Normal bladder function requires co-operative interaction of several cell types. Detrusor muscle exhibits nerve-evoked and spontaneous contractions (SCs) which are regulated by the release of neurotransmitters and spontaneous action potentials. Recent studies have revealed that the urothelium and lamina propria, collectively known as the mucosa, also exhibit spontaneous and agonist-induced contractile activity (1). Furthermore, it has been suggested that the contractility of detrusor muscle is linked to the mucosa layer via the possible release of diffusible agents. Nevertheless, the contractile properties of the mucosa and its impact on detrusor function have yet to be fully understood. This study aims to investigate the role of the mucosa in mediating detrusor contractile activity.

Female pig (Sus scrofa domestica ∼6-months old) bladders obtained from a local abattoir were placed in cold Krebs bicarbonate (KB) solution (mM: NaCl 118.4, NaHCO3 24.9, KCl 4.7, CaCl2 1.9, MgSO4 1.15, KH2PO4 1.15, glucose 11.7; pH 7.4 with 5% CO2;95% O2). From each bladder dome longitudinally-orientated preparations were prepared. These were: (a) intact (I; detrusor+mucosa); (b) denuded (D; detrusor with mucosa removed) (c) mucosa alone (M); (d) reconstructed (R; denuded with previously isolated mucosa reattached). (e) mucosa (M-D) and denuded (D-M) placed in the same organ bath but separated by a ~5 mm gap; all were under a fixed (20 mN) load in KB solution at 37°C for 45 min. SCs were recorded and assessed during the final 15-min of the experimental period. The tissues were stimulated with 10 µM carbachol (CCh) and tonic tensions were recorded. The tissues were superfused for 1 hour and then stimulated with 120 mM KCl (high-K). The amplitude (mg tension/mg tissue) and frequency (number of events in 5 mins) of SCs and tonic responses to CCh and high-K solutions (mg tension/mg tissue) were recorded and compared between the different preparations. All data are expressed as medians (25,75% interquartiles), n=number of preparations. Statistical analyses were carried out by non-parametric ANOVA and appropriate post hoc paired and unpaired tests.

Basal SCs were recorded in 100% of mucosa, intact and reconstructed pig bladder preparations. However, only 50% of denuded preparations (D and D-M) demonstrated basal SCs. The amplitude and frequency of basal SCs were significantly smaller in denuded preparations (both D and D-M) vs other preparations (Figure 1A&B). The greatest amplitude of SCs was detected in mucosa strips (Figure 1A). There was no significant difference in the amplitude or frequency of SCs between intact vs reconstructed preparations but these were higher than all denuded preparations (D and D-M) (Figure 1A&B). There was no significant difference in the amplitude or frequency of SCs between denuded (D) vs denuded in the presence of mucosa (D-M) preparations. This showed that a close proximity between the layers was needed to generate SCs. Tonic contractions to CCh and high-K solutions were significantly greater in denuded preparations (D and D-M) vs other preparations (Figure 1C&D) demonstrating that mucosa has an inhibitory effect on agonist-induced tonic contractions of detrusor.

The contractile properties of the bladder mucosa have been previously described and it was proposed that phasic activity of intact bladder strips may be influenced by this layer (1,2) This study demonstrated that the pig bladder detrusor alone has very little intrinsic ability to generate spontaneous activity and the interaction between mucosa and detrusor is required for generation of SCs. Dissecting the mucosa away from the muscle and then reattaching it to the detrusor immediately restored SCs.  However, placing the mucosa in close proximity to the detrusor, but separate from it, did not restore SCs. This implies that there is a diffusible factor released from the mucosa which can only diffuse over a short distance in order to induce detrusor SCs. Paradoxically, attachment of mucosa to the detrusor appears to inhibit the contractile responses of the detrusor to CCh and high-K solutions, demonstrating that a diffusible inhibitory factor is also released from the mucosa which also can only diffuse over a short distance.

Spontaneous contractions of pig bladder appear to originate from the mucosal layer possibly through the release of a diffusible factor which acts on the underlying detrusor to generate SCs. However, the contractile response of detrusor to CCh and high-K solutions is inhibited in presence of the mucosa, possibly due to release of an inhibitory diffusible factor, as previously described [3]. Further studies are needed to identify the source and nature of these diffusible factors.

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