that hormones only affect cells with the appropriate receptors ( on muscle and liver cells)
- Hormone-receptor binding initiates a signaling cascade within the target cell
- This signaling cascade amplifies the hormone's signal and leads to a specific cellular response (increased glucose uptake in response to )
- The cellular response to a hormone can involve changes in gene expression, protein activity, or cellular metabolism (increased in response to insulin)
- These changes ultimately result in the physiological effects associated with the hormone (blood glucose regulation by insulin)
Intracellular vs cell surface receptors
- Intracellular hormone receptors
- Located within the cytoplasm or nucleus of the target cell
- Typically bind to lipid-soluble hormones, such as and thyroid hormones (, , )
- Lipid-soluble hormones can diffuse through the cell membrane to reach intracellular receptors
- Hormone-receptor complexes act as transcription factors, directly regulating gene expression
- These complexes bind to specific DNA sequences, activating or repressing target genes ( receptor complex activating genes involved in female reproductive development)
- Cell surface hormone receptors
- Located on the plasma membrane of the target cell
- Bind to water-soluble hormones, such as and (insulin, )
- Water-soluble hormones cannot cross the cell membrane and must bind to surface receptors
- Hormone binding to cell surface receptors activates intracellular signaling pathways
- These pathways often involve , such as ###cyclic_AMP_()_0### or () (cAMP production in response to adrenaline binding to β-adrenergic receptors)
- Second messengers amplify the hormone's signal and lead to changes in cellular activity (increased heart rate and blood pressure in response to adrenaline)
Second messengers in hormone signaling
- Second messengers are small, diffusible molecules that relay signals from cell surface receptors to intracellular targets (cAMP, , )
- They amplify the hormone's signal, allowing a small number of activated receptors to produce a large cellular response
- Common second messengers include (cAMP), calcium ions (), and inositol trisphosphate (IP3)
- The binding of a hormone to a cell surface receptor activates an enzyme, such as or
- These enzymes catalyze the production of second messengers (adenylyl cyclase producing cAMP in response to binding)
- cAMP is produced by adenylyl cyclase and activates ()
- PKA phosphorylates various target proteins, modulating their activity (PKA phosphorylating , leading to glycogen breakdown)
- PKA can also regulate gene expression by phosphorylating transcription factors (PKA activating , which promotes the expression of genes involved in glucose metabolism)
- is released from the endoplasmic reticulum in response to IP3 production
- Elevated cytosolic levels activate calcium-dependent enzymes, such as () (PKC activation in response to binding to )
- PKC phosphorylates target proteins, leading to changes in cellular activity (PKC promoting smooth muscle contraction in blood vessels)
- Second messengers provide a means for hormones to induce rapid and specific cellular responses through cell surface receptors (rapid increase in blood glucose levels in response to glucagon)
Endocrine System and Hormone Regulation
- The consists of glands that secrete hormones directly into the bloodstream
- occurs in specialized endocrine cells, involving complex biochemical pathways
- Hormones play a crucial role in maintaining through mechanisms
- Negative feedback helps regulate hormone levels by inhibiting further hormone production when target effects are achieved
- Target cells possess specific receptors that allow them to respond to particular hormones
- pathways within target cells convert hormone signals into cellular responses, enabling precise control of physiological processes
Key Terms to Review (59)
Adenylyl cyclase: Adenylyl cyclase is an enzyme that converts ATP into cyclic AMP (cAMP), a critical second messenger in cellular signaling pathways. By facilitating the production of cAMP, adenylyl cyclase plays a vital role in amplifying signals from hormones and neurotransmitters, which in turn influences various physiological responses. This enzymatic action is essential for the propagation of signals across cells and is key to how hormones exert their effects on target tissues.
Adrenaline: Adrenaline, also known as epinephrine, is a hormone produced by the adrenal glands that plays a crucial role in the body's fight-or-flight response. When faced with stress or danger, adrenaline is released into the bloodstream, leading to various physiological changes such as increased heart rate, heightened alertness, and improved energy availability. This hormone prepares the body to respond quickly to threats, making it essential for survival and demonstrating the power of hormonal regulation in bodily functions.
