The dependence of electrical transport pathways in Malpighian tubules on ATP

Abstract

SUMMARY The relationship between the intracellular ATP concentration[ATP]i and the electrical properties of principal cells was investigated in Malpighian tubules of the yellow fever mosquito, Aedes aegypti. Under control conditions, [ATP]i was 0.91 mmol l-1, the input resistance of the principal cell(Rpc) was 334.1 kΩ, and the basolateral membrane was marked by a large K+-conductance and a membrane voltage(Vbl) of -75.8 mV. Peritubular cyanide (CN, 0.3 mmol l-1) reduced [ATP]i to 0.08 mmol l-1 in less than 2 min; however, Vbl dropped to -8 mV and Rpc increased to 3150.8 kΩ in 8 min, while the K+-conductance of the basolateral membrane disappeared. Upon washout of CN, Vbl and Rpc returned to control values within 2 min, and the basolateral membrane recovered its K+-conductance. The recovery of normal [ATP]i took 15 min. Dose-dependence and EC50 values for the CN-inhibition of Vbl and the increase in Rpc were strikingly similar (184.0 μmol l-1 and 164.4 μmol l-1). Similar effects of metabolic inhibition were observed with dinitrophenol (DNP), but the EC50 values were 50.3 μmol l-1 and 71.7 μmol l-1 for the effects on Vbl and Rpc, respectively. Barium, a blocker of K+-channels, significantly hyperpolarized Vbl to -89.1 mV and increased Rpc to 769.4 kΩ under control conditions, but had no effects during metabolic inhibition. These results illustrate a temporal relationship between[ATP]i and electrogenic and conductive transport pathways in principal cells that is consistent with the role of ATP as an integrator of transport steps at apical and basolateral membranes of the cell. When[ATP]i drops to levels that are 10% of control, the V-type H+-ATPase is inhibited, preventing the extrusion of K+to the tubule lumen. At the same time, basolateral membrane K+-channels close, preventing the entry of K+ from the hemolymph. Intracellular K+ homeostasis is thus protected during metabolic inhibition, allowing the cell to re-establish K+transport when ATP is synthesized again.