SUMMARYDexamethasone caused premature delivery when infused into foetal lambs at rates of 0\m=.\06-4\m=.\0 mg./24 hr. but it had no effect when administered to pregnant ewes at the rate of 4\m=.\0 mg./24 hr. Infusions into the foetus of deoxycorticosterone or corticosterone were ineffective; mixtures of dexamethasone and deoxycorticosterone did not cause parturition more rapidly than dexamethasone alone. Thus, the ability of corticosteroids to cause premature parturition appears to depend on glucocorticoid rather than mineralocorticoid activity.Parturition induced by dexamethasone was not delayed by administration of 100 mg. progesterone/24 hr. to the ewe or to the foetus. This suggests either that withdrawal of inhibitory effects of progesterone on the myometrium can occur independently of the progesterone concentration in peripheral plasma, or that the mechanism of parturition provoked by corticosteroids in the foetus can override any regulatory influence of progesterone on myometrial contractility.Partial aeration of the lungs was observed in lambs born vaginally at 117-123 days of gestation after receiving dexamethasone. It is suggested that this may be the result of accelerated appearance of surfactant activity.
The glucocorticoids, cortisol and corticosterone, have a unique function in the fetus in inducing a wide range of enzymes before birth that have little or no function during fetal life but on which survival after birth is dependent. The loss of the placenta at birth deprives the fetus of a source of oxygen, glucose and heat (among many other things) for which alternatives must be available immediately if survival is to be assured. In anticipation of these needs several organs undergo maturational changes in late pregnancy aimed at meeting these requirements. The lungs mature structurally and functionally, becoming distensible and capable of coping with high surface tension when air enters the alveoli with the first breath. In the liver, glycogen accumulates and gluconeogenesis is initiated to meet the demands for glucose until feeding begins. There is an increase in the production of tri-iodothyronine and catecholamines in preparation for the sharp increase in metabolic rate and thermogenesis associated with breathing and the cold environment. All these dramatic maturational events are regulated by cortisol as are numerous others in most organ systems that contribute to neonatal well-being but on which survival is less dependent. Pharmacological manipulation of these systems before birth has made a substantial contribution to improving human health.
SUMMARY1. In foetal lambs from 40 days gestation (0.27 of term) onwards delivered into a warm saline bath, apparently spontaneous breathing movements were present intermittently. They became deeper and more rapid with increasing age.2. In foetal lambs (from 0-66 of term) in which observations were made for many days after chronic implantation of tracheal, carotid and amniotic catheters, rapid irregular respiratory movements were present up to 40 % of the time, and brief gasps also were seen.3. The presence of these movements was unrelated to the foetal carotid blood gas values over a wide range of spontaneous variation.4. These foetal breathing movements were accompanied by comparatively small alterations of pulmonary volume recorded from a tracheal flowmeter, insufficient to clear the tracheal dead space. Occasionally a more prolonged expiration led to the outward flow of fluid.5. A description is given of sleep and wakefulness in foetal lambs from 0-78 of term.6. Rapid irregular breathing was associated with rapid eye movement sleep as seen in a warm saline bath or, in utero, as inferred from records of eye movements and electrocortical activity.7. Respiratory movements were often associated with relatively large variations in foetal heart rate, blood pressure and descending aortic blood flow.8. Rapid irregular foetal breathing was unaffected by section or blockade of the cervical vagi, but was abolished by general anaesthesia.
Changes in regional blood flow during simulated normobaric diving were studied in the conscious Antarctic Weddell seal (Leptonychotes weddelli) by injecting 25-microns radioactive microspheres into the left ventricle. Injections were performed before and 8--12 min after submersion of the head in iced seawater. Diving was associated with a fall in cardiac output from a mean control value of 39.8 +/- 10.2 to 5.6 +/- 3.4 l/min (mean +/- SD) and in heart rate from 52 +/- 15 to 15 +/- 4 beats/min. Blood flow to the splanchnic and peripheral vascular bed was reduced by more than 90%, cerebral blood flow was unchanged, right and left ventricular blood flow decreased by 85%, and adrenal blood flow decreased by 39%. The pulmonary fraction of the injected microsphere dose increased from 7.9 to 29.9% during diving. This may signify a large increase of peripheral arteriovenous shunting during the dive and/or increased bronchial artery blood flow. It is concluded that blood flow during diving is directed to organs and tissues according to their metabolic requirements.
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