Calcium ions: Calcium ions (Ca²⁺) are positively charged particles that play crucial roles in various physiological processes within living organisms. They are essential for signal transduction in cells, muscle contraction, hormone secretion, and fertilization, acting as important messengers in these processes and influencing cellular activity and communication.
CAMP: cAMP, or cyclic adenosine monophosphate, is a second messenger important in many biological processes. It plays a crucial role in transmitting signals from hormones and other signaling molecules to target cells, facilitating various physiological responses like gene expression, metabolism, and homeostasis.
CAMP-dependent kinase (A-kinase): cAMP-dependent kinase (A-kinase) is an enzyme that is activated by the molecule cyclic AMP (cAMP). It plays a crucial role in regulating various cellular processes by phosphorylating target proteins.
Catecholamines: Catecholamines are a group of hormones produced by the adrenal glands, primarily consisting of epinephrine (adrenaline), norepinephrine (noradrenaline), and dopamine. These hormones play a crucial role in the body's response to stress, affecting various physiological processes such as heart rate, blood pressure, and energy metabolism. They act as neurotransmitters in the nervous system, facilitating communication between nerve cells and influencing both physical and emotional responses to stimuli.
CREB: CREB, or cAMP response element-binding protein, is a cellular transcription factor that plays a critical role in regulating gene expression in response to various signals, particularly those involving cyclic AMP (cAMP). It acts as a mediator of cellular responses to hormones and other signaling molecules, influencing processes such as metabolism, growth, and memory formation.
Cyclic AMP: Cyclic AMP (cAMP) is a secondary messenger molecule that plays a crucial role in cellular signaling pathways, especially in response to hormones and neurotransmitters. It is synthesized from ATP by the enzyme adenylate cyclase, and it mediates the effects of various hormones by activating protein kinases, which ultimately leads to changes in cellular activity. This mechanism is essential for understanding how signals are propagated within cells and how hormones exert their effects on target tissues.
Cyclic AMP (cAMP): cAMP (cyclic adenosine monophosphate) is a second messenger important in many biological processes. It relays signals from extracellular molecules to intracellular targets, facilitating cellular responses.
Dephosphorylation: Dephosphorylation is the removal of a phosphate group from an organic molecule. This process is crucial in regulating cellular activities and signaling pathways.
Down-regulation: Down-regulation is the process by which a cell decreases the number of receptors for a particular hormone, reducing its sensitivity to that hormone. This mechanism helps maintain homeostasis when hormone levels are chronically elevated.
Endocrine glands: Endocrine glands are specialized organs that secrete hormones directly into the bloodstream, playing a critical role in regulating various physiological processes. These glands help maintain homeostasis, influence growth and development, and control metabolism by releasing hormones that act on distant target organs. Their function is essential for communication within the body, linking different systems and maintaining overall health.
Endocrine system: The endocrine system is a network of glands that produce and secrete hormones directly into the bloodstream to regulate various body functions. This system plays a crucial role in maintaining homeostasis, controlling metabolism, growth, reproduction, and responding to stress, thereby influencing nearly every aspect of our biology.
Estrogen: Estrogen is a primary female sex hormone responsible for the regulation of the reproductive system and secondary sexual characteristics. It plays a crucial role in the menstrual cycle and pregnancy.
Estrogen: Estrogen is a group of hormones that play a crucial role in the development and regulation of the female reproductive system and secondary sexual characteristics. These hormones are not only important for reproduction but also influence various body processes, including metabolism and bone density.
First messenger: A first messenger is a hormone or signaling molecule that binds to a cell surface receptor, initiating a cascade of intracellular events. It is the initial signal in the communication process between cells.
G protein-coupled receptors: G protein-coupled receptors (GPCRs) are a large family of cell surface receptors that play a crucial role in cellular communication by detecting extracellular signals and activating intracellular signaling pathways. They are involved in a wide range of physiological processes and respond to various signaling molecules, such as hormones, neurotransmitters, and sensory stimuli, making them essential for cell signaling and response mechanisms.
