Effect of filling rate on cystometric parameters in young and middle aged mice

Abstract

Objectives: To investigate the effect of changing the bladder filling rate during cystometry in younger (2-3 months) and older (13-14 months) C57BL/6J male mice.

Methods: Cystometry was performed on mice under anesthesia. Voiding cycles were established in each mouse at a pump delivery rate of 17 μl/min. After 30 min, the rate was increased sequentially to 25, 33, 41 and 49 μl/min. Each rate was maintained for 30 min. The following cystometric parameters were quantified: peak pressure amplitude, intercontractile interval (ICI), compliance, micturition pressure threshold and voiding efficiency.

Results: Bladder weights were significantly greater in older mice (42 mg vs. 27 mg, P < 0.01), but functional capacities were not different. The pressure amplitudes did not change as filling rate increased, nor did they differ between the 4-month and 13-month-old males. ICIs were not significantly different between young and mature mice. However, both groups exhibited a non-linear reduction in ICI with increasing filling rate, best described by a power curve (R² > 0.93). Compliance was higher in the older mice at low filling rates (17 and 25 μl/min) but this difference diminished at higher rates. Compliance decreased with increasing flow rate in a non-linear manner, again with greater effects at low filling rates. Micturition pressure thresholds increased with increasing flow rate in a linear manner and older mice began voiding at higher pressures than younger. Both young and old mice exhibited voiding efficiencies of ~70%.

Conclusions: The rate of volume delivery has complex effects on the timing of voiding and compliance. These findings argue for greater standardization of cystometry protocols and further investigation into afferent signaling to higher centers at different filling rates.

Keywords: compliance; cystometrogram; filling rate; mouse; urodynamics; volume delivery.

Figure 1. Mouse and bladder weights from 2–3 months old and 13–14 months old C57BL/6J males

A. Mouse weights were not significantly different. The average weight of young males (n = 4) was 29.5 ± 1.8 g compared to 34.1 ± 1.3 g for older males (n = 4). B. Bladder wet weights. Older mice had significantly heavier bladders (41.7 ± 0.9 mg vs. 27.0 ± 3.0 mg; mean ± SEM; **P < 0.01).

Figure 2. Void spot assays on five mice at 2–3 months and 13–14 months of age

A. Representative filters from 2 individual mice at the younger and older ages. Mouse #333 exhibits greater small spot dispersion which may be related to marking behavior or fur contamination. B–D. Data are from 5 mice tested three times each at each age and averaged. All spots with an area corresponding to < 5 μl were eliminated from the analysis. B. Total number of spots > 5 μl; C. Total urine volumes; D. Average volume of voids larger than 20 μl. Data are mean ± SEM. Paired t-test showed no difference in any parameter with age.

Figure 3. Cystometric parameters of 2–3 and 13–14 months old C57BL/6J males at incremental filling rates of 17, 25, 33, 41 and 49 μl/min

A. Pressure tracing of a 4 month old mouse. Arrows indicate the time points at which the flow rate was increased. It can be seen that voiding intervals diminish with fill rate. This effect is most pronounced at the slowest filling velocity. B–D. Effect of bladder filling rate on compliance during early third (B), middle third (C) and final third (D) of bladder filling phase. E. Pressure amplitudes at voiding (peak pressure to basal) in younger and older mice. F. Micturition pressure threshold at different flow rates in younger and older mice. G. Intercontractile interval (time between voids) at different filling rates. Both age groups showed a non-linear reduction in ICI with increasing filling rate best described by a power curve (R² > 0.93). Data is mean ± SEM, N = 4 mice/group. For each flow rate a minimum of 4 voiding cycles was analyzed and a mean calculated.

Figure 4. Efficiency of voiding

After continuous flow cystometry, the pump was stopped, the bladder was emptied and then a single fill CMG was performed. Upon initiation of a void the pump was stopped again. Following completion of the void, the remaining residual volume in the bladder was measured. Efficiency was calculated by ((inflow volume [flow rate × time of single fill] − residual volume)/inflow volume) × 100. Data is mean ± SEM; n = 4 for both groups.