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SUMMARY Using a protein-binding technique, progesterone concentrations in peripheral plasma (jugular vein) were measured throughout the oestrous cycle of 24 ewes. Examination of the specificity of the method by thin-layer chromatography indicated that interference from other steroids was not significant in sheep plasma. During the first 4 days of the cycle (days 0–3), plasma progesterone concentrations were below 0·4 ng./ml., increasing to a mean level of 1·5–2·5 ng./ml. between days 4 and 9, and remaining at this level for approximately 5 days, before declining rapidly on days 14 and 15 to reach a low level on the day before oestrus. The progesterone concentration on the day of oestrus was extremely low (0·1 ng./ml.), and was of the same order as that found in the plasma of wethers and anoestrous or ovariectomized ewes. Three ewes, superovulated with pregnant mare serum gonadotrophin, showed marked elevation of peripheral progesterone concentration during the luteal phase of the cycle, the concentration being proportional to the number of corpora lutea formed.

SUMMARY Growth hormone (GH) concentrations in the plasma of foetal lambs (range 20–150 ng/ml) were ten or more times those in the plasma of ewes during the last third of pregnancy. Blood samples obtained from foetuses with indwelling vascular catheters indicated that there was a steady rise in foetal plasma GH concentrations from 40–50 ng/ml at 100–110 days gestation to maximum values of 110–120 ng/ml between 130 and 140 days gestation and then a rapid decrease around the time of birth. Growth hormone disappeared from the foetal plasma after hypophysectomy of the foetus and there was no evidence for a placental source of the hormone. Ovine pituitary growth hormone given by intravenous injection disappeared from the foetal plasma at a similar rate to the endogenous hormone following hypophysectomy. The rate of GH removal from foetal plasma was slower than that in lambs 4–12 days after birth. Glucose infusion into foetal lambs did not alter their plasma GH concentration significantly, but infusion of the catecholamine, isoprenaline, decreased the foetal plasma GH concentration rapidly. Cessation of infusion was followed by very rapid restoration of the former high plasma GH concentration. The results indicate that the high foetal plasma GH concentration is maintained by active secretion of the hormone, but its function in the foetal lamb remains obscure.

The metabolic clearance rate of125I-labelled sheep growth hormone (GH) in intact sheep was calculated to be 61±7.8 l/24 h and in nephrectomized sheep 26±3.9 l/24 h. From these figures a production rate of GH of 122 μg/24 h was derived for intact sheep. After a single intravenous injection of labelled GH the kidneys extracted more than 90% within 4 h. The extracted GH was metabolized by the kidney, only a small amount (5%) of the radioactivity excreted in the urine could be precipitated by perchloric acid. Renal metabolism of GH was also indicated by the appearance in renal venous blood of labelled material that was not immunoprecipitable. The liver also removed labelled GH and degraded it. Four hours after the injection of labelled GH most of the radioactivity was in blood and muscle, but there were significant amounts in kidney, liver and fat. Radioactivity that could be precipitated with GH-antiserum was found only in the kidney, thyroid and blood. A high percentage of activity extracted from all these tissues could still be precipitated by perchloric acid.

SUMMARY Foetal lambs (100–150 days' gestation) with indwelling vascular catheters were used to study the regulation of the insulin concentration in the plasma of foetal lambs in utero. Immediately after the implantation of the catheters the insulin concentration in foetal plasma was significantly correlated with the foetal glucose and fructose concentrations and with the maternal glucose concentration. On the next day the foetal insulin concentration was significantly correlated only with the maternal glucose concentration. Both glucose and fructose, when infused i.v., increased the insulin concentration in foetal plasma, but the increases were slow and far less than those observed in newborn lambs infused with glucose or fructose. Intravenous infusion of isoprenaline or glucagon did not alter the plasma insulin concentration of foetal lambs, but both caused a rapid increase in the insulin concentration of newborn lambs. Glucagon did not potentiate the insulin response to glucose. Addition of aminophylline to a glucagon infusion failed to cause insulin secretion in foetal lambs. The results suggest the cyclic-3′,5′-AMP dependent part of the insulin secretory mechanism does not develop fully before the last week of gestation. Gel filtration of foetal plasma on Sephadex indicated that the immunoreactive material present was insulin. No significant amounts of proinsulin were found.

THE phospholipids of pulmonary surfactant1 cover the respiratory epithelium and because of their unique properties of lowering surface tension, spreading rapidly and sustaining high surface pressures, allow the lungs to expand easily and prevent their collapse. Deficiency of lung surfactant is the major cause of respiratory distress syndrome in premature babies and also contributes to the respiratory problems of paraquat poisoning and cardiac bypass surgery. It has generally been assumed that surfactant, whether in the lamellar bodies or alveoli, is in an aqueous environment and must, therefore, be hydrated. We show here, however, that fully hydrated surfactant is apparently inactive and that surfactant can only be effective as a surface monolayer if the source of the material is ‘dry’.

Renal clearances of inulin and chromium-51 ethylenediaminetetraacetate were compared in sheep. In 41 clearance periods from five animals the mean clearance ratio of the chelate to inulin was 0.95 ± 0.03 (S.D.). Part of the difference between the two sets of results may have been due to binding of the metal chelate by plasma protein since 1.5 to 2 percent of chromium-51 ethylenediaminetetraacetate in plasma could not be removed by dialysis.

Low birthweight in sheep is associated with increased perinatal mortality. The sheep uterus contains folds of endometrium (caruncles) which form the maternal cotyledon in pregnancy. Alexander (1964) removed caruncles & in a subsequent pregnancy observed a reduction of placental & fetal weight, increased incidence of premature labour & stillbirths. 4 small fetuses have been produced using this technique. Catheters were implanted into the ewe & the fetus between 105 & 110 d. Serial measurements of blood gases, pH, metabolites, cortisol & ACTH were made. The mean fetal carotid PaO2 & haematocrit were 15.9 ± 1.3 mm Hg & 37.4 ± 2.2% compared to 21.6 ± 1.0 mm Hg & 30.3 ± 0.6% in 7 control fetuses (P < 0.01). There were no significant changes in PaCO2 or pH. Plasma glucose (0.66 ± 0.03 μmol/ml), lactate (0.9 ± 0.02 μmol/ml) and a amino nitrogen (aNH2, 6.11 ± 0.25 μmol/ml) were lower than in control fetuses (glucose 1.19 ± 0.02 μmol/ml, lactate 1.47 ± 0.02 μmol/ml and a NH2 7.64 ± 0.02 μmol/ml)suggesting a limitation of placental transfer. We have previously observed increased ACTH with hypoxaemia or hypoglycaemia. However, the concentration of ACTH (69.2 ± 29.7 pg/ml) and cortisol (21 ± 8.2 ng/ml) in the small fetuses was not significantly different from controls.

SUMMARY Thyroid glands were surgically ablated from foetal sheep between 81 and 96 days of gestation in order to study the role of the thyroid gland in foetal development. Gestation of ewes bearing thyroidectomized foetuses was prolonged by about 5 days, and athyroid lambs failed to survive more than 24 h. Athyroid foetuses delivered by Caesarian section 1 week before term were characterized by low body weight, low thymus gland weight, and shortened limbs. Foetal thyroidectomy markedly retarded both the maturation and growth of wool follicles and osseous tissue. Plasma concentrations of growth hormone, calcium and phosphorus were normal in the athyroid foetuses, but cholesterol levels were increased. The absence of thyroxine in the foetal circulation agrees with previous findings that the ovine placenta is impermeable to maternal thyroxine.