G-protein: G-proteins are guanine nucleotide-binding proteins that act as molecular switches inside cells, involved in transmitting signals from various stimuli outside the cell to its interior. They play a crucial role in activating or inhibiting downstream signaling pathways in response to hormone binding to cell surface receptors.
Glucagon: Glucagon is a peptide hormone produced by the alpha cells of the pancreas that plays a critical role in maintaining blood glucose levels. It works primarily by promoting the conversion of stored glycogen in the liver into glucose, releasing it into the bloodstream when blood sugar levels are low, and also influences the metabolism of proteins and lipids, contributing to overall energy homeostasis.
Gluconeogenesis: Gluconeogenesis is the metabolic process through which glucose is synthesized from non-carbohydrate substrates, primarily in the liver. It plays a critical role in maintaining blood sugar levels during fasting or intense exercise.
Gluconeogenesis: Gluconeogenesis is the metabolic process by which organisms synthesize glucose from non-carbohydrate precursors, primarily occurring in the liver and to a lesser extent in the kidneys. This pathway is crucial for maintaining blood glucose levels during fasting or intense exercise, ensuring that vital organs, especially the brain, have a continuous supply of glucose as an energy source.
Glycogen phosphorylase: Glycogen phosphorylase is an enzyme that catalyzes the breakdown of glycogen into glucose-1-phosphate, playing a crucial role in energy metabolism. This enzyme is activated during times of energy demand, such as during exercise or fasting, to release stored glucose for use by cells. Its activity is regulated by various hormonal signals, making it essential for maintaining blood glucose levels and overall metabolic homeostasis.
Glycogen synthesis: Glycogen synthesis is the biochemical process by which glucose molecules are combined to form glycogen, a polysaccharide that serves as a primary energy storage molecule in animals. This process is crucial for regulating blood sugar levels and providing energy during periods of fasting or intense physical activity, connecting it closely to hormonal regulation in the body.
Homeostasis: Homeostasis is the process by which biological systems maintain a stable internal environment despite external changes. This dynamic equilibrium is essential for the survival of organisms, as it regulates factors like temperature, pH, and the concentration of ions and nutrients. It connects to various aspects of biology, including how organisms interact with their environment and the physiological processes that sustain life.
Hormone: Hormones are chemical messengers produced by glands in the endocrine system. They regulate various physiological processes by signaling organs and tissues to perform specific functions.
Hormone synthesis: Hormone synthesis refers to the biochemical processes through which hormones are produced and secreted by endocrine glands. This process involves the conversion of precursor molecules into active hormones, which can then be released into the bloodstream to regulate various physiological functions in the body. Hormone synthesis is essential for maintaining homeostasis, influencing growth, metabolism, and responses to stress and other stimuli.
Inositol triphosphate (IP3): Inositol triphosphate (IP3) is a secondary messenger molecule used in signal transduction and lipid signaling. It is produced by the cleavage of phosphatidylinositol 4,5-bisphosphate (PIP2) by the enzyme phospholipase C (PLC).
Insulin: Insulin is a hormone produced by the pancreas that regulates blood glucose levels by facilitating the uptake of glucose into cells. It plays a crucial role in maintaining homeostasis within the body.
Insulin: Insulin is a peptide hormone produced by the pancreas that regulates glucose levels in the blood and facilitates cellular uptake of glucose. It plays a vital role in maintaining energy balance by promoting the storage of glucose as glycogen and inhibiting the production of glucose by the liver, which connects it to various metabolic and physiological processes in the body.
Insulin Receptors: Insulin receptors are specialized proteins located on the surface of cells that bind insulin, a hormone crucial for regulating glucose levels in the blood. When insulin binds to these receptors, it triggers a series of cellular responses that help the body utilize glucose effectively, promoting its uptake by tissues, particularly muscle and fat cells. This process is vital for maintaining energy balance and overall metabolic health.
IP3: IP3, or inositol trisphosphate, is a second messenger molecule that plays a crucial role in cellular signaling pathways, particularly in response to certain hormones. It is produced from phosphatidylinositol 4,5-bisphosphate (PIP2) through the action of phospholipase C, which is activated by various hormones and neurotransmitters. IP3 primarily functions to increase intracellular calcium levels, triggering various physiological responses in target cells.
