Development and aldosterone regulation of sodium transport in the chick (Gallus domesticus) allantoic epithelium
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The chick allantoic epithelium, an extraembryonic membrane, encases the waste fluid generated by the kidney. Volume and ionic composition of the allantoic fluid change dramatically over development. The allantoic epithelium has long been established as a sodium (Na+ )-absorbing epithelium; however, the ontogeny and regulation of its transport capabilities had not been examined. I now report that transepithelial voltage and current (“short-circuit” current, Isc) increase about five-fold from day 9 to day 19 of development, with a highly significant surge in Isc magnitude occurring around day 13. Inhibition of the Isc by amiloride (Na+ channel blocker) and ouabain (Na+ /K+ -ATPase inhibitor) shows that greater than 80 percent of the current is carried by Na+ at all times. The active transport of Na+ is reflected in decreasing allantoic Na+ concentration, while plasma Na+ remains approximately the same. Amiloride sensitivity and presence of the aENaC subunit (Western blot) indicate that the Isc is largely mediated through ENaC-like apical Na+ channels, which operate in tandem with basolateral Na+ /K+ -ATPases to maintain cell composition. I also report that chick allantoic epithelium responds to aldosterone, the primary vertebrate regulator of Na+ balance, with an increase in amilorideinhibitable current. The aldosterone-induced response appears to be age-specific, increasing gradually from 9 to 18 days. Although some preparations exhibit a rapid Isc increase beginning within minutes, the response typically begins about 100 minutes after hormone addition, suggesting a predominantly genomic mechanism of action. In all ages examined, Isc at least doubles by 4 hours after initiation of a response, although the percent increase was greatest in 13-14 day membranes. Mineralocorticoid receptors are present (Western blot), and glucocorticoid receptors are assumed present, in allantoic tissue. Experiments with spironolactone (mineralocorticoid antagonist) and dexamethasone (glucocorticoid agonist) failed to definitively demonstrate through which receptor aldosterone preferentially exerts its effect. However, at experimental concentrations or lower, the aldosterone effect is probably predominantly mediated via mineralocorticoid receptors. This aldosterone-responsive surge in current may enhance allantoic water reabsorption, necessary for hydration of the growing embryo. The allantoic epithelium appears to be an excellent epithelial transport model in which to study basic processes of more complex systems.