Hypothesis / aims of study
A recent study revealed that histamine modulates the contractile activity of urinary bladder via H1 and H2 receptors [1]. However, physiological role of H2 receptor (H2R) subtype is still not fully understood. To disclose the physiological role of histamine and H2R in the bladder, we investigated the expression of H2R in the bladder, and examined effects of histamine and cimetidine, an H2R antagonist, on bladder function including the in vivo primary bladder single-unit afferent activities (SAAs) in rats.
Study design, materials and methods
Female Sprague-Dawley rats were used (9-11 weeks old).
The expression of H2R in the bladder was evaluated following immunohistochemical procedures. In the separate rats, effects of histamine and cimetidine on bladder function were investigated using SAAs and cystometry (CMG) measurements.
In the SAAs measurements under isoflurane anesthesia, the fine filaments were dissected from the left L6 dorsal roots and placed across a bipolar electrode for monitoring SAAs. Nerve fibers primarily originating from the bladder were identified by electrical stimulation of the left pelvic nerve and by bladder distension. Nerves with conduction velocities (CV) more than 2.5 m/s were designated as Aδ-fibers and those with CV less than 2.5 m/s as C-fibers. Saline instilled until the intravesical pressure reached 30 cmH2O. SAAs measurements were performed two protocols: first, to confirm the effect of histamine itself (10 – 1000 μM cumulative instillation); second, to confirm the effects of cimetidine (30 mg/kg, intravenously) and its relationship with histamine.
Continuous CMG measurements were performed with intravesical saline instillation under urethane anesthesia (1.2 g/kg, intraperitoneally). After baseline recording (> 1 h), vehicle or cimetidine (30 mg/kg, intravenously) administered. Then, histamine (100 μM) was continuously instilled into the bladder.
Results
H2Rs were strongly expressed in the urothelium layer rather than the smooth muscle layer.
SAAs of both Aδ- and C-fibers were dose-dependently increased during histamine instillation, which were significant at 100 and 1000 μM (figures 1AB). SAAs of C-fibers, not Aδ-fibers, were also increased by cimetidine administration (figures 1CD).
In results of the CMG, peak micturition pressure in vehicle group was significantly decreased during histamine instillation compared with that in cimetidine group (figure 2B). None of the other CMG parameters (basal pressure, threshold pressure, intercontraction interval and voided volume) showed significant differences between the vehicle and cimetidine groups (figures 2AB).
Interpretation of results
SAAs measurements suggested histamine (more than 100 μM) facilitates bladder sensory function, which was consistent with a previous mouse study [2]. Present study further revealed that H2R may contribute the bladder sensory function, especially via C-fiber. As harmonized with these results, H2R mainly expressed on the urothelium, which actively participates in sensory functions, expressing various receptors for neurotransmitters.
In contrast, CMG measurements showed that histamine itself decreased peak micturition pressure, which was counteracted by cimetidine, suggesting histamine decreased detrusor contractile activity via H2R. Although we cannot conclude this physiological mechanism, there were several possibilities e.g., different G protein-coupled receptors (H1R: Gq-coupled contraction, H2R: Gs-coupled relaxation), H2R antagonists as cholinesterase inhibitors [3], and experimental setup including anesthesia, etc.