Normal rats and rats with infravesical outflow obstruction were investigated with regard to micturition patterns and bladder pressure during the micturition cycle. Normal rats were found to have a micturition pattern characterized by a high micturition frequency and a small micturition volume. The most commonly occurring micturition intervals were between 50 and 75 min. The micturition volumes were between 0.2 and 0.4 d. The obstructed rats could be divided into two types with regard to their micturition patterns. The majority of the obstructed rats (75%) voided similar volumes as control rats (between 0.2 and 0.4 ml) but the intervals were shorter (between 10 and 20 min). A minority of the obstructed rats (25%) showed a micturition pattern consisting of large volumes (between 1 and 2 ml) and long intervals (> 200 min).
In control rats the bladder pressure was low during the micturition cycle and only minute bladder pressure fluctuations were noted until micturition. The obstructed rats, however, exhibited a marked spontaneous activity during the micturition cycle. The spontaneous activity became more and more pronounced until micturition occurred. Prior to micturition it exceeded 7 cm H20. The altered micturition pattern and the development of a pronounced bladder instability seen during the micturition cycle reveal clear similarities with findings in man with infravesical outflow obstruction.