Channels for water flow in epithelia: characteristics and regulation

Pos and Pd, osmotic and diffusive water permeability coefficients of isolated rabbit proximal tubule (PT) cells (expressed per cm2 of real cell membrane area, in microns/sec) are: Pos 396; Pd, 22; Pos/Pd, 18 (control), and after exposure to parachloromercuribenzenesulfonate (pCMBS): Pos, 32; Pd, 10; Pos/Pd, 3. The sulfhydryl reagent dithiothreitol (DTT) reverts pCMBS action. The activation energies (kcal/mol) are Pos, 3.2 (control); 9.2 (pCMBS); Pd, 5.2 (control) and 9.1 (pCMBS). Thus water channels pierce the control plasma membrane and are reversibly closed by pCMBS. High control PT permeabilities are comparable with those of amphibian urinary bladder and collecting tubules (CT) stimulated with antidiuretic hormone (ADH), and low PT (pCMBS) values with those of CT in the resting state, respectively. Transcellular permeability is regulated in PT by the state of sulfhydryl groups and in CT by ADH induced insertion (or, no ADH, suppression) of water channels. In PT (a) large extracellular markers are dragged by water flow indicating extracellular solute-water interaction, (b) transepithelial Pos is much higher than transcellular Pos. Therefore water also flows paracellularly, in addition to the transcellular flow. In PT paracellular permeability is increased if urea in lumen is higher than in blood. It is reduced in the reverse situation. In CT paracellular permeability is virtually zero (resting condition). It may be increased by high lumen urea. Thus, paracellular permeability (which is significant in control PT and zero in CT) can be regulated by changes in the transepithelial urea concentrations. Transcellular permeability depends on the number of channels/cm2 epithelium, their probability of being opened and their individual permeability.(ABSTRACT TRUNCATED AT 250 WORDS)