Cell water content in carp kidney tissue slices as influenced by various osmotic agents.

: Determination of water content in the surviving kidney tissue of the carp (Cyprinus carpio) is a convenient method for studying epithelial cell volume regulation under the influence of various osmotic agents and inhibitors. Artificial media of abnormal tonicities were used both as a purely experimental tool and as an attempt to model the behaviour of the tissue under extreme (physiological or pathological) conditions. In a medium hypertonic with mannitol the inhibition of the sodium-potassium pump by ouabain does not change the water content of the tissue (the same as in normal Krebs-Henseleit saline). On the other hand, in media which are hypotonic or hypertonic by addition of NaCl, KCl, Tris chloride or sodium benzenesulphonate the same inhibition invariably results in an increased water content. Thus, in contrast to the situation in normal or mannitol-hypertonic media, the ouabain-sensitive sodium-potassium pump does regulate the cell volume in artificial salines which are hypotonic or hypertonic by addition of electrolytes. 2,4-dinitrophenol often brings about a more pronounced swelling of the tissue than ouabain, suggesting the presence of an additional cell-volume regulating mechanism, ATP-dependent but ouabain-insensitive. The additional swelling is not always accompanied by a gain of sodium; hence it may not be correct to call the above mechanism a "second sodium pump". On the whole, the carp kidney tissue appears to behave as an osmometer with a certain incompressible volume, gaining or losing either sodium or potassium chloride in various abnormal media or with the two above inhibitors.