Leucokinin increasess shunt conductance and anion selectivity in malpighian tubules of the Yellow Fever mosquito Aedes aegypti

M.-J. YU (Home Page) & K. W. Beyenbach

Dept. of Biomedical Sciences, Cornell University, VRT 8014, Ithaca, NY 14853, USA

In Malpighian tubules of the yellow fever mosquito, leucokinin-VIII (LK-VIII) opens a transepithelial Cl - conductance located outside principal cells. The parallel drop of the transepithelial voltage and resistance to values less than 10% of control (self short-circuit), symmetrical transepithelial Cl - diffusion potentials, and the dependence of the shunt conductance on the Cl - concentration in the extracellular solutions (both sides), all suggest that leucokinin increases the Cl - conductance of the paracellular pathway delineated by septate junctions (Pannabecker et al., J. Membr. Biol. 132, 63, 1993). The present study was undertaken to investigate the properties of this septate junctional pathway. In vitro microperfusion techniques were utilized to determine the halide selectivity sequence of the shunt conductance by measuring transepithelial bi-ionic diffusion potentials. Relative permeability ratios (PX/PCl) were estimated using the Goldmann constant-field equation. Under control conditions, PX/PCl for iodide and bromide were 1.59 and 1.23 respectively, revealing Eisenman halide seletivity sequence I (I - >Br - >Cl - >F - ). The same selectivity sequence was more clearly observed after inhibiting active transcellular transport with dinitrophenol (DNP). LK-VIII significantly decreased transepithelial voltage from 40.8 mV (control) to 2.7 mV, and transepithelial resistance from 12.8 K?cm to 2.1 K?cm (the self short-circuit). In parallel, the shunt anion selectivity sequence shifted to Eisenman series III (Br - >Cl - >I - >F - ), in the absence or presence of DNP. PX/PCl now were 1.13, 0.35 and 0.22 for Br - , I - and F - respectively. High concentrations of F - (142.5 mM) in the peritubular Ringer bath duplicated the effects of LK-VIII on shunt resistance and selectivity sequence, suggesting a role of G-protein in mediating the effects of LK-VIII. The shunt pathway induced by LK-VIII (or F - ) was blocked by I - and F - itself. Since under control conditions selectivity sequence I is the mobility of the four halides in free solution, we conclude that the shunt pathway has a low electrical field strength, that selects halides on the basis on their hydrated size. In contrast, LK-VIII (or F - ) induces a shunt pathway with moderate electrical field strength such that coulombic interactions between the shunt and halides exceed halide hydration energies, thereby selecting small, dehydrated halides for passage. Thus leucokinin increases the shunt conductance while optimizing the Cl - filter. Coulombic interactions between shunt and halides may be strong enough to cause binding, as indicated by the block of the shunt Cl - conductance by I - and F - . (NSF IBN 9604394).

Index terms: leucokinin, shunt pathway, halide selectivity

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