ION-TRANSPORT ACROSS THE EARLY CHICK-EMBRYO .2. CHARACTERIZATION AND PH SENSITIVITY OF THE TRANSEMBRYONIC SHORT-CIRCUIT CURRENT

The ectoderm of the one-day chick embryo generates dorsoventrally orie nted short-circuit current (I-sc) entirely dependent on extracellular sodium. At the dorsal cell membrane, the I-sc was modified reversibly and in a concentration-dependent manner by: amiloride (60% decrease at 1 mM, with 2 apparent IC(50)s: 0.13 and 48 mu M), phlorizin (0.1 mM) or removal of glucose (30% decrease, additive to that of amiloride), S ITS (1 mM, 13% decrease). Acidification or alkalinization of the dorsa l (but not ventral) superfusate produced, respectively, decrease or in crease of I-sc with a pH(50) of 7.64. Ba2+ (0.1-1 mM) from either side of the ectoderm decreased the I-sc by 30%. Anthracene-9-carboxylic ac id, furosemide and inducers of cAMP had no effect on electrophysiologi cal properties of the blastoderm. The chick ectoderm is therefore a hi ghly polarized epithelium containing, at the dorsal membrane, the high and low affinity amiloride-sensitive Na+ channels, Na+-glucose cotran sporter, K+ channels and pH sensitivity, and, at the ventral membrane, the Na+, K+-ATPase and K+ channels. The Na+ transport reacts to pH, b ut lacks the cAMP regulatory system, well known in many epithelia. The active Na+ transport drives glucose and fluid into the intraembryonic space, across and around the blastoderm which, in the absence of bloo d circulation, could secure renewal of extracellular fluid and disposa l of wastes and thus maintain the cell homeostasis.