Chapter 5 Chemical messengers

Chapter 5: Chemical Messengers

Intercellular Communication

  • Mechanisms for communication between cells.

  • Two modes of communication: Direct and Indirect.

  • Involves the sharing of information and responses between cells.

Direct Communication

  • Gap Junctions:

    • Involves connexins and connexons.

    • Communicates ions and small molecules.

    • Examples: Heart muscle, smooth muscle, neurons.

    • Response: Synchronizes function and electrically couples cells.

    • Structure:

      • 1 connexon in a cell equals a hemichannel.

      • Two hemichannels (from two cells) form a gap junction.

Indirect Communication

  • Transport via Interstitial Fluid:

    • Dependent on concentration of the messenger, number of receptors, sensitivity, and receptor identity.

    • Functional classes of chemical messengers include:

      • Paracrine: Acts on nearby cells.

      • Autocrine: Acts on the same cell that secretes it.

      • Neurotransmitters: Released by neurons (e.g. acetylcholine).

      • Hormones: Released into the bloodstream.

Paracrine and Autocrine Chemical Messengers

  • Paracrine:

    • Diffuses through extracellular fluid to target nearby cells.

    • Examples: Growth factors, cytokines, clotting factors.

  • Autocrine:

    • Secreted by a cell and acts on itself.

Neurotransmitters

  • Released by neurons at the axon terminal.

  • Diffuse across the synapse to postsynaptic cells.

  • Example: Acetylcholine impacts skeletal muscle.

Hormones

  • Synthesized and released by endocrine cells into interstitial fluid.

  • Move through blood to target cells (only those with receptors respond).

  • Example: Insulin from pancreas impacts insulin receptors in various cells.

  • Neurohormones: Secreted by specialized neurons (Example: Vasopressin).

Chemical Classification of Messengers

  • Characteristics:

    • Lipophilic: Lipid soluble; crosses membranes; cannot be freely transported in plasma.

    • Lipophobic: Water soluble; requires transport mechanisms; can be transported in plasma.

Types of Chemical Messengers

  • Amino Acid Messengers:

    • Lipophobic/hydrophilic. (e.g., Glutamate, GABA)

    • Synthesized in neurons and stored in vesicles.

  • Amine Messengers:

    • Primarily lipophobic/hydrophilic (e.g., Dopamine, Serotonin).

    • Derived from amino acids; synthesized in secretory cells.

  • Peptide/Protein Messengers:

    • 2-100 amino acids; lipophobic/hydrophilic; synthesized in RER.

  • Steroid Messengers:

    • Lipophilic/hydrophobic; derived from cholesterol; synthesized on demand.

  • Eicosanoid Messengers:

    • Lipophilic/hydrophobic; derived from phospholipids; synthesized on demand.

Transport Mechanisms for Messengers

  • Hydrophilic Messengers:

    • Freely soluble in interstitial fluid.

    • Released near target cells and degrade quickly.

    • Hormones and peptides may bind to carrier proteins for transport.

  • Hydrophobic Messengers:

    • Not very soluble; often bound to carrier proteins.

    • Example: Steroid hormones, thyroid hormones.

Receptors for Chemical Messengers

  • Located in the plasma membrane, cytosol, or nucleus.

  • Properties of messenger-receptor interactions:

    • Specific, transient, reversible.

    • A single messenger can bind to multiple receptors; a cell can respond to many messengers.

Factors Influencing Target Cell Response

  • Concentration of the messenger.

  • Number and affinity of receptors.

  • Higher receptors increase chance of binding.

  • Receptor Agonists: Induce a response upon binding.

  • Receptor Antagonists: Block responses by competing with agonists.

    • Example: Naloxone as an opioid antagonist.

Target Cell Responses

  • Lipophilic Messengers:

    • Freely enter cytosol; bind to intracellular receptors.

    • Initiate transcription and translation processes, leading to delayed cellular responses.

  • Lipophobic Messengers:

    • Trigger rapid cellular responses through channel-linked receptors, enzyme-linked receptors, or G-protein linked receptors.

    • Examples include acetylcholine and insulin interactions.