No abstract

It is well known that a high external Ca2+ concentration increases the spontaneous activity of certain smooth muscle preparations in vitro, whereas a high Mg2+ concentration decreases it. The effects of these ions on the responses to stimulants vary; for example, a rise in the external [Mg2+] increases the sensitivity of the guinea-pig uterus to oxytocin, and much more so to vasopressin, but decreases the responses to histamine and ergobasine (Fraser, 1939).We have noticed that a high [Mg2+] in the suspension medium decreases the sensitivity of the uterus to component A of the menstrual stimulant (Clitheroe & Pickles, 1961), and have confirmed that a high [Mg2+] often increases the sensitivity to vasopressin. This observation may give some clue to the different modes of action of the two stimulants at the cellular level.Since Mg2& might act either in antagonism or synergism with Ca2+, or independently of it, the two ions have been administered together in many different proportions, in order to study their effects upon the following three properties of the uteri: (1) the spontaneous rhythmical activity, (2) the sensitivity to vasopressin, and (3) the sensitivity to the menstrual stimulant component A. 'Sensitivity' here means the reciprocal (expressed on a logarithmic scale) of the dose of stimulant necessary to give a standard tension response; but a few observations have also been made on the varying tensions produced by standard doses of stimulant, and on the forms of the tension response.The results show that whereas calcium ions increase all three indices of uterine activity, and may therefore act at a site controlling both the spontaneous contractions and the responses to the two stimulants, magnesium ions act in some respects synergically and in others antagonistically Physiol. 167

The administration of oestradiol to pregnant animals can, in certain instances, cause a delay in the onset of parturition (Hain 1935). One of the effects of excess oestradiol may be on the hypothalamus, impeding the release of oxytocin. The purpose of the present study was to note the effect of oestradiol monobenzoate on the activities of certain hypothalamic enzymes which are capable of inactivating oxytocin. In the pregnant animal, oestradiol depressed enzyme activity, and this is interpreted as indicating an inhibitory action of the hormone on the hypothalamus. The inhibitory action is opposite to that produced by the hormone in the non-pregnant animal (Hooper 1968a), and involvement of the corpus luteum or placenta seems probable.