Transport of model peptides across Ascaris suum cuticle

Several FMRFamide-related peptides (FaRPs) Found in nematodes exert potent excitatory or inhibitory effects on the somatic musculature of Ascaris suum and other nematode species when injected into the pseudocoelom or applied directly to isolated: neuromuscular preparations. These peptides, however, generally fail to induce detectable effects on the neuromusculature when ap plied externally to intact nematodes. The apparent lack of activity for the se peptides when administered externally in whole-organism assays is likely a function of both absorption and metabolism. To delineate the factors tha t govern transport of peptides across the cuticle/hypodermis complex of nem atodes, we measured the rates of absorption of a series of structurally rel ated model peptides using isolated cuticle/hypodermis segments from A. suum and two-chamber diffusion cells. [C-14]-Labeled peptides were prepared fro m D-phenylalanine: with the amide nitrogens sequentially methylated to give AcfNH(2,) Acf(3)NH(2), Acf(NMef)(2)NH2 and Ac(NMef)(3)NHMe. These model pe ptides were designed to allow systematic analysis of the influence of pepti de size, hydrogen bonding and lipophilicity on transport. Results of these studies show that. within this series, permeability across the cuticle incr eases with addition of each methyl group. The permeability coefficient of A c(NMef)(3)NHMe, with four methyl groups, was 10-fold greater than that of t he smaller peptide. AcfNH(2), even though both peptides contain five hydrog en bonds. When compared with vertebrate membranes, transport of the model p eptides across A. suum cuticle was about 10-fold slower. A biophysical mode l for transcuticular transport of peptides predicted that nematode FaRPs, w hich are larger, less methylated and less lipophilic than the model peptide s tested, would not be absorbed across the cuticle of nematodes. This predi ction was confirmed for the excitatory FaRP, AF2 (KHEYLRFamide), which did not diffuse across the cuticle/hypodermis complex, but diffused rapidly acr oss lipid-extracted cuticle preparations. (C) 2000 Elsevier Science B.V. Al l rights reserved.