Restoration of lung liquid volume following its acute alteration in fetal sheep.

1. The experiments were aimed at determining the means by which lung liquid volume is controlled in fetal sheep. Six unanaesthetized chronically catheterized fetuses between 121 and 144 days of gestation were used in experiments in which lung liquid volume was acutely reduced or increased. 2. The effects on lung liquid volume, secretion and flow in the trachea were continuously monitored until the original volume was restored. Tracheal pressure, breathing activity, electrocortical activity and laryngeal adductor muscle activity were also monitored. 3. Following a mean reduction in lung liquid volume of 54.1% (40.0 +/- 4.3 ml) there was a reduction in tracheal pressure of 1.91 +/- 0.26 mmHg. There was no change in the rate of lung liquid secretion and there was a near cessation in its rate of efflux from the trachea. Influx of liquid from the upper airway was rarely seen. 4. 5 h after its reduction, lung liquid volume had returned to control values. Tracheal pressure and the rate of liquid flow in the trachea returned to control values over 7-8 h. 5. In the hour following the reduction in lung liquid volume the incidence and amplitude of fetal inspiratory muscle electromyogram (e.m.g.) activity were reduced by 19% and 28% respectively. There was no change in laryngeal adductor muscle activity or in the duration of fetal sleep states. 6. When lung liquid volume was increased by 25.4 +/- 3.1 ml tracheal pressure increased by 2.01 +/- 0.17 mmHg. Although there was no change in the rate of liquid secretion there was a rapid efflux of liquid from the trachea principally during epochs of fetal breathing activity thus returning lung liquid volume to its control value. 7. Following the increase in lung liquid volume there were no changes in the incidence or amplitude of fetal inspiratory muscle activity, the activity of laryngeal adductor muscles or in the duration of sleep states. 8. It is concluded that following its alteration in fetal sheep lung liquid volume is restored by passive means. There was no evidence of volume-receptive neural reflexes being activated in defence of lung liquid volume.