The Effect of Mestranol and Lynoestrenol on the Sensitivity of the Isolated Rat Uterus to the Inhibitory Action of Sympathomimetic Agents

Abstract

The effect of oral treatment of rats for 12 consecutive days with the contraceptive steroids, mestranol (0\m=.\08mg/rat/day) and lynoestrenol (2\m=.\5mg/rat/day), separately or in combination, on the sensitivity of the isolated spontaneously contracting rat uterus to the inhibitory action of isoprenaline, adrenaline and noradrenaline was investigated. The sensitivity to isoprenaline did not change significantly after any of the hormonal treatments. The sensitivity to adrenaline was significantly reduced after treatment with mestranol and lynoestrenol separately and together. Only mestranol treatment also resulted in a significant decrease in sensitivity to noradrenaline while the decrease after lynoestrenol or mestranol+ lynoestrenol was not significant. Treatment with only one or with a combination of the steroids resulted in the appearance of excitatory \g=a\-adrenoceptors which most probably contributed to the decrease in the sensitivity of the uterus to the inhibitory action of the sympathomimetic agents. The reactivity of the isolated rat uterus to the inhibitory action of some sym¬ pathomimetic agents depends on the types of adrenoceptors, the rate of binding, the enzymatic inactivation and the functional state of the uterus (see AbdelAziz & Bakry, 1973). After oestrogen treatment and during early pregnancy, there is a decrease in the sensitivity of the isolated rat uterus to some sympatho¬ mimetic agents (Abdel-Aziz & Bakry, 1973).The present investigation deals with the effect of treatment with two contra¬ ceptive steroids, mestranol and lynoestrenol, on the sensitivity of the isolated rat uterus to the inhibitory action of some sympathomimetic agents.Forty virgin female albino rats with a mean body weight ofl55 + 6-0g were allocated equally to four groups. The animals selected showed regular oestrous cycles. The contraceptive steroids were suspended in 0-5% (w/v) carboxymethylcellulose (CMC) and oral treatment with 0-2 ml CMC/day was con¬ tinued for 12 consecutive days. The first group received 0-08 mg mestranol/rat/ day, the second group received 2-5 mg lynoestrenol/rat/day and the third * Present address: