Introductory Remarks

Water movement occurs to a variable degree across every epithelial layer of cells and plays a determinant role in the homeostasis of body fluid balance. Is such water transport the result of a difference of water activity or osmotic pressure across the epithelium? The answer to that question is more often negative as demonstrated by the phenomenon of isotonic water movement. Many epithelia transport water in near perfect isosmotic concentration, i.e., at total solute concentrations equal to both the blood and the apical surface, in the absence of an osmotic gradient across the cell layer, and in close correlation to and coupled with solute transport. Although water flow can occur from a compartment of low activity to a compartment of higher activity, water movement has not been found to be an independent or active process. Any model explaining isotonic water movement must reconcile these three experimental facts: 1. isosmotic absorption or secretion. 2. absence of external osmotic or hydrostatic pressure differences. 3. close coupling between solvent and solute fluxes. Water movement is thus conceived of as driven by osmotic or hydrostatic forces generated within the epithelium itself, such forces being the consequence of primary active transport of solute. The models proposed to date and reviewed in this symposium have essentially used the framework ofan intraepithelial compartment in order to obtain the necessary interaction between solute and solvent flow. Several questions may be raised. 1. What is the anatomical localization of these crucial intraepithelial compartments? 2. What are the passive and active transport parameters of the barriers confining these compartments? 3. What are the solute activities, electrical potential, osmotic and hydrostatic pressure within these compartments? 4. Most importantly how does the interplay ofthese terms result in solute and solvent flow which are an almost perfect isosmotic match. The speakers of this symposium will cover some aspects of these questions. The session is divided into two parts: isotonic water absorption and isotonic water secretion. Isotonic water absorption, i.e., movement from the external environment to the blood, is found in the gallbladder, small intestine and proximal renal tubule. Dr. Stanley G. Schultz will review the role of the paracellular pathways in isotonic fluid transport by absorbing epithelial. Together with Henry Sackin, I will discuss mathematical models of isotonic absorption using membrane parameters particularly appli-