Fig. 4. Effects of bumetanide on fluid secretion rates (mean ± S.E.M.) of tubules bathed in (A) control saline, (B) K⁺-free saline or (C) Na⁺-free saline. Tubules were exposed to the vehicle (; 0.1% ethanol) alone or to vehicle plus bumetanide at the following concentrations: , 10^–6 mol l^–1; , 0.5x10^–5 mol l^–1; , 10^–5 mol l^–1; , 10^–4 mol l^–1. Serotonin (10^–6 mol l^–1) was added at t=0 min. Fluid secreted between 0 and 15 min was discarded and the first droplet was collected at t=20 min. Arrows indicate the time of addition of bumetanide. The number of tubules per group is indicated in parentheses.