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Fig. 1. Transepithelial fluid secretion in isolated Malpighian tubules of female yellow fever mosquitoes studied by the method of Ramsay (1953). Cumulative secreted volume is plotted against time first under control conditions and then in the presence of (A) Ba²⁺ and (B) Ba²⁺ plus bumetanide. After the usual initial 30 min control period, fluid secreted by the tubules was removed for analysis with an electron microprobe, and 5 mmol l^–1 BaCl₂ (A) or 5 mmol l^–1 BaCl₂ plus 100 µmol l^–1 bumetanide (B) was added to the peritubular Ringer solution. Fluid secreted under experimental conditions was collected at the end of the experiment, 25–70 min after introducing the agent of interest to the peritubular Ringer bath. The slope of regression lines yields the flow rate from the open end of the tubule, which equals the rate of transepithelial fluid secretion. Each tubule is used as its own control. Values are means ± S.E.M.

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