Lipolysis: Lipolysis is the metabolic process through which stored fats, primarily triglycerides, are broken down into free fatty acids and glycerol. This process is crucial for energy production, especially during periods of fasting or intense physical activity, connecting to how hormones regulate energy balance in the body.
Negative feedback: Negative feedback is a regulatory mechanism in biological systems that helps maintain homeostasis by reversing changes from a set point. This process involves detecting deviations from a normal range and initiating responses that counteract those deviations, ensuring stability in various physiological functions.
Negative feedback loop: A negative feedback loop is a biological process where the output of a system suppresses or diminishes its own activity to maintain homeostasis. It helps stabilize internal conditions by counteracting deviations from a set point.
Nuclear receptors: Nuclear receptors are a class of proteins found within cells that act as transcription factors, regulating the expression of specific genes in response to hormones and other signaling molecules. These receptors play a critical role in how hormones exert their effects on various physiological processes, such as metabolism, development, and immune responses, by interacting with hormone-response elements in the DNA.
Peptide hormones: Peptide hormones are signaling molecules composed of amino acid chains that regulate physiological functions. They are synthesized in endocrine glands and act on specific target cells to elicit responses.
Peptide hormones: Peptide hormones are signaling molecules made up of chains of amino acids that play a crucial role in various physiological processes by transmitting messages between cells. These hormones can affect growth, metabolism, and homeostasis, and they bind to specific receptors on target cells to initiate cellular responses. They are an essential class of hormones, alongside steroids and amines, with unique mechanisms of action in the body.
Phosphodiesterase: Phosphodiesterase is an enzyme that breaks down cyclic nucleotides like cAMP and cGMP, which are important secondary messengers in cellular signaling. It regulates the amplitude and duration of signal transduction.
Phospholipase C: Phospholipase C is an enzyme that plays a crucial role in cellular signaling by hydrolyzing phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol trisphosphate (IP3) and diacylglycerol (DAG). This process is integral to the response of cells to various signals, particularly in the action of hormones and neurotransmitters, leading to diverse physiological responses.
Phosphorylation: Phosphorylation is the biochemical process of adding a phosphate group (PO4) to a molecule, typically a protein, which can alter the function and activity of that molecule. This process is essential in regulating various cellular activities, including metabolism, signaling, and gene expression.
PKA: PKA, or protein kinase A, is an important enzyme that plays a critical role in cell signaling by transferring phosphate groups to specific proteins, thus modifying their function. This phosphorylation process can activate or deactivate various cellular pathways, influencing diverse physiological responses such as metabolism, gene expression, and muscle contraction. PKA is activated by cyclic AMP (cAMP), which acts as a secondary messenger in hormone signaling pathways.
PKC: Protein Kinase C (PKC) is a family of protein kinases that play vital roles in cellular signaling processes, particularly in response to hormones and growth factors. PKC is activated by diacylglycerol (DAG) and calcium ions, leading to various cellular responses, including cell growth, differentiation, and survival. This mechanism is crucial for understanding how hormones exert their effects on target cells, influencing numerous physiological functions.
Plasma membrane hormone receptors: Plasma membrane hormone receptors are proteins located on the surface of cells that bind to specific hormones, initiating a cellular response. These receptors play a crucial role in signal transduction and regulating physiological processes.
Protein kinase A: Protein kinase A (PKA) is an important enzyme that phosphorylates specific serine and threonine residues on target proteins, which plays a crucial role in various cellular signaling pathways. It is activated by cyclic AMP (cAMP) and is a key player in the propagation of signals initiated by hormones and other signaling molecules, thus influencing processes like metabolism, gene expression, and cell growth.
Protein kinase C: Protein kinase C (PKC) is a family of enzymes that play crucial roles in various cellular signaling pathways by phosphorylating specific serine and threonine residues on target proteins. This phosphorylation process can lead to diverse effects such as cell growth, differentiation, and apoptosis, making PKC essential for responding to signals from hormones and other stimuli.
