Mechanisms of K+ transport across basolateral. membranes of principal cells in Malpighian tubules of the yellow fever mosquito, Aedes aegypti

摘要

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 (V-s) 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 (V-t) from 19.4 mV to 17.2 mV, increased the TEP resistance (R-t) from 6.4 kOmegacm to 6.9 kOmegacm, hyperpolarized the basolateral membrane voltage of principal cells (V-bl) from -75.2 mV to -88.2 mV and increased the cell input resistance from 363.7 kOmega to 516.3 kOmega. These effects of Ba2+ reflect the block of K+ channels that, apparently, are also permeable to Na+. Bumetanide (100 mumol l(-1)) had no effect on TEP fluid secretion and electrical resistance but significantly decreased TEP K+ secretion, consistent with the inhibition of electronentral 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+.