In humans the desire to void is influenced by urinary pH - the desire being experienced at lower bladder volumes when the urine is more acidic (1). Detrusor contractions can also be provoked in many patients by instillation of acidic solutions into the bladder (2). The working mechanisms for these effects remain to be clarified. The aim of the present experimental study in the rat was to modulate the micturition threshold by infusion of fluid at low pH and to identify associated changes in bladder afferent and efferent activity.

Methods

Thirteen female rats, anaesthetised by a-chloralose and paralysed by pancurone bromide, were used for the experiments. Micturition threshold volume was determined by repeated transurethral cystometries at low infusion speed (0.06 ml/min). Saline, titrated to pH 3, 4, 5, 6 and 7 by acetic acid, was used for infusions. Bladder afferent and efferent activity was recorded, simultaneously with cystometry, from exposed pelvic nerve branches to the bladder. After an initial equilibrium period, four control cystometries were performed with saline at pH 7. The different acidic solution were then tested in random order until at least 4 cystometries at each pH were obtained. A resting interval of at least 3 minutes, with the bladder open and empty, was interposed between successive cystometries.

Bladder pressure, afferent or efferent nerve activity together with the full-wave rectified and integrated nerve responses were continuously recorded on a HIOKI chart recorder and subsequently analysed off line with a PC based system. The following parameters were studied: afferent and efferent threshold volumes, micturition threshold volume and pressure, afferent activity at micturition threshold, peak afferent and efferent activity, bladder pressure at peak contraction, afferent pressure sensitivity. The used recording technique resolved primarily activity in myelinated Ad afferents from bladder mechanoreceptors (3).

Results

A total of 509 cystometries were performed. There was a small gradual decrease in micturition threshold volume with acidic solutions from a mean volume 0.49 ml at pH 7 to 0.42 ml at pH 3. The change was only significant for the two lowest pH levels (pH 4 and 3; p < 0.01). At these low pH levels there was also a small decrease in mean bladder compliance (from 5.3 to 4.8 ml/ cm H2O; p < 0.01) and peak contraction pressure (from 41 to 39 cm H2O; p < 0.5). There was no detectable change in afferent or efferent pelvic nerve activity or in other measured parameters.

Conclusion

Cystometry with acidic solutions induced a significant decrease in the micturition threshold volume of anaesthetised rats. The decrease occurred without a detectable change in the firing properties of bladder Ad mechanoreceptor afferents. It is proposed that proton sensitive bladder receptors with unmyelinated afferents were stimulated by the acidic solution and that the micturition reflex was facilitated by afferent inflow from such receptors.

Reference

(1) Neurourol Urodyn 16:396-397, 1997
(2) Neurourol Urodyn  9: 355-356, 1990
(3) Neurourol Urodyn  17:543-553, 1998