Study Notes on Chemical Messengers

Definition: Gap junctions are specialized channels that allow for direct communication between neighboring cells.
Components:
- Cell 1 and Cell 2: Two adjacent cells connected via a channel called Connexon.
- Function: Allow ions and small molecules to pass directly from one cell to another, facilitating the synchronization of cellular activities.

Chemical Messengers: Substances secreted by a cell to convey signals to target cells.
Components:
- Secretory Cell: The cell that releases the chemical messenger.
- Receptor: A protein on the surface of the target cell that binds to the chemical messenger.
- Target Cell: The cell that receives the message, leading to a biological response.

Definition: Paracrines are a type of chemical messenger that act locally on neighboring cells.
Components:
- Secretory Cell: Releases the paracrine.
- Target Cell: The neighboring cell with receptors for the paracrine.
- Mechanism: Signaling molecules diffuse across a short distance to reach target cells.

Definition: Neurotransmitters are chemicals used for communication between neurons.
Components:
- Axon Terminal: End of a neuron where neurotransmitters are released.
- Synapse: The gap between the presynaptic neuron (secretory cell) and the postsynaptic neuron (target cell).
- Mechanism: Neurotransmitters are released into the synapse and bind to receptors on the target cell to initiate a response.

Definition: Hormones are long-distance signaling molecules secreted into the bloodstream.
Components:
- Secretory Cell/Endocrine Cell: The cell that produces and releases hormones.
- Blood Vessel: Carries hormones to their target cells.
- Nontarget Cell: Cells that do not have receptors for specific hormones (thus have no response).
- Mechanism: Hormones circulate through the blood and bind to specific receptors on target cells.

Structure: Derivatives of the amino acid Tyrosine and include neurotransmitters like Dopamine, Norepinephrine, and Epinephrine.
Synthesis Steps:
- Tyrosine is converted to L-Dihydroxyphenylalanine (L-Dopa) by the enzyme Tyrosine Hydroxylase.
- Dopamine is formed from L-Dopa via Dopa Decarboxylase.
- Norepinephrine is synthesized from Dopamine via Dopamine β-Hydroxylase.
- Epinephrine is produced from Norepinephrine by the enzyme Phenylethanolamine N-Methyl Transferase (PNMT).

Process Overview:
- Prepropeptide is synthesized in the rough endoplasmic reticulum (RER).
- This molecule is then processed into a Propeptide and transported to the Golgi apparatus.
- After additional modifications, the final Peptide Hormone is formed and stored in secretory vesicles until release.
Example:
- The synthesis of Parathyroid hormone (PTH) involves the cleavage of the Preproparathyroid hormone into active Parathyroid hormone (84 amino acids).

Source: Derived from Cholesterol.
Key Intermediates: Includes various nonactive intermediates leading to active steroid hormones such as Cortisol, Aldosterone, and Testosterone.
Pathway: Starting from Cholesterol, it is converted through different pathways to generate cortisol, aldosterone, and sex steroids.

Pathways: Eicosanoids are derived from membrane phospholipids via two primary pathways—Cyclooxygenase and Lipoxygenase.
Types of Eicosanoids:
- Cyclooxygenase Pathway produces Prostaglandins, Prostacyclins, and Thromboxanes.
- Lipoxygenase Pathway produces Leukotrienes.

Characteristics: Secreted by exocytosis and dissolve in blood.
Mechanism: Typically bind to surface receptors on target cells to initiate responses.

Characteristics: Secreted via diffusion, often bound to carrier proteins in the blood (over 99% bound).
Example: Steroid hormones that pass through cell membranes to bind intracellular receptors.

Concept: The specificity of receptors ensures that only target cells with appropriate receptors respond to a given messenger.
Example: Receptor A may bind Messenger 1 while Receptor B binds Messenger 2, indicating selective interaction.

Graph Analysis: Reflects the percentage of receptors occupied based on the concentration of the messenger ( $[M]$ ).
Significance: The concentration influences the biological effect of the messenger on target cells.

Receptor Concentration: The number of receptors present can affect the response; higher concentrations can lead to more binding.
Receptor Affinity: Higher affinity receptors bind messengers more tightly while lower affinity may require a higher concentration for effective binding.
Graph Analysis: Illustrated effects of varying affinities and concentrations on receptor binding.

Mechanism: Lipophilic hormones often diffuse through membranes and bind to cytoplasmic receptors, translocating to the nucleus to influence gene expression.
Process:
1. Hormone enters the cell.
2. Binds to a cytosolic receptor, forming a hormone-receptor complex.
3. Complex translocates to the nucleus, interacts with DNA, leading to mRNA synthesis.
4. mRNA is translated into proteins.

Transcription Activation: Binding of steroid hormones to their receptors activates genes that lead to various cell responses.
Dimerization: Steroid hormones form dimers upon binding which increases binding affinity to DNA.

Function: Serve as ion channels that can open or close in response to messenger binding, allowing ions such as Na+, K+, and Cl- to change the electrical properties of the cell.

Role in Cellular Response: Rapidly open channels allow for calcium influx, affecting muscle contraction, secretion, and metabolism through interactions with proteins such as Calmodulin.

Mechanism: Receptors with intrinsic kinase activity trigger phosphorylation of substrate proteins in response to messenger binding, leading to downstream cellular effects, such as altered metabolism or protein synthesis.

Function: G proteins relay signals from neurotransmitter receptors to effector proteins, influencing cellular activities.
Mechanism: Upon activation by a messenger, G proteins exchange GDP for GTP, leading to various intracellular changes in electrical properties and other signaling cascades.

Types: Involves both electrical and chemical signaling for communication between neurons or between neurons and other target cells.
Components:
- Neuron: The signaling cell that releases neurotransmitters.
- Target Cell: The cell that receives the neurotransmitter.
- Synapse: The junction where the signal is transmitted, involving neurotransmitter release and receptor interaction.