• Calcium enters the cell when:
  • A change in membrane potential opens calcium channels, or
  • A hormone binds to a membrane receptor that opens calcium channels.
• Once inside, Ca²⁺ binds to calmodulin, a protein with 4 calcium-binding sites.
• When 3 or 4 sites bind Ca²⁺, calmodulin changes shape and becomes active.
• Active calmodulin can produce several effects, including activating or inhibiting protein kinases.
• Calmodulin-dependent protein kinases phosphorylate proteins, which can activate or inhibit the proteins responsible for the cell’s response to the hormone.
• One important example is myosin light chain kinase, which calmodulin activates.
  • This kinase acts on myosin in smooth muscle.
  • The result is smooth muscle contraction (Fig. 8.3).
• Normally, intracellular Ca²⁺ is only 10⁻⁸ to 10⁻⁷ mol/L, which is too low to activate calmodulin.
• When Ca²⁺ rises to 10⁻⁶ to 10⁻⁵ mol/L, enough Ca²⁺ binds to calmodulin to activate its intracellular effects.
• This calcium change is almost the same as the change needed to activate troponin C and cause skeletal muscle contraction.
• Troponin C and calmodulin are similar in both their function and protein structure.

KEY CONCEPT

Ca²⁺ enters → binds calmodulin → calmodulin changes shape → activates/inhibits protein kinases → proteins are phosphorylated → cell response occurs.

CONCEPTUAL EXAMPLES

• Smooth muscle: Ca²⁺ → calmodulin → myosin light chain kinase → contraction.
• Skeletal muscle: Ca²⁺ activates troponin C → contraction.

HORMONES THAT ACT MAINLY ON THE GENETIC MACHINERY OF THE CELL

Steroid Hormones Increase Protein Synthesis

• Steroid hormones mainly act by causing new protein synthesis in target cells.
• These new proteins can act as:
  • Enzymes
  • Transport proteins
  • Structural proteins
• These proteins then produce the hormone’s effects in the cell.
• The sequence of steroid hormone action is as follows (Fig. 75.6):
  1. Steroid hormone crosses the cell membrane and enters the cytoplasm.
  2. It binds to a specific receptor protein.
  3. The hormone–receptor complex enters the nucleus.
  4. It binds to specific sites on DNA, activating specific genes to produce mRNA.
  5. The mRNA moves into the cytoplasm and directs ribosomes to make new proteins.
• Example: Aldosterone
  • Aldosterone enters renal tubular cells and binds to the mineralocorticoid receptor.
  • This starts the same sequence of events.
  • After about 45 minutes, new proteins begin to appear.
  • These proteins increase sodium reabsorption from the tubules and potassium secretion into the tubules.
  • Therefore, steroid hormone effects are usually slow, beginning after at least 45 minutes and sometimes taking hours or days.
• This is different from some peptide and amino acid–derived hormones, such as vasopressin and norepinephrine, which can act much more rapidly.

KEY CONCEPT

Steroid hormone → cell membrane → receptor → nucleus → DNA → mRNA → ribosome → new proteins → cell response.

CONCEPTUAL EXAMPLES

• Aldosterone → renal tubular cell → new proteins → ↑ Na⁺ reabsorption + ↑ K⁺ secretion.
• Steroid hormones: slower action because they must change gene activity and make new proteins.
• Vasopressin/norepinephrine: can produce rapid effects compared with steroid hormones.

Thyroid Hormones Increase Gene Transcription in the Cell Nucleus

• The thyroid hormones thyroxine (T₄) and triiodothyronine (T₃) increase transcription of specific genes in the cell nucleus.
• These hormones bind directly to receptor proteins inside the nucleus (Fig. 77.5).
• These receptors act as transcription factors within the chromosomes and control gene promoters.
• Thyroid hormone action in the nucleus has two important features:
  1. They activate genes that produce many types of intracellular proteins, probably 100 or more.
  2. Many of these proteins are enzymes, which increase metabolic activity in almost all cells of the body.
• After thyroid hormones bind to their intranuclear receptors, their effects can continue for days or even weeks.

KEY CONCEPT

T₃/T₄ → nuclear receptor → increased gene transcription → more proteins/enzymes → increased cellular metabolism.

CONCEPTUAL EXAMPLES

• T₃/T₄ + nuclear receptor → gene activation → enzyme production → ↑ cellular metabolism.
• Long-lasting effect: once bound to nuclear receptors, thyroid hormones can control cell function for days to weeks.

Measurement of Hormone Concentrations in the Blood

• Hormones are present in the blood in extremely tiny amounts, sometimes as low as 1 picogram/mL.
• Because these amounts are so small, ordinary chemical methods were difficult to use.
• Radioimmunoassay (RIA) was developed by Rosalyn Yalow and Solomon Berson in 1959, making measurement of tiny amounts of hormones much easier.
• Other methods, such as enzyme-linked immunosorbent assays, are also used for accurate, high-throughput hormone measurement.

