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Stopped flow microperfusion technique ( Am. J. Physiol. 195: 563, 1958) was used to study water movement across the proximal tubular wall of Necturus kidney. In 23 experiments, net water movement was measured from perfusion solutions containing 50, 62.5, 75 and 100 mEq. NaCl/1. which were made isosmotic with Necturus plasma by addition of mannitol. Water movement was shown to depend upon luminal NaCl concentration. Studies of the relationship between net solute flux and water flux demonstrated a linear relationship: net water flux (mµl/sec.) = 9.4 x net solute flux + 0.003. Net water flux is statistically zero when net solute flux is zero. Under these experimental conditions no force is important for water movement other than that arising from solute movement. It is concluded that net movement of Na has taken place up an electrochemical potential gradient, indicating active transport of this ion. Furthermore, movement of water from the tubule is considered to be passive since net water flux may be accounted for quantitatively in terms of osmotically induced forces arising from net solute movement.
Windhager et al. (Sat,) studied this question.