Leucokinin Increases Paracellular Permeability in Insect Malpighian Tubules

Abstract

There are two major transport pathways across epithelia: a transcellular pathway through cells and a paracellular pathway between cells. Previous electrophysiological studies in mosquito Malpighian tubules suggested that the neuropeptide leucokinin-VIII (LK-VIII) increases the chloride permeability of the paracellular pathway. To test the effect of LK-VIII on the paracellular pathway further, we measured transepithelial permeabilities of inulin and sucrose in isolated Malpighian tubules from the mosquito Aedes aegypti. Cell membranes are impermeable to inulin and sucrose, leaving the paracellular pathway as the only route for their transepithelial permeation. LK-VIII (10(-6) mol l-1) significantly increased transepithelial permeability to both inulin (by 73.8 %) and sucrose (by 32.4 %) in parallel with a significant increase in rates of transepithelial fluid secretion (by 75­90 %). Cyclic adenosine monophosphate (cyclic AMP, 10(-4) mol l-1), which is known to stimulate transcellular transport, also increased rates of transepithelial fluid secretion (by 57­59 %), but it did so without increasing the permeability to sucrose and inulin. Thus, LK-VIII increases the permeability of the paracellular pathway whereas cyclic AMP does not.

Inorganic and organic anion transport by insect renal epithelia

“…Future studies, perhaps using the cell-attached patch technique or perforated patch whole-cell recording, could directly assess the possibility that the apical Cl À channels in the stellate cells of Malpighian tubules are controlled through increases in intracellular calcium in response to leucokinin. In contrast to the view that stellate channels play a cardinal role in transepithelial Cl À flux, there has also been a suggestion of a paracellular route for Cl À through septate junctions in Aedes tubules stimulated with leucokinin [39,40].…”