7 Hormones

Hormones

Definition:
- Hormones are chemical signaling molecules, usually proteins or steroids, secreted by endocrine cells.
- They regulate specific physiological processes in animals; plants and fungi also have hormones which are different in structure.
Endocrinology:
- The study of the endocrine system in animals.
- Involves glands that release hormones for internal chemical communication.
Chemical Communication:
- One of the two major communication types within an organism (the other being the nervous system).
- Endocrine and nervous systems can interact through neuroendocrine signaling.

Consists of glands and cells that release hormones into bodily fluids (blood, interstitial fluid).
Fat tissue may have endocrine functions.
Some organs (like the pancreas) have both endocrine (insulin, glucagon) and non-endocrine (producing digestive enzymes) functions.

General Rule:
- The nervous system is involved in instantaneous communication (e.g., reflex actions).
- The endocrine system operates slower but can have quick responses (e.g., adrenalin).
Many endocrine effects are long-term, such as developmental control of multicellular organisms.
Example: Amphibian metamorphosis driven by hormones.

Hormone: A circulating chemical signal of a multicellular organism, formed in specialized cells, affecting distant parts of the organism at low concentrations.
Insulin: A peptide hormone regulating blood glucose, composed of two linked polypeptide chains (quaternary structure).

Peptide Hormones:
- Composed of linked amino acids (e.g., insulin).
- Typically hydrophilic and larger than hormones derived from cholesterol.
Steroid Hormones:
- Synthesized from cholesterol, based on a steroid backbone.
- Hydrophobic lipids (e.g., testosterone, estradiol).
Amine Hormones:
- Derived from single amino acids (e.g., epinephrine from tyrosine, melatonin from tryptophan).
- Majority are hydrophilic, except thyroid hormones which are hydrophobic.

Hormones are made in specialized cells of specific organs and secreted through exocytosis into bodily fluids.
Types of cells involved:
1. Endocrine Cells: Part of ductless endocrine glands, releasing hormones into extracellular spaces.
2. Neurosecretory Cells: Specialized nerve cells that release hormones into bodily fluids upon activation (e.g., cells in hypothalamus).

Hormones circulate in bodily fluids but only affect target cells with corresponding receptors.
Hormone Receptors: Proteins located either on the plasma membrane or inside the cell (cytosol or nucleus).
- Specificity is crucial; a cell must have the correct receptor to respond.

For Hydrophilic Hormones:
- Cannot diffuse through the plasma membrane; bind to receptors on the outside.
- Example: Epinephrine activates glycogen breakdown via a distinct signaling pathway.
For Steroid Hormones:
- Hydrophobic; can cross the plasma membrane and bind to intracellular receptors, leading to gene transcription.

Diffusion of the hormone into cytosol.
Binding with receptor to form a hormone-receptor complex.
The complex enters the nucleus and activates transcription of specific genes, resulting in protein synthesis.

Epinephrine binds to its receptor, activating G protein.
G protein activates adenylyl cyclase, converting ATP to cyclic AMP (cAMP).
cAMP activates protein kinases, leading to enzyme activations through phosphorylation, eventually resulting in glycogen breakdown.

The signaling pathway allows for amplification of response, requiring few hormone molecules for a significant effect (e.g., glycogen breakdown).
Mechanisms exist to turn off hormonal effects, preventing continuous activation.