Leucokinin activates Ca(2+)-dependent signal pathway in principal cells of Aedes aegypti Malpighian tubules

The role of CA(2+) in mediating the diuretic effects of leucokinin-VIII was studied in isolated perfused Malpighian tubules of the yellow fever mosquito, Aedes aegypti. Peritubular leucokinin-VIII (1 microM) decreased the transepithelial resistance from 11.2 to 2.6 kOmega. cm, lowered the transepithelial voltage from 42.8 to 2.7 mV, and increased transepithelial Cl(-) diffusion potentials 5.1-fold. In principal cells of the tubules, leucokinin-VIII decreased the fractional resistance of the basolateral membrane from 0.733 to 0.518. These effects were reversed by the peritubular CA(2+)-channel blocker nifedipine, suggesting a role of peritubular CA(2+) and basolateral CA(2+) channels in signal transduction. In CA(2+)-free Ringer bath, the effects of leucokinin-VIII were partial and transient but were fully restored after the bath CA(2+) concentration was restored. Increasing intracellular CA(2+) with thapsigargin duplicated the effects of leucokinin-VIII, provided that peritubular CA(2+) was present. The kinetics of the effects of leucokinin-VIII is faster than that of thapsigargin, suggesting the activation of inositol-1,4,5-trisphosphate-receptor channels of intracellular stores. Store depletion may then bring about CA(2+) entry into principal cells via nifedipine-sensitive CA(2+) channels in the basolateral membrane.