Mineralocorticoid concentrations in unstressed female rabbits and embryonic sodium transport

Plasma aldosterone, corticosterone, and cortisol were measured during the first week of pseudopregnancy or pregnancy in New Zealand White rabbits to determine whether any sustained elevations of adrenal steroids occur. There were no pregnancy-specific alterations in circulating adrenal steroid concentrations during the preimplantation stages of embryonic development. Elevation of plasma aldosterone in vivo did not induce amiloride-sensitive Na+ transport across the embryonic trophectoderm. It therefore seems unlikely that an increase in maternal adrenal steroid concentrations is necessary for the development of amiloride-sensitive Na+ transport in rabbit blastocysts. Sodium efflux from Day 6 post coitum (p.c.) blastocysts was lower than Na+ influx. By day 7 p.c. Na+ efflux was equivalent in magnitude to the component of Na+ influx not inhibited by amiloride. This suggests that between Days 6 and 7 p.c. the amiloride-sensitive component of Na+ influx becomes essential for blastocyst expansion.