Hormone Receptors: Examples, Notes, Receptors And Mechanism

What Are Hormone Receptors?

Hormone receptors are specific protein molecules either on the surface or inside the target cells. They bind to specific hormones and bring about a series of cellular events that lead to a physiological response. Each receptor is specific to any particular hormone, providing for targeted and effective action of the hormones.

Examples Of Hormone Receptors:

Insulin Receptors:

These receptors are located on the surface of cells and bind to insulin to facilitate glucose uptake.

Estrogen Receptors:

Found in the nucleus and associated with estrogen for transcriptional regulation of genes related to reproductive functions and secondary sexual characteristics.

Adrenergic Receptors:

These are cell surface receptors that associate with adrenaline or epinephrine and execute the fightorflight response.

Classification Of Hormone Receptors

Hormone receptors are classified according to their location and the type of hormone they associate with. This includes:

Cell Surface Receptors:

Location: Integrated into the plasma membrane of target cells.

Function: To interact with watersoluble hormones such as peptide hormones and amino acidderived hormones.

Mechanism: The binding of a hormone brings about a signal transduction pathway via second messengers, leading to cellular responses.

Intracellular Receptors:

Location: Found within the cytoplasm or the nucleus of target cells.

Function: Bind to lipidsoluble hormones such as steroid and thyroid hormones.

Mechanism: The hormones diffuse across the cell membrane and find some intracellular receptors that, in turn, affect gene transcription and protein synthesis.

Mechanism Of Action

Hormones themselves do not exert their effects on target cells but trigger all of their effects via receptormediated mechanisms:

Cell Surface Receptors:

1. Binding:

The hormone binds to the receptor on the cell surface.

2. Signal Transduction:

Binding activates the signal transduction pathway with second messengers.

3. Cellular Response:

The second messengers initiate a cascade of events to produce the cellular response.

Intracellular Receptors:

1. Diffusion:

Lipidsoluble hormones diffuse through the plasma membrane.

2. Binding:

Hormones bind to receptors within the cytosol or nucleus.

3. Gene Regulation:

The hormonereceptor complex acts on DNA to initiate gene transcription and the subsequent synthesis of proteins.

4. Cellular Response:

The newly synthesized proteins mediate their desired physiological responses.

Control Of Hormone Receptors

The number and affinity of hormone receptors can be controlled under various conditions. For example,

Receptor Sensitivity:

The cells through positive/negative feedbacks modulate the sensitivity of receptors to these circulating hormones, thereby altering the strength of the response.

Receptor Density:

The number of receptors may increase or decrease, thus changing the cell's sensitivity to the hormone.

Desensitization:

When a hormone is continuously present, the receptor becomes desensitized to the presence of the hormone, and a decreased cellular response ensues.

Dynamics Of Hormone receptor Interactions

The dynamics of hormone receptor interactions are mentioned below:

ReceptorBinding Affinity:

Definition: It is the strength with which a hormone binds to its receptor.

Impact: The strength of binding has a huge effect; high-affinity binding results in more effective signalling, while low-affinity binding may lead to weak responses.

Receptor Agonists And Antagonists:

Agonist:

A molecule binds to the receptor to produce an effect similar to that of the hormone.

Antagonist:

A molecule that binds to the receptor but may block or dampen the effect of the hormone.

Receptor Internalization And Downregulation:

Internalization:

The hormone receptor complex is internalized into the cell. This process generally leads to the cessation of signalling. This mechanism controls the duration and amplitude of the hormonal response.

Downregulation:

It occurs in response to continuous exposure to a hormone by reducing the quantity of the receptors or by decreasing their sensitivity, thus diminishing cellular responsiveness and preventing overstimulation of the cells.

Mutations Of Hormone Receptors

Genetic mutations:

Consequence: This may alter hormone binding or signalling of the respective receptors, leading to a panoply of endocrine disorders.

Examples: Insulin receptors have mutations that confer insulin resistance; those in thyroid hormone receptors disturb thyroid function.

Clinical Implications

Diagnosis:

The identification of receptor mutations helps diagnose certain endocrine disorders.

Treatment:

Targeted therapies may have to be instituted to address receptor mutations and to reconstitute normal function.

Crosstalk Between Receptor Pathways

Pathway Interactions:

Definition: The interaction of different signalling pathways that may modify the activity of hormone receptors.

Examples: Crosstalk between insulin signalling and growth factor pathways can impact metabolic and growth responses, respectively.

Functional Implications:

Synergistic Effects: Augmented or changed physiological responses because of pathway interactions

Antagonistic Effects: The effect of one pathway is inhibited by another.

Conclusion

Receptor proteins play a vital role in the specificity and, therefore, in the efficiency of the action of hormones. They transduce the message in a wide range of physiological responses. Their regulation and function are thus important for homeostasis and the response to environmental change.

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