Factors affecting the compartmentalization of sodium ion within rabbit ileum in vitro.

(1) Net Na+ loss from rabbit ileum, stripped of its serosal muscle layers, into ice cold choline chloride is consistent with loss from two separate pools (rate constants 0.102 and 0.011 min-1). Since cell K+ is lost with a single rate constant, 0.0062 min-1) and inulin, a good extracellular marker, is lost with a single rate constant 0.082 min-1, it is inferred that the fast rate constant of Na loss characterizes loss from an extracellular pool and the slow constant, loss from an intracellular pool. (2) The [Na+] in the inulin space (extracellular) was calculated to be 180 +/- 13 (S.D.) mequiv. and the [Na+] in the intracellular space 30.4 +/- 4.1 (S.D.) medquiv., this provides evidence that the paracellular spaces are, at least 80 mosmol hypertonic to the external Ringer. (3) There is a saturable galactose-dependent increase in both the intracellular and extracellular [Na+]. Extracellular [Na+] is increased to 236 +/- 22 (S.D.) mequiv. Whilst intracellular [Na+] is increased to 42.6 +/- 8.8 (S.D.) mequiv. when Ringer [galactose] is 10 mM. Galactose-dependent increases in total tissue [Na+] can thus be attributed mainly to the increase in extracellular [Na+]. (4) Extracellular hypertonicity, both in the presence and absence of galactose, is dependent upon the [Na+] of the bathing Ringer. 0.1 mM ouabain abolishes the extracellular hypertonicity. This observed extracellular hypertonicity in normally functioning tissue may provide the driving force for transcellular convective flow of salt, water and sugars.