1. The effect of oestradiol and progesterone pre-treatment on the contractile response of isolated rat detrusor muscle to electrical field stimulation was investigated. The response to direct administration of 2 μmol/l progesterone and 2 μmol/l diethylstilboestrol in the organ bath was also examined.

2. Virgin female Wistar rats were injected sub-cutaneously with oestradiol benzoate (150 μg/kg) for 3 days followed by 1 day of progesterone (160 μg/kg). This cycle was repeated once. Control rats received no injections.

3. In controls, progesterone significantly reduced the maximum contractile response of rat detrusor muscle in vitro by 12% (P < 0.01). The EF50 was significantly increased compared with control. When 2 μmol/l diethylstilboestrol, was added, the maximum contractile response was significantly reduced by 42% (P < 0.01) and the frequency-response curve showed a further increase in EF50.

4. Progesterone had no effect on the atropine-resistant component of electrical field stimulation, but progesterone and diethylstilboestrol reduced the atropine-resistant response by 16% (P < 0.01).

5. Detrusor muscle from pretreated rats showed a non-significant increase in maximum contractile response compared with untreated controls. The addition of 2 μmol/l progesterone to the bath chamber had no effect on this response, but the further addition of 2 μmol/l diethylstilboestrol reduced the maximum contraction.

6. Pretreatment with oestradiol and progesterone had no effect on the atropine- or tetrodotoxin-sensitive response to electrical field stimulation.

7. In conclusion, the direct effect of progesterone and diethylstilboestrol inhibited the contractile response of detrusor muscle to electrical field stimulation and the effects of each summated. Atropine blocked this effect of progesterone, but not that of diethylstilboestrol. Pretreatment with progesterone and oestradiol had no significant effect on rat detrusor contractile response. Since treatment with oestradiol alone has been shown to have a significant inhibitory action on contractile response, the addition of progesterone would appear to alter this effect of oestradiol.

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