Membrane conductances of principal cells in Malpighian tubules of Aedes aegypti

Two-electrode voltage clamp (TEVC) methods were used to explore conductive transport pathways in principal cells, the dominant cell type in Malpighian tubules of the yellow fever mosquito. The basolateral membrane of principal cells had a voltage (Vbl) of –85.1 mV in 49 principal cells under control conditions. Measures of the input resistance Rpc together with membrane fractional resistance yielded estimates of the conductance of the basolateral membrane (gbl = 1.48 μS) and the apical membrane (ga = 3.13 μS). K+ channels blocked by barium accounted for 0.94 μS of gbl. Estimates of transference numbers yielded the basolateral membrane Na+ conductance of 0.24 μS, leaving 0.30 μS (20%) of gbl unaccounted. The secretagogue db-cAMP (0.1 mM), a known activator of the basolateral membrane Na+ conductance, significantly depolarized Vbl to –65.0 mV and significantly increased gbl from 1.48 μS to 2.47 μS. The increase was blocked with amiloride (1 mM), a known blocker of epithelial Na+ transport. The inhibition of metabolism with di-nitrophenol significantly depolarized Vbl to –9.7 mV and significantly increased Rpc from 391.6 kΩ to 2612.5 kΩ. Similar results were obtained with cyanide, but it remains unclear whether the large increases in Rpc stem from the uncoupling of epithelial cells and/or the shutdown of conductive transport pathways in basolateral and apical membranes. Our results indicate that the apical membrane of principal cells is more than twice as conductive as the basolateral membrane. Partial ionic conductances suggest the rate-limiting step for transepithelial Na+ secretion at the basolateral membrane.