37.2 How Hormones Work

Hormone Signaling Mechanisms

Hormones are the body's long-distance messengers. Secreted by endocrine glands, they travel through the bloodstream to reach target cells that carry the right receptors. Once a hormone binds its receptor, it kicks off a signaling cascade that amplifies the message and drives a specific cellular response. How that signal gets transmitted depends on whether the hormone can cross the cell membrane or not.

Hormone Binding and Cellular Responses

Hormones don't affect every cell in the body. They only act on target cells that express the matching receptor. This receptor specificity is what makes hormone signaling precise. For example, insulin receptors are found on muscle and liver cells, so those are the cells insulin acts on.

Here's the general sequence:

1. An endocrine gland secretes a hormone into the bloodstream.
2. The hormone travels to a target cell and binds its specific receptor.
3. Binding triggers a signaling cascade inside the cell, amplifying the original signal.
4. The cascade produces a cellular response, such as changes in gene expression, enzyme activity, or metabolism.

Using insulin as a concrete example: insulin binds receptors on muscle cells, which triggers increased glucose uptake and glycogen synthesis. The end result is lower blood glucose levels.

Intracellular vs. Cell Surface Receptors

The chemical nature of a hormone determines where it binds. This is one of the most important distinctions in endocrine signaling.

Intracellular receptors

• Found in the cytoplasm or nucleus of the target cell.
• Bind lipid-soluble hormones like steroid hormones (testosterone, estrogen) and thyroid hormones (triiodothyronine, or $T_3$).
• Because these hormones are lipid-soluble, they can diffuse directly through the plasma membrane to reach their receptor inside the cell.
• Once the hormone binds, the hormone-receptor complex acts as a transcription factor. It binds to specific DNA sequences and activates or represses target genes.
  • For example, the estrogen-receptor complex activates genes involved in female reproductive development.
• Because these hormones alter gene expression, their effects tend to be slower but longer-lasting.

Cell surface receptors

• Found on the plasma membrane of the target cell.
• Bind water-soluble hormones like peptide hormones (insulin, glucagon) and catecholamines (adrenaline/epinephrine).
• Water-soluble hormones can't cross the hydrophobic lipid bilayer, so they need a receptor on the outside of the cell.
• Binding activates intracellular signaling pathways that often rely on second messengers (covered below).
• These responses tend to be faster but shorter-lived compared to intracellular receptor signaling.

Second Messengers in Hormone Signaling

Second messengers solve a key problem: the hormone can't enter the cell, so something inside the cell has to carry the message forward. These are small, diffusible molecules that relay and amplify the signal from a surface receptor. A small number of activated receptors can generate a huge number of second messenger molecules, producing a large cellular response.

The three most common second messengers are:

• Cyclic AMP (cAMP)
• Calcium ions ($Ca^{2+}$)
• Inositol trisphosphate ($IP_3$)

The cAMP pathway

1. A water-soluble hormone (e.g., glucagon) binds a cell surface receptor.
2. The receptor activates the enzyme adenylyl cyclase.
3. Adenylyl cyclase converts ATP into cAMP.
4. cAMP activates protein kinase A (PKA).
5. PKA phosphorylates target proteins, changing their activity.
   • Example: PKA phosphorylates glycogen phosphorylase, which breaks down glycogen into glucose. This is how glucagon raises blood sugar.
   • PKA can also phosphorylate transcription factors like CREB, which then promotes expression of genes involved in glucose metabolism.

The $IP_3$/$Ca^{2+}$ pathway

1. A hormone (e.g., vasopressin) binds a cell surface receptor.
2. The receptor activates the enzyme phospholipase C.
3. Phospholipase C produces $IP_3$ (among other products).
4. $IP_3$ triggers the release of $Ca^{2+}$ from the endoplasmic reticulum into the cytoplasm.
5. Elevated $Ca^{2+}$ activates calcium-dependent enzymes like protein kinase C (PKC).
6. PKC phosphorylates target proteins, altering cellular activity.
   • Example: vasopressin binding to $V_1$ receptors on blood vessel smooth muscle activates PKC, which promotes smooth muscle contraction.

The key takeaway: second messengers allow water-soluble hormones to produce rapid, amplified, and specific responses without ever entering the cell.

Endocrine System and Hormone Regulation

The endocrine system maintains stable internal conditions through negative feedback. Here's how it works: when a hormone produces its target effect, that effect signals back to inhibit further hormone release. This prevents the hormone from overshooting.

For example, when insulin successfully lowers blood glucose, the drop in glucose signals the pancreas to reduce insulin secretion. The system self-corrects.

A few core principles to remember:

• Endocrine glands secrete hormones directly into the bloodstream (no ducts, unlike exocrine glands).
• Hormone synthesis occurs in specialized endocrine cells through specific biochemical pathways.
• Target cells must have the correct receptor to respond to a given hormone.
• Signal transduction pathways convert the hormone's signal into a precise cellular response, giving the body fine control over processes like metabolism, growth, and reproduction.