Rat uterine contractility and the activities of uterine adenyl cyclase and phosphodiesterase during the estrus cycle

A study has been made of the activities of rat uterus adenyl cyclase and phosphodiesterase, and the inhibitory effect of theophylline on uterine contractions at various stages of the estrus cycle. The activities of adenyl cyclase and phosphodiesterase at proestrus were found to be 2.52 ± 0.28 pmoles/min/mg of protein and 1.90 ± 0.30 nmoles/min/mg of protein, respectively. Activities of both the enzymes increased from proestrus to peak values at metestrus (early metestrus for adenyl cyclase and late metestrus for phosphodiesterase) and then fell until the following proestrus. Theophylline inhibition of oxytocin-induced maximum uterine contractions was found to be greatest at early metestrus. Similarly, the oxytocin concentration producing 40 per cent maximum contraction in the presence and absence of the theophylline

$\text{Oxytocin}\text{(E}{}_{40}\text{)+}\text{theophylline}\text{Oxytocin}\text{(E}{}_{40}\text{)}$

was also highest at early metestrus. These findings indicate that, at early metestrus, cellular turnover of cyclic AMP may be high and that the exaggerated inhibition of oxytocin- induced maximum contraction and the high value of

$\text{Oxytocin}\text{(E}{}_{40}\text{)+}\text{theophylline}\text{Oxytocin}\text{(E}{}_{40}\text{)}$

could be due to extensive accumulation of cyclic AMP produced from theophylline inhibition of phosphodiesterase.