26/09/2026
Smooth muscle (β2): RELAXATION / DILATION
PKA inhibits Myosin Light Chain Kinase (MLCK) → myosin can’t be activated → muscle relaxes
Result: bronchodilation, vasodilation in skeletal muscle vessels, uterine relaxation
Where the β receptors act:
• β1: Heart (SA node, AV node, ventricles), kidney (renin release)
• β2: Bronchi, skeletal muscle blood vessels, uterus, liver (glycogenolysis)
• β3: Bladder detrusor (relaxation), adipose tissue (lipolysis)
Why the opposite effects?
Cardiac muscle contraction is driven by Ca²⁺ binding troponin, so more Ca²⁺ means more force. Smooth muscle contraction is driven by MLCK, so blocking MLCK means relaxation.
Same messenger. Opposite endpoint. This is why a salbutamol inhaler opens the airways but can cause palpitations.
Save this for your next Pharmacology revision.
NEETPG
