Urethral dysfunction contributes to conditions such as stress urinary incontinence (SUI), yet underlying regulatory mechanisms remain incompletely understood (1). While 5-HT has been shown to modulate urethral smooth muscle activity (2), its effects on the urethral mucosa have not been investigated. Given the recognised functional role of the mucosal layer in other regions of the lower urinary tract, such as the bladder (3), this study aimed to characterise the effects of 5-HT on the urethral mucosa and compare these responses with those of the underlying smooth muscle.

Urethral tissue strips were prepared in three groups: mucosa-intact smooth muscle, denuded smooth muscle, and mucosal layer. Tissues were mounted in organ baths containing Krebs-bicarbonate solution, maintained at 37°C, and gassed with 95% O2 and 5% CO2. Tissue strips were allowed to equilibrate for 45 minutes with washout every 15 minutes. After a stable pattern of contractile activity was generated, 5-HT was added in a cumulative manner (10 nmol/L – 100 μmol/L). In a separate set of experiments, the smooth muscle and mucosal tissue strips were incubated in the presence and absence of 5HT receptor selective antagonists GR113808 (5-HT4, 1 mmol/L), SB269970 (5-HT7, 10 nmol/L), SB399885 (5-HT6, 10 nmol/L), ondansetron (5-HT3, 30 nmol/L) and ketanserin (5-HT2, 30 nmol/L). The tissue responses were recorded using isometric transducers via a Powerlab recording system and Labchart® software. Paired Student’s t-tests were performed, with p-values <0.05 considered statistically significant.

When subjected to 5-HT, the urethral mucosal strips generated a concentration-dependent increase in tone, while the smooth muscle strips concentration-dependently relaxed. Mucosa-intact smooth muscle strips relaxed significantly more compared to the denuded smooth muscle strips (p < 0.05). 5HT2 receptor antagonist ketanserin (30 nmol/L) significantly attenuated the 5-HT-induced tonic contractions in the mucosal strips (p < 0.05) but did not affect the 5-HT-induced relaxatory responses in urethral smooth muscle strips (p > 0.05). 

When subjected to 5-HT, the urethral mucosal strips also generated a concentration-dependent increase in frequency of phasic spontaneous contractions. Ketanserin (30 nmol/L) did not affect the 5-HT induced increase in phasic spontaneous contractions in the mucosal strips (p > 0.05).

Antagonism of 5-HT4, 5-HT7, 5-HT6 and 5-HT3 receptors with GR113808, SB269970, SB399885 and ondansetron did not affect the relaxatory responses of the urethral smooth muscle strips or the contractile response of the mucosal strips (p > 0.05).

These findings indicate that 5-HT exerts opposing effects in the urethral mucosa versus the muscle, inducing contraction in the mucosa via 5-HT2 receptors while promoting relaxation in smooth muscle through alternative 5-HT receptor pathways. The attenuation of smooth muscle relaxation in mucosa-intact preparations confirms that the mucosa modulates underlying smooth muscle tone, potentially via release of urothelium-derived factors. 5-HT also concentration-dependently increased the frequency of phasic contractile activity in the urethral mucosa; this effect was not altered by ketanserin, suggesting involvement of non-5-HT2 receptor mechanisms.

5-HT elicits opposing, tissue-specific responses in the urethra, driving contraction in the mucosa via 5-HT2 receptors while inducing relaxation in smooth muscle through different mechanisms. These findings confirm the urethral mucosa as an active modulator of 
the smooth muscle and identify it's potential contribution to urethral tone, highlighting tissue-specific serotonergic pathways as potential therapeutic targets in urethral dysfunction, including SUI.

Yang B et al (2022) Medicina (Kaunas) 59:5
Kamo et al (2021) J Pharmacol Exp Ther 378:60-68
Chau et al (2016) Auton Neurosci 200:49-56

Continence 19S (2026) 102658
DOI: 10.1016/j.cont.2026.102658