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Abstract

EPITHELIA such as that of Necturus gallbladder consist of a pavement-like sheet of uniform cuboidal cells of 20×20×20 µm. As seen in the electron microscope, the clefts between the cells (lateral intercellular spaces, LIS) are ∼0.1–1 µm wide. They are closed against the gallbladder lumen by so-called ‘tight’ junctions — which, however, allow the passage of small ions — and at the other end communicate more freely with the interstitial fluid compartment of the body through slits of > 30–50 nm width. The lateral membranes of the epithelial cells which face the LIS are highly folded (the surface amplification factor in Necturus gallbladder is around 20 (ref. 2) but the basal membranes which face the interstitial fluid compartment directly are not, and so it is evident that virtually all matter which is reabsorbed from the lumen after passing the cell membranes (or the tight junctions) must enter the LIS before it reaches the interstitial fluid compartment proper. It is generally assumed that the LIS have a central role in isotonically transporting epithelia by acting as the main coupling site between salt and water transport. Although elaborate models have been used in attempts to describe the salt and water coupling in detail mathematically, the results have been inconclusive, mainly because there was no definitive experimental proof. A central question, whether or not the LIS fluid is hypertonic as postulated by the standing gradient hypothesis of isotonic fluid absorption, remained open, until recently, when the first direct measurements of the ion concentrations in the lateral spaces were published. These data were obtained by means of electron-probe analysis of frozen tissue sections of rabbit ileum and revealed a substantial hypertonicity. We have tried the alternative approach of measuring the ion concentrations in the LIS directly using ion-selective microelectrodes. Our observations, which are reported here, indicate that as far as the K⁺ concentration is concerned the ion accumulation in the LIS of Necturus gallbladder is much lower than suggested by the electron-probe data from rabbit ileum.