Effect of pregnancy on cooling tone and rhythmic contractions of the rat urinary bladder.

OBJECTIVES: Pregnancy is a physiological alteration that can affect urinary bladder. Cooling of urinary bladder smooth muscle is known as a potent stimulus to micturition due to an increase in muscle tone. The current study investigates the effects of pregnancy on cooling tone and on the rhythmic contractions of the urinary bladder.

METHODS: Twenty-four rats were used in this study as control group (non-pregnant) and pregnant group (18-20-day pregnancy). Isolated rat urinary muscle strips were suspended in organ baths containing Krebs' solution for isometric tension recording.

RESULTS: Cooling from 37 to 5 °C induced a rapid and reproducible increase in basal tone, proportional to cooling temperature. Cooling also increased the rhythmic activity (amplitude and frequency) at 30 and 25 °C, then decreased at 20 °C, and abolished at 15-5 °C. These responses were more pronounced in pregnant group than in control group. Rhythmic contractions were abolished in calcium-free, EGTA (1 mM)-containing Krebs' solution and in the presence of nifedipine, while they were not affected by CPA or TTX in both groups. Our investigation showed that the influx of extracellular calcium is important in inducing the rhythmic contractions.

CONCLUSIONS: Pregnancy increases cooling-induced contraction in pregnant rat urinary preparations and its rhythmic contractions including amplitude and frequency than non-pregnant rat. Rhythmic contractions are myogenic in nature and highly extracellular calcium dependent. They may play a crucial role in urinary bladder overactivity and incontinence during pregnancy.