Membrane dynamics in relation to fluid absorption in reptilian proximal renal tubules

Net fluid absorption (J(v)), cell volume, and cell membrane area have been studied in isolated, perfused snake (Thamnophis spp.) proximal tubules. With Na⁺ in both perfusate and bathing medium, J(v) averages ~ 0.9 nl · min^-1 · mm^-1. When choline replaces Na⁺ in perfusate, J(v) nearly ceases. When choline also replaces Na⁺ in bathing medium, so that both solutions are identical, J(v) returns to the control rate. The results are the same when tetramethylammonium replaces Na⁺, when sucrose replaces Na⁺ and the equivalent amount of Cl^-, and when methyl sulfate replaces Cl^- alone. However, when Li⁺ replaces Na⁺ in perfusate alone or in both perfusate and bathing medium, J(v) is unchanged. J(v) at control rates is isosmotic and can be partially inhibited by cold and cyanide. When choline replaces Na⁺ in perfusate and bathing medium, cell volume doubles, and intercellular space volume nearly quintuples. The areas of the lateral and apical cell membranes also approximately double, so that the surface area-to-volume ratio remains constant. These morphological changes in the absence of Na⁺ occur concomitantly with maintenance of J(v), suggesting that they may play a permissive role in such maintenance.