Fig. 1. Exogenous 8-bromo-cyclic AMP stimulates secretion of Na⁺-rich urine by An. gambiae Malpighian tubules. Fluid secretion and urine Na⁺ and K⁺ concentrations were initially measured over a 30 min control period and then at 10–15 min intervals after the addition of 1 mmol l^–1 8-bromo-cyclic AMP. Data points show the means ± S.E.M. for five tubules. (A) Fluid secretion (solid line) and the tubule fluid [Na⁺]:[K⁺] ratio (broken line) increase after the addition of 8-bromo-cyclic AMP. (B) This reflects the selective stimulation of transepithelial Na⁺ transport (solid line) compared with K⁺ transport (broken line). Arrows show the time of addition of 8-bromo-cyclic AMP.