Receptors: Cellular receptors are proteins located on the cell surface or within cells that bind to specific signaling molecules. This binding triggers a response in the cell, facilitating communication and regulation of various physiological processes.
Second messengers: Second messengers are intracellular signaling molecules released by the cell in response to exposure to extracellular signaling molecules. They help amplify and propagate the signal within the cell, leading to various cellular responses.
Second messengers: Second messengers are intracellular signaling molecules that are released in response to the activation of cell surface receptors by signaling molecules. They play a crucial role in transmitting signals from the cell membrane to various intracellular targets, leading to a cellular response. This process is essential for how cells communicate and respond to external signals, such as hormones and neurotransmitters.
Signal transduction: Signal transduction is the process by which cells convert external signals into functional responses, allowing them to communicate and adapt to their environment. This involves a series of molecular events, including the reception of signaling molecules, propagation of the signal through cellular pathways, and eventual cellular responses that influence activities such as growth, metabolism, and immune reactions.
Steroid hormones: Steroid hormones are a class of hormones derived from cholesterol, characterized by their lipid-soluble nature, allowing them to easily pass through cell membranes and bind to intracellular receptors. They play critical roles in regulating various physiological processes, including metabolism, immune response, and reproductive functions, by influencing gene expression and protein synthesis within target cells.
Target cells: Target cells are specific cells in the body that are affected by particular hormones. These cells have receptors that bind to the hormones, allowing them to respond to hormonal signals. The interaction between hormones and their target cells is crucial for regulating various physiological processes, including growth, metabolism, and homeostasis.
Testosterone: Testosterone is a steroid hormone primarily produced in the testes in males and in smaller amounts by the ovaries in females. It plays a key role in the development of male reproductive tissues and secondary sexual characteristics.
Testosterone: Testosterone is a steroid hormone produced primarily in the testes in males and in smaller amounts in the ovaries and adrenal glands in females. It plays a critical role in the development of male reproductive tissues, promoting secondary sexual characteristics such as increased muscle and bone mass, and influencing libido. This hormone is also involved in various bodily processes and functions beyond reproduction.
Thyroid hormone: Thyroid hormone refers to a group of hormones produced by the thyroid gland, primarily thyroxine (T4) and triiodothyronine (T3). These hormones are critical for regulating metabolism, growth, and development in the body. They influence various physiological processes, including energy expenditure, heart rate, and the overall metabolic rate, making them essential for maintaining homeostasis.
Triiodothyronine: Triiodothyronine, commonly known as T3, is a thyroid hormone that plays a crucial role in regulating metabolism, growth, and development in the body. It is one of the main hormones produced by the thyroid gland, alongside thyroxine (T4), and is essential for maintaining energy levels and overall metabolic function. T3 influences many physiological processes, including heart rate, body temperature, and the rate at which the body uses fats and carbohydrates.
Up-regulation: Up-regulation is the process by which a cell increases the number of receptors for a particular hormone, enhancing its sensitivity to that hormone. This mechanism helps cells respond more effectively to hormonal signals during periods of increased demand.
V1 receptors: V1 receptors are a type of vasopressin receptor primarily found in vascular smooth muscle and the liver, playing a crucial role in regulating blood pressure and fluid balance. These receptors are activated by the hormone vasopressin (also known as antidiuretic hormone), leading to vasoconstriction and increased vascular resistance, which directly affects blood pressure levels. V1 receptors are an essential part of the body's response to dehydration and low blood volume, showcasing how hormones can influence physiological processes.
Vasopressin: Vasopressin, also known as antidiuretic hormone (ADH), is a peptide hormone that regulates water balance in the body by increasing water reabsorption in the kidneys. It is produced in the hypothalamus and released from the posterior pituitary gland.
Vasopressin: Vasopressin, also known as antidiuretic hormone (ADH), is a peptide hormone produced by the hypothalamus and released from the posterior pituitary gland. It plays a crucial role in regulating water balance and blood pressure, influencing how the kidneys manage water reabsorption and maintaining osmotic balance within the body.