Development of sodium and chloride transport across fetal and newborn rat stomach in vitro.

1. Unidirectional and net Na+ and Cl- fluxes were determined across isolated fetal rat stomach 19-21 days post-coitum, and across the stomach of newborn rats aged 5 or 12 days. 2. On fetal day 19, absorption of both Na+ and Cl- was greater than the short-circuit current, Isc (net Na+ flux, JnetNa = 4.7 +/- 1.0 and net Cl- flux, JnetCl = 5.4 +/- 1.4 mu equiv cm-2 h-1 vs. Isc = 0.9 +/- 0.1 mu equiv cm-2 h-1). Mucosal addition of 10 microM-amiloride did not significantly alter JnetNa, JnetCl, Isc or total conductance. 3. However, on fetal day 20, neutral absorption of NaCl was no longer observed but amiloride had inhibited electrogenic absorption of Na+, and significant active secretion of Cl- was observed (JnetCl = -1.3 +/- 0.6 mu equiv cm-2 h-1). On day 21 (i.e. 24 h before birth), values for JnetNa, JnetCl, and Isc were not different from those determined on adult gastric mucosa. 4. After birth, NaCl transport continued to exhibit its prenatal characteristics on day 5 but not on day 12, when Na+ and Cl- were both absorbed; on that day, JnetNa-Isc was equal to both JnetCl and to the amiloride-insensitive component of Isc, indicating that neutral NaCl absorption had resumed. 5. These data show that in rat stomach, NaCl transport differentiates on fetal day 20, when H+ secretion is first observed, and thereafter undergoes biphasic development. 6. The significant Cl- absorption observed on post-natal day 12 was concomitant with the inhibition of net H+ secretion.