Signaling Molecules

Signaling Molecules (Ligands)

Released by signal-producing cells that transfer biological signals to target cells, initiating specific responses; act via autocrine, paracrine, or endocrine mechanisms.

First Messenger

The extracellular signaling molecule that binds to a receptor to begin a signaling pathway.

Extracellular First Messengers

Categories include proteins/glycoproteins/peptides (hormones, cytokines), amino acids & derivatives (gly, glu, adrenaline, thyroxine), lipids (sex hormones, glucocorticosteroids), prostaglandins, nucleosides/nucleotides (ATP, GTP), and gases (NO, CO).

Cellular Response Specificity

Each cell responds only to specific combinations of extracellular signal molecules determined by its receptor set and intracellular machinery.

Apoptosis

Programmed cell death that occurs when a cell is deprived of essential survival signals.

Signal Specificity

The same signaling molecule can generate different responses in different cells due to variations in receptors and intracellular machinery.

Neurotransmitters

Chemical messengers produced by neurons that relay nerve impulses between cells; act via synaptic signaling; bind to postsynaptic membranes to produce excitatory or inhibitory responses.

Hormones

Extracellular signaling molecules produced by endocrine glands that regulate growth, differentiation, and metabolism through endocrine signaling.

Amine Hormones

Include thyroxine, epinephrine, and melatonin; derived from amino acids.

Peptide and Protein Hormones

Include somatostatin, oxytocin, vasopressin, insulin, erythropoietin, TSH, FSH, somatotropin, EGF, ACTH, CRH, LH, and parathormone.

Steroid Hormones

Sex hormones (progesterone, estrogen, testosterone) and corticosteroids (glucocorticoids, mineralocorticoids); synthesized from cholesterol.

Ecdysone

Insect steroid hormone triggering metamorphosis from larva to adult.

Hydrophobic Hormones

Include steroid hormones, thyroid hormones, and vitamin D3; can cross membranes and act on intracellular receptors.

Hydrophilic Hormones

Include peptide hormones (e.g., insulin) and small charged molecules (e.g., epinephrine); act on cell surface receptors via second messengers.

Steroid Hormone Action

Steroids enter cells, bind cytoplasmic/nuclear receptors, expose DNA-binding sites, and regulate gene transcription—producing long-lasting effects.

Peptide Hormone Action

Peptides bind to surface receptors, activate second messengers, and trigger enzyme cascades—producing short-term effects.

Primary vs Secondary Response Genes

Primary-response proteins activate secondary-response genes; inhibitors of protein synthesis block secondary but not primary responses.

Carrier Proteins for Steroid Hormones

Bind hydrophobic hormones for transport in blood; e.g., albumin, which also buffers pH and maintains osmotic pressure.

Neurotransmitters vs Hormones

Neurotransmitters are small, water-soluble molecules for synaptic signaling; hormones are larger molecules for endocrine signaling.

Epinephrine

Dual-role molecule functioning as a neurotransmitter (CNS) and hormone (adrenal gland).

Eicosanoids

Locally acting biologically active lipids derived from arachidonic acid; include prostaglandins, prostacyclin, thromboxanes, and leukotrienes.

Eicosanoid Function

Regulate blood platelet aggregation, inflammation, and smooth muscle contraction; act in autocrine/paracrine manner.

Arachidonic Acid

20-carbon fatty acid precursor of eicosanoids; formed by phospholipase A2 hydrolysis of membrane phospholipids.

Eicosanoid Pathways

COX pathway → prostaglandins, prostacyclin, thromboxanes; LOX pathway → leukotrienes.

Nitric Oxide (NO)

A gaseous paracrine signaling molecule produced from arginine by nitric oxide synthase (NOS); acts locally due to short half-life.

NO in Blood Vessel Dilation

NO synthesized in endothelial cells diffuses to smooth muscle, causing relaxation and vasodilation; nitroglycerin works by releasing NO.

NO Mechanism Triggered by Neurotransmitters

Acetylcholine from nerve endings stimulates endothelial NO production, leading to muscle relaxation and increased blood flow.

Carrier Example: Albumin

Main transport protein for hormones and fatty acids in blood; buffers pH and maintains osmotic balance.

Long-lasting vs Short-term Responses

Steroid hormones alter gene expression (long-lasting); peptide hormones act through second messengers (short-term).

Hormone Solubility Summary

Hydrophobic hormones bind intracellular receptors; hydrophilic hormones bind cell surface receptors.