How do we know that the drug going act only on this receptor for eg no

How do we know that the drug going act only on this receptor for eg nor epinephirne it act on both alpha and beta what will the effect

Reading File

Finding Sources

Reading File

Excellent - I now have solid textbook content. Let me compile a thorough answer.

This is a great pharmacology concept question. Here is a thorough explanation:

Why Drugs Don't Always Act on Just ONE Receptor - The Norepinephrine (NE) Example

The Core Problem: Receptor Selectivity vs. Specificity

Most drugs are not perfectly selective for a single receptor subtype. Whether a drug activates one receptor or many depends on:

Receptor affinity - how strongly the drug binds to each receptor type
Drug concentration / dose - higher doses recruit lower-affinity receptors
Tissue distribution - which receptors are present in a given organ

Norepinephrine: A Classic Multi-Receptor Drug

Norepinephrine (NE) acts on five adrenergic receptor subtypes:

α₁, α₂ (alpha receptors)
β₁, β₂, β₃ (beta receptors)

But it does NOT act equally on all of them. As Guyton & Hall states:

"Norepinephrine excites mainly alpha receptors but excites the beta receptors to a lesser extent as well."

Guyton and Hall Textbook of Medical Physiology

And Schwartz's Surgery gives the receptor affinity hierarchy:

α receptors: epinephrine > norepinephrine >> isoproterenol
β₁ receptors: isoproterenol > epinephrine = norepinephrine
β₂ receptors: isoproterenol > epinephrine >> norepinephrine (very weak)

So NE has high affinity for α receptors, moderate affinity for β₁, and very low affinity for β₂.

What Are the Effects When NE Acts on Both Alpha and Beta?

Here is what happens organ by organ when NE activates both receptor types:

Receptor Activated	Organ	Effect
α₁	Blood vessels (skin, gut, kidney)	Vasoconstriction → raises BP
α₂	Presynaptic nerve terminals	Inhibits further NE release (feedback)
β₁	Heart	Increases heart rate & contractility
β₂	Blood vessels (skeletal muscle), bronchi	Vasodilation, bronchodilation (weak with NE)
β₃	Adipose tissue	Thermogenesis, lipolysis

Net clinical effect of NE given IV:

Strong vasoconstriction (dominant α₁ effect) → marked rise in peripheral resistance
Increased BP (both systolic and diastolic)
Reflex bradycardia - the rise in BP triggers baroreceptors that slow the heart, which can override the direct β₁ stimulation
Minimal bronchodilation (β₂ effect is weak with NE)

This is the key insight: the dominant receptor type in a tissue determines the net response, even if the drug can bind multiple receptors.

How Do We Know Which Receptor a Drug Acts On? (The Core of Your Question)

Pharmacologists use several methods to figure this out:

1. Selective Agonists and Antagonists

Using drugs that activate or block ONLY one receptor type to map out effects:

Isoproterenol - acts ONLY on β receptors (no α effect) - so if NE's effect is blocked by a β-blocker like propranolol, that effect is β-mediated
Phenylephrine - acts ONLY on α₁ receptors - so if its effects mimic part of NE's effects, that part is α₁-mediated
Phentolamine - blocks all α receptors. If NE is given after phentolamine, the vasoconstriction disappears but β effects remain
Propranolol - blocks all β receptors. If NE is given after propranolol, heart rate/contractility effects are blocked

2. Receptor Affinity Data (Binding Studies)

Radioligand binding assays measure how tightly a drug binds to cloned/isolated receptors
The Ki (inhibition constant) or Kd (dissociation constant) tells you the affinity - a lower Kd means tighter binding

3. Dose-Response Curves

At LOW doses, only high-affinity receptors are activated
At HIGH doses, lower-affinity receptors are also recruited
This explains why dose matters enormously - low-dose dopamine activates dopamine receptors (renal vasodilation), mid-dose activates β₁ (cardiac), high-dose activates α₁ (vasoconstriction)

4. Tissue Receptor Distribution

Tissues express different receptor subtypes. The effect of NE in the heart is mainly β₁-mediated because that is the dominant receptor there
In blood vessels of skin, α₁ dominates, so NE causes constriction there

Why This Matters Clinically

This is why we design selective drugs:

Salbutamol (albuterol) - selective β₂ agonist - gives bronchodilation without much cardiac β₁ stimulation
Metoprolol - selective β₁ blocker - slows heart without blocking bronchial β₂ receptors
Prazosin - selective α₁ blocker - lowers BP without blocking the presynaptic α₂ feedback

But even "selective" drugs lose selectivity at high doses, because the structural differences between receptor subtypes are only partial.

The Key Principle (from Ganong's Review)

"The action of a chemical mediator on its target structure is more dependent on the type of receptor on which it acts than on the properties of the mediator per se."

Ganong's Review of Medical Physiology

This is the fundamental principle: the receptor in the tissue, not just the drug, determines the response. The same NE molecule causes vasoconstriction in skin (α₁-rich vessels) but increases heart rate (β₁-rich cardiac tissue).

Summary:

NE acts on α > β receptors, with α₁ being dominant
We determine which receptor mediates which effect by using selective blockers/agonists, binding assays, and dose-response studies
The net clinical effect is a sum of all receptor activations, weighted by receptor density and affinity in each tissue
This is why selective drugs (that target only one subtype) are preferred in clinical medicine

This is a shared conversation. Sign in to Orris to start your own chat.