BIOPHYSICAL PROPERTIES OF EPITHELIAL WATER CHANNELS

The biophysical models describing the structure of water pores or chan nels have evolved, during the last forty years, from a pure 'black box ' approach to a molecular based proposal. The initial 'sieving pore' i n which water and other molecules were moving together was replaced by a more restrictive model, where water is moving alone in a 'single fi le' mode. Aquaporins discovery and cloning [G.M. Preston, T.P. Carroll , W.B. Guggino, P. Agre, Science 256 (1992) 365] leaded to the 'hour-g lass model' and other alternative proposals, combining information com ing from molecular biology experiments and two dimensional crystallogr aphy. Concerning water transfers in epithelial barriers the problem is quite complex, because there are at least two alternative pathways: p aracellular and transcellular and three different driving forces: hydr ostatic pressure, osmotic pressure or 'transport coupled' movements. I n the case of ADH-sensitive epithelia it is more or less accepted that regulated water channels (AQP2), that can be inserted in the apical m embrane, coexist with basolateral resident water channels (AQP3). The mechanism underlying the so-called 'transport associated water transfe r' is still controversial. From the classical standing gradient model to the ion-water co-transport, different hypothesis are under consider ation. Coming back to hormonal regulations, other than the well-known regulation by neuro-hypophysis peptides, a steroid second messenger, p rogesterone, has been recently proposed [P. Ford, G. Amodeo, C. Capurr o, C. Ibarra, R. Dorr, P. Ripoche, M. Parisi, Am. J. Physiol. 270 (199 6) F880]. (C) 1997 Elsevier Science B.V.