Radioimmunoassay

• First, a highly specific antibody against the hormone being measured is produced.
• A small amount of this antibody is mixed with:
  • The fluid containing the natural hormone being measured.
  • A known amount of purified radioactive hormone.
• There must be too little antibody to bind both hormones completely, so the natural hormone and radioactive hormone compete for the antibody’s binding sites.
• The amount of each hormone that binds depends on its concentration in the fluid.
• After binding reaches equilibrium, the antibody–hormone complex is separated from the remaining solution.
• The amount of radioactive hormone bound to the antibody is then measured.
• Large amount of radioactive hormone bound → small amount of natural hormone was present.
• Small amount of radioactive hormone bound → large amount of natural hormone was present, because it competed more strongly for antibody binding sites.
• For accurate measurement, the same procedure is performed using standard solutions containing known hormone concentrations.
• These results are used to make a standard curve (Fig. 75.9).
• The radioactive count from the unknown sample is compared with this curve to determine its hormone concentration.
• The method can measure hormone concentrations with an error of about 10%–15%.
• It can detect amounts as tiny as billionths or even trillionths of a gram.

KEY CONCEPT

Natural hormone + radioactive hormone compete for limited antibody sites → measure radioactive hormone bound → compare with standard curve → determine natural hormone concentration.

CONCEPTUAL EXAMPLES

• More natural hormone → more competition → less radioactive hormone binds.
• Less natural hormone → less competition → more radioactive hormone binds.

Aldosterone Radioimmunoassay Graph

Big idea:
👉 More aldosterone in the sample = less radioactive aldosterone can bind to the antibody.

Think of the antibody as having limited seats 🪑.

• Radioactive aldosterone = labeled passenger
• Patient/sample aldosterone = unlabeled passenger
• Antibody = limited seats

They compete for the same seats.

📊 Understand every part

Y-axis: “% of antibody bound with radioactive aldosterone”

→ How much of the radioactive aldosterone is attached to the antibody.

• 100% = lots of radioactive aldosterone is bound
• 0% = almost none is bound

X-axis: “Aldosterone concentration in test sample (ng/dL)”

→ How much unlabeled aldosterone is present in the sample.

From left → right:

2 → 4 → 8 → 16 → 32 → 64 → 128 ng/dL

So, aldosterone concentration is increasing.

🔴 What does the downward curve mean?

At 2 ng/dL → about 94% radioactive aldosterone is bound.

At 8 ng/dL → about 72% is bound.

At 32 ng/dL → about 41% is bound.

At 128 ng/dL → only about 15% is bound.

⭐ WHY?

Because increasing sample aldosterone competes with radioactive aldosterone for antibody binding sites.

More sample aldosterone ↑
↓
More competition ↑
↓
Less radioactive aldosterone binds ↓
↓
% radioactive aldosterone bound decreases ↓

🔥 The most important concept

RIA is an inverse relationship:

Sample aldosterone ↑ → radioactive aldosterone bound ↓

Therefore:

Less radioactivity bound = MORE aldosterone in the patient’s sample.

🧠 One-line memory trick

“More real aldosterone kicks out more radioactive aldosterone.” 🚪

So this graph is a standard curve used to determine an unknown aldosterone concentration by looking at how much radioactive aldosterone remains bound.

Enzyme-Linked Immunosorbent Assay

• ELISA can measure almost any protein, including hormones.
• It combines:
  • Specificity of antibodies
  • Sensitivity of enzyme assays
• ELISA is often performed in plastic plates with 96 small wells (Fig. 75.10).
• Each well is coated with AB1, an antibody specific for the hormone.
• The sample or standard is added to the well.
• Then AB2 is added, which also recognizes the hormone but binds to a different site.
• Next, AB3 is added; it recognizes AB2 and is attached to an enzyme.
• The enzyme changes a suitable substrate into a detectable product, measured by color or fluorescence.
• Each enzyme molecule can produce thousands of product molecules, so even a small amount of hormone can be detected.
• Unlike competitive radioimmunoassay, ELISA uses excess antibodies, so essentially all hormone molecules are captured in antibody–hormone complexes.
• Therefore, more hormone → more product formed.
• ELISA is widely used because:
  1. It does not use radioactive isotopes.
  2. 96-well plates allow much of the test to be automated.
  3. It is cost-effective and accurate for measuring hormone levels.

KEY CONCEPT

Hormone captured by antibodies → enzyme attached → substrate converted to detectable product → more hormone = more product.

CONCEPTUAL EXAMPLES

• Small hormone amount → small amount of product → weak signal.
• Large hormone amount → more product → stronger signal.

This is Guyton Figure 75.10: Enzyme-Linked Immunosorbent Assay (ELISA).
ISA is a laboratory test used to measure how much hormone is present in a sample.

Think of it as a hormone-catching + color-making test.

The basic idea:

More hormone → more enzyme → more colored product → stronger signal

That is the key concept.

🎯 FIRST: WHAT ARE WE TRYING TO FIND?

We want to know:

How much hormone (H) is in the patient’s sample?

The hormone may be present in a very small amount, so we use antibodies to find it.

🧩 UNDERSTAND EVERY PART OF THE FIGURE

1️⃣ AB₁ = First antibody 🪤

At the bottom of the well are many:

AB₁ antibodies

These antibodies are fixed to the plastic surface of the well.

