Which smooth muscles are affected by tadalafil?

Smooth Muscles Affected by Tadalafil

Tadalafil acts on smooth muscle throughout the body by inhibiting phosphodiesterase type 5 (PDE5), which is widely distributed in vascular and visceral smooth muscle, causing relaxation and vasodilation in multiple organ systems. 1

Primary Smooth Muscle Targets

Genitourinary System

• Corpus cavernosum – PDE5 is highly concentrated in penile smooth muscle, where tadalafil enhances nitric oxide-mediated vasodilation, increasing blood flow and enabling erection during sexual stimulation. 1
• Prostate – Tadalafil relaxes prostatic smooth muscle, reducing lower urinary tract symptoms associated with benign prostatic hyperplasia, though the exact mechanism for symptom improvement has not been fully established. 1
• Bladder and bladder neck – The drug affects smooth muscle in the bladder wall and bladder neck, contributing to improved urinary flow and reduced storage symptoms. 1
• Urethra – PDE5 is present in urethral smooth muscle, where tadalafil may reduce tone and resistance. 1

Vascular Smooth Muscle

• Pulmonary arteries – Tadalafil causes significant pulmonary vasodilation by increasing cGMP in pulmonary vascular smooth muscle, which is the basis for its FDA approval in pulmonary arterial hypertension at 40 mg daily. 2, 3, 4
• Systemic blood vessels – The drug acts on vascular smooth muscle throughout the arterial and venous systems, causing mild systemic vasodilation (mean maximal decrease in blood pressure of 1.6/0.8 mm Hg supine and 0.2/4.6 mm Hg standing at 20 mg dose). 1
• Penile arteries – Specific vasodilation of penile arteries increases corporal blood flow during sexual arousal. 1

Other Smooth Muscle Sites

• Anococcygeus muscle – This muscle, part of the erectile machinery in male rodents, demonstrates concentration-dependent relaxation with tadalafil, though clinical relevance in humans is uncertain. 5
• Ureteral smooth muscle – In vitro studies show tadalafil can reverse tension in isolated human ureteral smooth muscle via cGMP-mediated pathways, though clinical applications remain investigational. 6

Mechanism of Smooth Muscle Relaxation

• Tadalafil selectively inhibits PDE5, preventing the breakdown of cyclic guanosine monophosphate (cGMP) in smooth muscle cells. 2, 1
• Elevated cGMP levels activate protein kinase G, which reduces intracellular calcium and causes smooth muscle relaxation. 2
• This mechanism requires initial nitric oxide release from nerve terminals or endothelial cells—without sexual stimulation or endogenous nitric oxide, tadalafil has minimal effect on erectile tissue. 1

Tissue Distribution of PDE5

• PDE5 is found in smooth muscle of the corpus cavernosum, prostate, bladder, blood vessels (both vascular and visceral), skeletal muscle, urethra, and multiple organs including platelets, kidney, lung, cerebellum, heart, liver, testis, seminal vesicle, and pancreas. 1
• The highest concentrations relevant to therapeutic effects are in penile erectile tissue, prostatic smooth muscle, and pulmonary vascular smooth muscle. 1

Selectivity and Off-Target Effects

• Tadalafil is >10,000-fold more selective for PDE5 than for PDE1, PDE2, PDE3, PDE4, PDE7, PDE8, PDE9, and PDE10. 1
• It is 700-fold more selective for PDE5 than PDE6 (found in the retina), which explains the low incidence (<0.1%) of visual disturbances. 1
• Tadalafil is only 14-fold more selective for PDE5 than PDE11A1 (found in prostate, testes, skeletal muscle, and adrenal cortex), meaning it does inhibit PDE11 at therapeutic concentrations, though the clinical significance remains undefined. 1

Clinical Implications of Multi-System Smooth Muscle Effects

• The widespread distribution of PDE5 explains why tadalafil has FDA-approved indications for erectile dysfunction, benign prostatic hyperplasia, and pulmonary arterial hypertension—all conditions involving smooth muscle dysfunction. 3, 4
• Combination therapy with alpha-blockers (which also relax prostatic and bladder neck smooth muscle) produces additive effects, as demonstrated in studies showing enhanced inhibition of neurogenic contractions in human prostate and bladder neck tissue when tadalafil is combined with tamsulosin or alfuzosin. 7, 8
• The systemic vasodilatory effects on vascular smooth muscle create the absolute contraindication with nitrates, which can cause potentially fatal hypotension due to excessive smooth muscle relaxation in systemic arteries. 3, 1