Membrane potential and ion content in cat and guinea-pig myometrium and the response to adrenaline and noradrenaline.

1. Cats, virgin and 17 days pregnant, and guinea-pigs, virgin and 14-60 days pregnant, or treated for 1-8 days with oestradiol+progesterone, were used. The response of the uterus to adrenaline and noradrenaline was observed and, in pieces from the same tissues, the resting and active membrane potentials were recorded and the ionic content was determined.2. Adrenaline and noradrenaline relaxed the virgin cat uterus, adrenaline being 20-100 times more potent in vivo and about 10 times or less in vitro.3. Adrenaline and noradrenaline caused contraction of the early pregnant cat uterus, the ratio of potency being about 1.4. Adrenaline and noradrenaline had a biphasic effect on the guinea-pig uterus in all conditions. The ratio of potency was about 1.5. The mean membrane potential was 48 mV in virgin cat uterus and 64 mV on the seventeenth day of pregnancy.6. In guinea-pigs the average membrane potential increased from 38 mV in the virgin uterus to 58 mV on the thirtieth day of pregnancy. A similar increase was produced by eight daily injections of 5 mug oestradiol and, on the last 4 days, additional 1.5 mg progesterone.7. In the cat, no significant change in K and Na content was observed during pregnancy. The intracellular chloride content, however, rose from 51.5 m-moles/1. fibre water in the virgin uterus to 89 m-moles in the early pregnant uterus. As a result, the calculated chloride equilibrium potential changed from - 25 mV in virgin uterus to - 11 mV in pregnant uterus.8. In the guinea-pig no significant change in ion content was observed and the calculated potassium and chloride equilibrium potentials remained both unaltered during pregnancy.9. In contrast to guinea-pig uterus in all conditions, and to virgin cat uterus, early pregnant cat uterus was not spontaneously active and excess calcium caused no hyperpolarization.10. The reversal of the uterine response to adrenaline as a result of pregnancy is discussed in relation to the increase of the intracellular chloride content which was only observed in the cat.