Fig. 4. Representative time course of the short-circuit current (I_sc) across a proximal split lamella of a posterior gill with NaCl saline on both sides. The following manipulations are shown: 1, addition of ouabain (2 mmol l^–1) to the internal bathing medium; 2, substitution of Na⁺ on both sides of the preparation. Substitution and readministration of Na⁺ resulted in fast, transient current overshoots (to approximately –40 and 60 µA cm^–2, respectively). These I_sc transients are due to concentration gradients resulting from the non-simultaneous replacement of the external and internal bathing media. The vertical current deflections are due to short voltage pulses and reflect the preparation conductance (G_te; see upper right corner for scale).