Factors affecting the wash-out rate of calcium from the myometrium

Abstract

In experiments on the isolated rat uterus maintained in a Ca-free medium, the effects of hormonal status and temperature on the rate of decline in contractility to a standard oxytocin stimulus were studied. If Ca-free medium replaced one containing 0.8 mM Ca, the response to oxytocin gradually decreased and then disappeared. Between 17 and 30° the rate of decrease was slower the higher the temperature, but between 30 and 42° the reverse trend held. This temperature relationship was more marked in uteri from oestrogen-treated than in uteri from progesterone-treated rats, and at 30°, the difference between the two types was significant. Calculations of activation energies suggest that there is an enzymatic system which maintains Ca in the cell membrane. This system is apparently activated by oestrogen, since in tissue from castrated animals there was no biphasic temperature effect on the reactivity decline rate. After disappearance of reactivity, Ca was gradually replaced in the medium. The oestrogenised tissue reached full reactivity in the temperature range 23–30° at 0.2 mM external Ca, whereas in progesteronised tissue full reactivity did not return at 0.4 mM Ca.