SUMMARY The mechanisms of K+ entry from the hemolymph into principal cells of Malpighian tubules were investigated in the yellow fever mosquito, Aedes aegypti. The K+ channel blocker Ba2+ (5 mmol l–1) significantly decreased transepithelial (TEP) fluid secretion (Vs) from 0.84 nl min–1 to 0.37 nl min–1 and decreased the K+ concentration in secreted fluid from 119.0 mmol l–1 to 54.3 mmol l–1 with no change in the Cl– concentration. Even though the Na+ concentration increased significantly from 116.8 mmol l–1 to 144.6 mmol l–1, rates of TEP ion secretion significantly decreased for all three ions. In addition,Ba2+ had the following significant electrophysiological effects: it depolarized the TEP voltage (Vt) from 19.4 mV to 17.2 mV,increased the TEP resistance (Rt) from 6.4 kΩcm to 6.9 kΩcm, hyperpolarized the basolateral membrane voltage of principal cells (Vbl) from –75.2 mV to –88.2 mV and increased the cell input resistance from 363.7 kΩ to 516.3 kΩ. These effects of Ba2+ reflect the block of K+ channels that, apparently, are also permeable to Na+. Bumetanide (100μmol l–1) had no effect on TEP fluid secretion and electrical resistance but significantly decreased TEP K+ secretion,consistent with the inhibition of electroneutral Na+/K+/2Cl– cotransport. TEP Na+ secretion significantly increased because other Na+entry pathways remained active. Bumetanide plus Ba2+ completely inhibited TEP electrolyte and fluid secretion, with fast and slow kinetics reflecting the Ba2+ block of basolateral membrane K+channels and the inhibition of Na+/K+/2Cl– cotransport, respectively. The single and combined effects of Ba2+ and bumetanide suggest that(1) K+ channels and Na+/K+/2Cl– cotransport are the primary mechanisms for bringing K+ into cells, (2) K+ channels mediate a significant Na+ influx, (3) Na+ has as many as four entry pathways and (4) the mechanisms of TEP K+ and Na+ secretion are coupled such that complete block of TEP K+ renders the epithelium unable to secrete Na+.

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