G7ii / 20A02: Describe the pharmacology of glyceryl trinitrate

20A02: Exam Report

Describe the pharmacology of glyceryl trinitrate (GTN).

79% of candidates passed this question.

GTN is a commonly used ‘level 1’ drug. The most comprehensive answers included information on available drug preparations, indications, mechanism of action, pharmacodynamics and pharmacokinetics and its side-effect profile. It was expected that significant detail be included in the pharmacodynamic section (e.g. preferential venodilation, reflex tachycardia, effects on myocardial oxygen demand etc). Common omissions included tachyphylaxis, dosing and its metabolism. Many answers didn’t mention the first pass effect.

G7ii / 20A02: Describe the pharmacology of glyceryl trinitrate (GTN)


An organic nitrate vasodilator


  1. Hypertension
  2. Angina
  3. LVF 2° AMI


SL tablets

Oral spray

Transdermal patch

Clear solution for injection 50mg/10mL vial


IV: 10 – 400mcg/min

PO/spray: 400 – 600mcg

Patch: 5 – 10mcg/hr

MoA (mechanism)

PRODRUG → needs to be denitrated to produce active NO i.e. reacts with sulfylhydryl groups/other enzyme reactions to liberate NO

NO → diffuses into smooth m. cell → binds to & activates GUANYLYL CYCLASE → GTP  cGMP → ↑cGMP

  1. Inhibits Ca2+ entry into smooth m. cell
  2. Activates K+ channels → hyperpolarizes cell (inactive)
  3. Stimulates “cGMP dependent protein kinases” → activates MLC phosphatases → dephosphorylates MLC → Smooth muscle relaxation



  • Low dose = veno VD
  • High dose = veno + art VD
    • Vein: ↓HR, ↓LVEDP, ↓PCWP (preL)
    • Art: ↓SVR, ↓afterL
  • Reflex ↑HR
  • Facilitates subendocardial BF & redistribution to ischaemic areas


  • Cerebral VD = ↑ICP


  • ↓LES pressure
  • Paralytic ileus



OBA 3%

High 1st pass

Rapid & efficient SL bioavailability


60% PPB

VD 3L/kg


Enters RBC → metabolized by hydrolysis into inactive compounds


80% urine

t ½ 1 – 2 mins

Adverse Effects

  • 80% dose absorbed by giving sets
  • CVS: ↓BP & ↑HR
  • CNS: headache
  • GI: nausea & vomiting
  • Haem: MetHb, platelet dysfunction
  • Tachyphylaxis