CURRENT-NOISE ANALYSIS OF NA ABSORPTION IN THE EMBRYONIC COPRODEUM - STIMULATION BY ALDOSTERONE AND THYROXINE

The mechanism and regulation of sodium transport in the embryonic copr odeum of chicken were investigated with isolated epithelia in vitro by electrophysiological techniques. Electrogenic sodium transport (I(Na) ) was measured in Ussing chambers by the short-circuit current (I(sc)) technique and identified by the diuretic amiloride or by removal of s odium from the apical medium. Apical sodium channels and the kinetics of amiloride binding were investigated by current-noise analysis. I(sc ) and I(Na) were measured under control conditions and under the influ ence of in vitro incubation with aldosterone and thyroxine. At 20 days the embryonic coprodeum has an I(sc) of 12.6 +/- 1.4 muA/cm2 and a tr ansepithelial resistance of 519 +/- 40 OMEGA . cm2. Amiloride blocks 9 .0 +/- 1.3 muA/cm2 of the I(sc) which represents electrogenic Na+ abso rption and can be inhibited by serosal ouabain. Aldosterone does not s timulate I(sc) or I(Na), whereas thyroxine increases I(sc) and I(Na) a bout threefold. Aldosterone in combination with thyroxine increases I( sc) and I(Na) further to about five- to sixfold. In both cases the hor monal stimulation can be totally blocked by spironolactone. Current-no ise analysis of the apical Na+ entry step reveals amiloride-sensitive Na+ channels with a single-channel current of approximately 2.3 pA and a channel density of 9-16 million/cm2 under stimulated conditions. Ha lf-maximal amiloride block occurs at 0.8-1 muM. The hormones stimulate Na+ absorption by increasing the Na+ channel density and not the sing le-channel current. This study provides the first analysis of ion tran sport in avian embryonic large intestine and shows that embryonic Naabsorption already occurs by the classical mechanism of apical Na+ cha nnels and a basolateral Na-K-ATPase. It furthermore presents the first characteristics of embryonic colonic Na+ channels and reveals a syner gistic interaction of aldosterone and thyroxine.