Renal pelvic pressure isolation from active and passive bladder pressures in the rat: the facilitating effect of urine flow rate.

The circumstances facilitating hydrostatic pressure communication between the bladder and renal pelvis were evaluated. In particular, we compared active detrusor contractions occurring spontaneously during voiding and evoked contractions produced during electrical pelvic nerve stimulation with passive abdominal compression. These variables were evaluated with respect to urine flow rate and bladder capacity. Waveform analysis of the pacemaker characteristics of the renal pelvis shows that the rat pelvis has a narrow frequency band of spontaneous contraction at 0.5 +/- 0.06 Hz. The baseline waveform and pressure of the pelvis is effected by urine flow and bladder capacity. Significant elevations in pelvic pressure of 25.8 +/- 7.9 and 28.7 +/- 5.8 cm H2O were observed for bladder pressures of 30 cm H2O (n = 8) at flow rates in the range of 2.5-5.0 and over 5.0 ml/kg/h respectively (p less than 0.05). At flow rates between 2.5 and 5.0 ml/kg/h, spontaneous contractions produced an increase in pelvic pressure in all rats, while electrical stimulation produced an increase in pelvic pressure in 63%. At flow rates above 5 ml/kg/h, abdominal compression produced sharp elevations in pelvic pressure in 75%. At the same flow rate, electrical stimulation produced a slow increase in 75% and spontaneous contraction produced a slow increase in 86%. These results suggest that the exposure of the upper urinary tract to vesical pressures occurring during active and passive contractions is facilitated by flow.