Leucokinin and the modulation of the shunt pathway in Malpighian tubules

Transepithelial secretion in Malpighian tubules of the yellow fever mosquito (Aedes aegypti) is mediated by active transport of Na+ and K+ through principal cells and passive Cl− transport through the shunt. Permeation through the shunt was assessed by measuring transepithelial halide diffusion potentials in isolated perfused Malpighian tubules, after first inhibiting active transport with dinitrophenol. Diffusion potentials were small under control conditions, revealing Eisenman selectivity sequence I (I−>Br−>Cl−>F−) which is the halide mobility sequence in free solution. Accordingly, electrical field strengths of the shunt are small, selecting halides for passage on the basis of hydrated size. Leucokinin-VIII (LK-VIII) significantly increased the shunt conductance from 57.1 μS/cm to 250.0 μS/cm. In parallel, the shunt selectivity sequence shifted to Eisenman sequence III (Br−>Cl−>I−>F−), revealing increased electrical field strengths in the shunt, now capable of selecting small, dehydrated halides for passage. High concentrations of peritubular F− (142.5 mM) duplicated the effects of LK-VIII on shunt conductance and selectivity, suggesting a role for G-protein. In the presence of LK-VIII (or F−), coulombic interactions between the shunt and I− and F− may be strong enough to cause binding, thereby blocking the passage of Cl−. Thus, LK-VIII increases both shunt conductance and selectivity, presumably via G-protein.