Think of AB₁ as:

A trap attached to the floor.

Its job is to catch the hormone.

2️⃣ H = Hormone 🎯

The patient’s sample contains:

H = Hormone

When the sample is added:

Hormone binds to AB₁

So:

AB₁ 🪤 + H → AB₁—H

The hormone is now captured.

3️⃣ AB₂ = Second antibody 🔎

Next, another antibody is added:

AB₂

AB₂ recognizes the hormone at a different binding site.

So now:

AB₁ ↓Hormone ↓AB₂

The hormone is literally sandwiched between two antibodies.

That’s why this is often called a:

Sandwich-type ELISA

Think:

🥪 Antibody — Hormone — Antibody

4️⃣ AB₃ = Antibody against AB₂

Now comes the clever part.

We add:

AB₃

AB₃ recognizes and binds to:

AB₂

So the structure becomes:

AB₁ ↓H ↓AB₂ ↓AB₃

5️⃣ E = Enzyme 🧪

AB₃ has an:

Enzyme (E)

attached to it.

So now we have:

AB₃ + enzyme

The enzyme is going to create a colored product.

This is what allows us to detect the hormone.

6️⃣ S = Substrate

Now we add:

S = Substrate

The enzyme acts on the substrate.

Think:

Substrate = raw material

The enzyme converts it into:

P = Product

And the product is colored/fluorescent.

7️⃣ P = Colored Product 🌈

The figure shows:

S → P

where:

S = substrate

P = colored product

The enzyme converts the colorless/less detectable substrate into a detectable product.

Then we measure the amount of this product using an optical instrument.

🔥 THE MOST IMPORTANT RELATIONSHIP

The figure tells you:

Amount of colored product ∝ Amount of hormone

In simple words:

More hormone captured → more antibody complex → more enzyme → more colored product.

Therefore:

🌈 More color = more hormone

🌈 Less color = less hormone

provided the antibodies are in excess, as Guyton specifically states.

🧠 FOLLOW THE FIGURE FROM BOTTOM TO TOP

Let’s read every level:

AB₁ ↓Catches HORMONE ↓H ↓AB₂ ↓AB₃ ↓ENZYME (E) ↓SUBSTRATE (S) ↓COLORED PRODUCT (P) ↓MEASURE COLOR ↓CALCULATE HORMONE AMOUNT

That’s ELISA.

🥪 THE EASIEST STORY

Imagine a sandwich shop.

AB₁ = bottom bread 🍞

It catches the hormone.

H = filling 🥩

The hormone sits on AB₁.

AB₂ = top bread 🍞

It attaches to another part of the hormone.

So:

AB₁ — H — AB₂

Then:

AB₃ = label 🏷️

AB₃ attaches to AB₂.

E = color-producing machine 🎨

The enzyme attached to AB₃ makes color.

Therefore:

Find the color → estimate the amount of hormone.

🔬 WHY USE THREE ANTIBODIES?

This is a common point of confusion.

AB₁

Catches the hormone.

AB₂

Recognizes the hormone at another site.

AB₃

Recognizes AB₂ and carries the enzyme.

So memorize:

AB₁ catches → AB₂ identifies → AB₃ brings enzyme

🚨 DON’T CONFUSE AB₂ AND AB₃

AB₂ binds:

Hormone

AB₃ binds:

AB₂

Very important.

🎯 WHAT DOES THE ENZYME ACTUALLY DO?

The enzyme does not directly measure the hormone.

Instead:

Enzyme + substrate → colored product

Then we measure the color.

So the chain is:

Hormone → Enzyme → Color

The color is our indirect measurement of hormone concentration.

📈 WHY DOES MORE HORMONE PRODUCE MORE COLOR?

Suppose sample A has:

10 hormone molecules

Only 10 hormone molecules get captured → fewer antibody complexes → fewer enzymes → less product.

Sample B has:

100 hormone molecules

More hormone gets captured → more complexes → more enzymes → more product.

Therefore:

More hormone → more color

🧠 THE ENTIRE FIGURE IN 10 SECONDS

AB₁ catches hormone

⬇️

AB₂ binds hormone

⬇️

AB₃ binds AB₂

⬇️

AB₃ carries enzyme

⬇️

Enzyme converts S → P

⬇️

P gives color

⬇️

Measure color

⬇️

Color tells us hormone concentration

⭐ HIGH-YIELD EXAM TABLE

Component	Job
H	Hormone being measured
AB₁	Captures hormone
AB₂	Binds hormone at another site
AB₃	Binds AB₂
E	Enzyme attached to AB₃
S	Substrate for enzyme
P	Colored/fluorescent product
Optical measurement	Measures product/color
Final purpose	Determine hormone concentration

🔑 GOLDEN MEMORY LINE

“AB₁ catches, AB₂ sandwiches, AB₃ brings the enzyme, enzyme makes color, and color tells hormone.”

Or even shorter:

🪤 Catch → Sandwich → Enzyme → Color → Measure

That’s the whole Guyton ELISA figure.