LEUCOKININ INCREASES PARACELLULAR PERMEABILITY IN INSECT MALPIGHIAN TUBULES

There are two major transport pathways across epithelia: a transcellul ar pathway through cells and a paracellular pathway between cells. Pre vious electrophysiological studies in mosquito Malpighian tubules sugg ested that the neuropeptide leucokinin-VIII (LK-VIII) increases the ch loride permeability of the paracellular pathway. To test the effect of LK-VIII on the paracellular pathway further, we measured transepithel ial permeabilities of inulin and sucrose in isolated Malpighian tubule s from the mosquito Aedes aegypti. Cell membranes are impermeable to i nulin and sucrose, leaving the paracellular pathway as the only route for their transepithelial permeation, LK-VIII (10(-6) mol l(-1)) signi ficantly 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 75-90%). Cyclic adenosine monophosphate (cyclic AMP, 10(-4) mol l(-1)), which is known to stimu late transcellular transport, also increased rates of transepithelial fluid secretion (by 57-59%), but it did so without increasing the perm eability to sucrose and inulin. Thus, LK-VIII increases the permeabili ty of the paracellular pathway whereas cyclic AMP does not.