Lecture Notes: Communication by Chemical Messengers and Signal Transduction

These cards cover the key concepts of chemical signaling, receptor types, second messengers, signaling pathways, and regulatory mechanisms as described in the notes.

What are the two main types of physiological signals used for body communication?

Electrical signals and chemical signals (ligands)

What is resting membrane potential?

A polarized state where the inside of the cell is negative and the outside is positive

What determines which receptor a ligand binds to and elicits a response?

Receptor specificity—the receptor binds only specific ligands or a limited set of related ligands

What distinguishes hydrophobic (lipophilic) ligands in terms of receptor location and response?

Hydrophobic ligands pass through the plasma membrane and bind intracellular receptors (cytoplasm or nucleus), often leading to slower, sustained responses and changes in gene expression

What distinguishes hydrophilic (lipophobic) ligands in terms of receptor location and response?

Hydrophilic ligands cannot enter the cell; receptors are on the plasma membrane and generally produce faster, short-lived responses

What processes do intracellular receptors typically influence when activated by lipophilic ligands?

Gene expression via transcription factors that regulate mRNA synthesis and protein production

Which type of ligands typically causes faster responses, hydrophilic or hydrophobic, and why?

Hydrophilic ligands cause faster responses because they act at membrane receptors and trigger rapid signaling cascades

Give two examples of hydrophobic ligands.

Steroid hormones and thyroid hormones

Name a common class of membrane receptors for hydrophilic ligands.

Ion channel receptors, receptor-enzyme (tyrosine kinase) receptors, enzyme-associated (e.g., JAK) receptors, and G protein-coupled receptors (GPCRs)

How does an ion channel receptor work when a ligand binds?

The receptor acts as a channel and opens to allow specific ions to pass, changing the cell’s membrane potential and triggering a response

What is a classic example of an ion channel receptor?

Acetylcholine receptor (Ach receptor) on skeletal muscle

What happens in receptor-enzyme signaling, especially for tyrosine kinases?

In receptor-enzyme signaling (e.g., receptor tyrosine kinases), ligand binding activates the receptor’s intrinsic kinase activity, causing autophosphorylation and phosphorylation of substrates, which initiates downstream signaling cascades (e.g., insulin receptor)

Which cell type produces insulin and where is its receptor located?

Insulin is produced by beta cells of the pancreas; it binds to membrane-bound insulin receptors (tyrosine kinase receptors)

What distinguishes Type 1 and Type 2 diabetes?

Type 1: autoantibodies destroy beta cells, little/no insulin; Type 2: target cells are insulin resistant despite insulin presence

How does epinephrine cause diverse tissue responses?

Epinephrine acts on different receptors in different tissues, causing liver glucose release, smooth muscle contraction, increased heart rate, and skeletal muscle vasodilation

In GPCR signaling, what is the role of the alpha subunit after receptor activation?

The alpha subunit dissociates and activates an effector (adenylyl cyclase or phospholipase C) in the membrane

What is the key second messenger produced by adenylyl cyclase activation?

cAMP (cyclic AMP)

What cellular effects does cAMP have once activated?

Activates protein kinase A (PKA) and triggers various responses including metabolism, gene expression, and ion transport

How is the cAMP signal terminated?

Phosphodiesterase enzymes degrade cAMP to AMP, ending the signal

Describe the IP3/DAG pathway and its enzymes.

G protein activates phospholipase C, which cleaves PIP2 into IP3 and DAG; IP3 releases Ca2+ from the endoplasmic reticulum, while DAG activates PKC

What is the role of IP3 as a second messenger?

IP3 diffuses to the ER and triggers Ca2+ release into the cytosol

What is the role of DAG as a second messenger?

DAG remains in the membrane and activates protein kinase C (PKC) to propagate signaling

What is the role of calcium (Ca2+) as a second messenger?

Ca2+ activates calmodulin and various calcium-dependent kinases; it also participates in PKC pathways when coupled with DAG

What are eicosanoids and how are they formed?

Eicosanoids are signaling molecules derived from arachidonic acid; formed via cyclooxygenase (prostaglandins, thromboxanes) and lipoxygenase (leukotrienes) pathways

What are COX inhibitors and what do they do?

NSAIDs (e.g., ibuprofen, aspirin) inhibit cyclooxygenase (COX) to reduce prostaglandin and thromboxane production

What is NO and what is its signaling characteristic?

Nitric oxide (NO) is a short-lived paracrine/autocrine signaling molecule with local action

What is a gap junction and its importance?

Gap junctions are direct cytoplasmic connections between adjacent cells, enabling chemical and electrical signaling, crucial in heart and smooth muscles

Differentiate paracrine and autocrine signaling.

Paracrine signals act on neighboring cells; autocrine signals act on the same cell that secreted them

What is meant by long-distance signaling in the nervous and endocrine systems?

Neurotransmitters (chemical and electrical) act across synapses; hormones are secreted into blood to reach distant targets; neurohormones are released by neurons into blood

What is the difference between receptor saturation and receptor affinity?

Saturation is the fraction of receptors bound by ligand (0-100%), while affinity is the strength of ligand binding to the receptor; higher affinity means tighter binding

What is an antagonist and give an example?

An antagonist binds a receptor without triggering a response, blocking the natural ligand (e.g., antihistamines, beta blockers)

What is an agonist?

A molecule that binds and triggers the receptor’s response (e.g., morphine acting on endomorphin receptors)

What are up-regulation and down-regulation?

Up-regulation: increase in receptor number due to low ligand, leading to supersensitivity. Down-regulation: decrease in receptor number due to high ligand, reducing responsiveness

Describe the insulin secretion mechanism from beta cells in response to glucose.

Glucose metabolism raises ATP, closing ATP-sensitive K+ channels, depolarizing the membrane, opening voltage-gated Ca2+ channels, Ca2+ influx triggers exocytosis of insulin

What is transcription and translation in the context of ligand effects?

Transcription converts DNA to mRNA; translation uses the mRNA to synthesize proteins; lipophilic ligands often influence this via intracellular receptors

What is meant by signal amplification in signal transduction?

One ligand can trigger a cascade that produces many downstream molecules, amplifying the response