CHANGES IN LUNG LIQUID DURING SPONTANEOUS LABOR IN FETAL SHEEP

The goals of this study were 1) to examine changes in lung liquid form ation and composition during spontaneous labor in fetal lambs and 2) t o determine the importance of beta-adrenergic stimulation and transepi thelial Na+ flux in removing liquid from the lung lumen near birth. We measured net production of lung liquid (J(v)), lung liquid compositio n, and transpulmonary electrical potential difference (PD) before and during labor in fetal sheep with chronically implanted tracheal and va scular catheters. We determined J(v) by measuring rate of change in lu ng liquid concentration of I-125-albumin, an impermeant tracer that wa s mixed in lung liquid at the start of each study. In 17 paired experi ments, J(v) decreased from 11 +/- 2 ml/h (J(v) > 0 = secretion) before labor to -1 +/- 2 ml/h (J(v) < 0 = absorption) during labor; in 5 pai red experiments, PD changed from -7 +/- 1 mV (lumen negative) before l abor to -12 +/- 1 mV during labor. To determine whether absorption of lung liquid during labor is the result of beta-adrenergic stimulation, we studied the effect of propranolol on J(v) during labor. When propr anolol (40 mu M) was added to lung liquid during active labor, J(v) de creased from -2 +/- 2 to -8 +/- 3 ml/h (n = 9). Thus, propranolol did not inhibit lung liquid absorption during labor. To determine whether transepithelial Na+ movement provides the driving force for lung liqui d clearance during labor, we tested the effects of amiloride, an Na+ t ransport inhibitor, on J(v) and PD. When amiloride (100 mu M) was mixe d in lung liquid during labor, J(v) increased from 0 +/- 2 to 9 +/- 3 ml/h (n = 7) and PD changed from -7 +/- 1 to -2 +/- 1 mV (n = 4). Thes e findings indicate that spontaneous labor is associated with a reduct ion in rate of formation of lung liquid in fetal sheep, often resultin g in net liquid absorption. This decrease or reversal of lung liquid f low is driven by Na+ flux out of lung lumen and does not require mu-ad renergic stimulation.