The role of amiloride‐blockable sodium transport in adrenaline‐induced lung liquid reabsorption in the fetal lamb.

Abstract

SUMMARY1. Adrenaline was infused intravenously at rates of 0 1-10 tg/min into chronically catheterized fetal lambs (125-141 days gestation) to induce slowing of secretion or reabsorption of lung liquid.2. There was an electrical potential difference (p.d.) of -0 3 to -9-5 mV (mean -3.4 mV) between lung liquid and plasma (lung liquid negative) during control lung liquid secretion. In response to adrenaline infusion, the p.d. increased (lung lumen more negative) and this change was greatest (1-8+0±3 mV) in experiments in which reabsorption occurred.3. Measurements were made of bidirectional fluxes of Na+ and Cl-across the pulmonary epithelium during control lung liquid secretion and during adrenaline infusion. Adrenaline-induced reabsorption of lung liquid was associated with an increase in Na+ flux from lung lumen to plasma. Similar but smaller changes occurred when the adrenaline response was slowing of secretion.

β-Adrenoceptor-mediated control of apical membrane conductive properties in fetal distal lung epithelia

“…Most importantly, our model predicts that this initial transient would be followed by a slowly developing but persistent rise in I sc that is due, very largely (Ͼ90%), to increased Na ϩ absorption. There is thus excellent agreement between the observed and predicted responses, and so the changes in G Cl Ϫ and G Na ϩ that we now report can explain the well-documented effects of isoprenaline upon these cells (23,30,(36)(37)(38). Moreover, in A6 cells, a Na ϩ -absorbing cell line derived from the amphibian kidney, cAMP-dependent agonists have been reported to elicit a biphasic response similar to that described above (7).…”

“…However, this liquid must be removed from the lungs if the newborn infant is to breathe at birth. The absorption of this liquid occurs during the final stages of gestation and is dependent upon the active withdrawal of Na ϩ from the lung lumen, a process that can be controlled via ␤-adrenoceptors (23,30,(36)(37)(38). The means by which this control is achieved are not fully understood, but there is evidence that the process involves a rise in apical Na ϩ conductance (G Na ϩ) (15,20).…”

Invited Review: Clearance of lung liquid during the perinatal period

“…The absorptive response to infused epinephrine can be inhibited by mixing the Na ϩ -channel blocker amiloride into lung liquid (52), and the inhibitor constant of 4 ϫ 10 -6 M provides evidence that the effect is mediated by apically located amiloride-sensitive Na ϩ channels. Given that the threshold for absorption in response to infused epinephrine was similar to that resulting from the catecholamine surge in the fetus during spontaneous labor (18) it seemed probable that the epinephrine-induced activation of apical Na ϩ channels and the resulting increase in transepithelial Na ϩ transport were largely responsible for lung liquid clearance at birth.…”

Proceedings of the Physiological Society, 23‐25 September 1992, Cambridge Meeting: Poster Communications

“…The fetal alveolar epithelium switches from fluid secretion to absorption in response to the high concentrations of circulating fetal catacholamines during labour. Although there is evidence to show that fluid reabsorption is sodium driven (Olver et al 1986), to date, no direct measurements of a sodium-selective conductance in the alveolar-epithelium have been made. We have used the patch-clamp technique to record sodium channels in freshly isolated type II pneumocytes.…”