title: Transport of water in proximal kidney tubules from whole tubules to single channels: length and section of the selectivity filter of aquaporin-1.
abstract: Proximal straight tubule (PST) were dissected from rabbit kidneys, held with crimping pipettes in a chamber bathed in a buffered mannitol isosmotic solution (MBS, 295 mOsm/kg). Tubule cell volume changes with time (dV/Adt) after steps in MBS osmolality (delta Cs) were monitored on line with an inverted microscope, a TV camera and an image processor. Reflection coefficients sigma and osmotic permeability coefficients, Pos, for several solutes were measured using two methods. Method 1: sigma was calculated from the delta Csiso of impermeant and permeant solutes at which (dV/Adt)t-->0 = 0 (i.e., by a null point method). It is denoted as sigma 1. sigma 1 = 1.00 for mannitol (M), raffinose (R), sucrose (S), glycerol (G), acetamide (A) and urea (U). With formamide (F), sigma 1, Formamide = 0.62 +/- 0.05. These findings confirm our previous value of dp = 4.5 A for the diameter of the selectivity filter of the basolateral PST cell membrane water channel AQP1. Method 2: PST were exposed for 20 s to MBS made hyperosmotic by addition of a delta Cs of 35 mOsm/kg of R, S, M, G, A and U. Cells shrunk within 500 ms of t = 0 to their osmometric volume and remained shrunk for the 20 s of the osmotic challenge. Pos was measured from the shrinking curves. P(os) = 3000 +/- 25 microns/s with R, S, M, G, A and U. Method 2 also allowed to calculate sigma, denoted as sigma 2. sigma 2 = 1.00 for R, S, M, G, A and U. By contrast, the shrinking curve produced by a delta Cs of 35 mOsm/kg F was 1/5th to 1/6th slower and smaller (i.e., subosmometric) than that produced by a delta Cs of 35 mOsm/kg R, S, M, G, A and U. Furthermore, with F cells did not remain shrunk but recovered their original volume within 3 s. P(os) (measured with F) is denoted as P(os)*, P(os)* = 480 +/- 30 microns/s. sigma 2, Formamide = 0.16 +/- 0.01. Use of sigma 1, sigma 2 and P(os)* values in Hill's equations for the bimodal theory of osmosis leads to n = 2-9. Where n is the number of water molecules single filling within the channel selectivity filter, whose length must lie within 6 to 27 A, a value significantly lower than our previous value calculated from the P(os)/Pd* ratio.
authors.affiliation: Instituto Venezolano de Investigaciones Científicas, IVIC, Caracas, Venezuela.
authors.forename: G
authors.initials: G
authors.lastname: Whittembury
authors.forename: E
authors.initials: E
authors.lastname: González
authors.forename: C S
authors.initials: CS
authors.lastname: Hernández
authors.forename: A M
authors.initials: AM
authors.lastname: Gutiérrez
authors.forename: M
authors.initials: M
authors.lastname: Echevarría
mesh_terms: D000818:Animals; D051398:Aquaporin 1; D020346:Aquaporins; D002478:Cells, Cultured; D007473:Ion Channels / Q000502:physiology*; D007687:Kidney Tubules, Proximal / Q000502:physiology*; D011817:Rabbits; D014882:Water-Electrolyte Balance / Q000502:physiology*
chemical_list: D020346:Aquaporins; D007473:Ion Channels; D051398:Aquaporin 1