Biochimica et Biophysica Acta, 448 (1976) 411--425 Elsevier/North-Holland BiomedicalPress BBA 77463 FACTORS AFFECTING THE COMPARTMENTALIZATION ION WITHIN RABBIT ILEUM IN VITRO OF SODIUM N. L. SIMMONS and R. J. NAFTALIN Department of Physiology, King's College, London, WC2.R 2LS (U.K.) (Received March 22nd, 1976) SUMMARY (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 rain- 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 30.44-4.1 (S.D.) mequiv., 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 extracellulax [Na + ]. Extracellular [Na + ] is increased to 2364-22 (S.D.) mequiv. whilst intracellular [Na + ] is increased to 42.6-4-8.8 (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. INTRODUCTION Transcellular flow of water through epithelial cells is presumed to be a response to osmotic pressure acting across the basal-lateral cell borders resulting from deposi- tion of hypertonic NaCI within the lateral spaces by Na+-pump activity [1 ]. Good evidence for the existence of such a mechanism would be the finding of extracellular hypertonicity that was dependent on (Na++K+)-ATPase activity. Machen and Diamond have made an indirect measurement of the concentration of NaCI within the lateral spaces of rabbit gall-bladder; they conclude that the lateral space is some 80 mosmol more concentrated than the bathing media [2]. Similarly Zybler et al. [3, 4] have inferred that a high concentration of Na ÷ is localized within