Effect of oestrogen and progesterone on sodium, potassium and water balance in sheep.

The effects of oestrogen and progesterone, injected alone or during successive periods, were studied in mature ovariectomised ewes on sodium intakes ranging from 1 to 4 mmol kg-1 d-1. Progesterone had no effect on fluid and electrolyte balance. Oestrogen (17 beta oestradiol) inhibited drinking but had little effect on water excretion whereas it reduced both food intake and urinary potassium concentration. Since the resulting changes in potassium intake and excretion were very similar, oestrogen had little net effect on potassium balance; neither change lasted beyond the treatment period. Oestrogen had a biphasic effect on urinary and faecal sodium excretion, an initial enhancement of salt loss being followed by sodium retention. Faecal sodium retention and the biphasic effect on renal sodium excretion were independent of changes in food intake. These observations are discussed in relation to the pattern of change of electrolyte balance during the reproductive cycle and the influence of changes in food intake and factors affecting sodium preference. The fact that a variety of responses is involved in a coordinated change of sodium balance is emphasised and its possible importance is considered.