Peripheral concentrations of progesterone, cortisol, aldosterone, sodium and potassium in the plasma of the Tadmit ewe during pregnancy and parturition.

Abstract

The peripheral blood concentrations of progesterone, cortisol, aldosterone and electrolytes were measured simultaneously in 16 multiparous ewes of the Tadmit Algerian breed during pregnancy and parturition. Plasma progesterone concentrations were similar to those of the luteal phase of the oestrous cycle during the first third of gestation. Between 60 and 135 days after mating, the levels of plasma progesterone increased steadily to values eight times those found in early pregnancy. However, no change was observed from days 43 to 28 before parturition. Mean progesterone concentrations declined during the 17 days preceding lambing, decreasing more quickly during the last 3 days. Pregnancy did not increase the levels of plasma cortisol and the mean values during pregnancy did not exceed 0·5 μg/100 ml until 90 days after mating. It then tended to decrease between days 57 and 17 before parturition, before rising slightly on both day 7 and on the day of lambing. Concentrations of plasma aldosterone were low during early pregnancy (about 2 ng/100 ml). Thereafter, as with progesterone, they rose from days 58 to 43, did not change from days 43 to 28 and increased again until maximum levels were reached 17 days prepartum and then levels decreased sharply until 7 days before lambing. At this stage, progesterone levels continued to decrease during the last 7 days while aldosterone concentrations once more increased. Sodium and potassium concentrations changed little during pregnancy, although the sodium: potassium ratio tended to increase during the last 17 days of pregnancy. It is suggested that changes in the levels of plasma aldosterone during gestation in the ewe are the result of the competitive inhibition of aldosterone by progesterone at a renal level and that the rise of aldosterone during the last few days of pregnancy is largely due to the stimulation of renin substrate production by oestrogens whose values are high at this period.