Fig. 6. Time-course of the transepithelial potential difference (V_te) across an isolated perfused posterior gill (representative of five trials). (A) Theophylline sustains the elevated V_te caused by forskolin. (B) Theophylline partially sustains the elevated V_te caused by Hyposmotic saline. Arrows indicate application of the different treatments. Iso + Theo: Isosmotic saline plus theophylline (2.5 mmol l^-1); Iso + Forsk: Isosmotic saline plus forskolin (0.01 mmol l^-1); Hypo + Theo: Hyposmotic saline plus theophylline (2.5 mmol l^-1).