fmrfamide and Ascariasis

Helminth infections are of significant concern in veterinary and human medicine. The drugs available for chemotherapy are limited in number and the extensive use of these drugs has led to the development of resistance in parasites of animals and humans (Geerts and Gryseels, 2000; Kaplan, 2004; Osei-Atweneboana et al., 2007). The cyclooctadepsipeptide, emodepside, belongs to a new class of anthelmintic that has been released for animal use in recent years. Emodepside has been proposed to mimic the effects of the neuropeptide PF1 on membrane hyperpolarization and membrane conductance (Willson et al., 2003). We investigated the effects of PF1 on voltage-activated currents in Ascaris suum muscle cells. The whole cell voltage-clamp technique was employed to study these currents. Here we report two types of voltage-activated inward calcium currents: transient peak (I(peak)) and a steady-state (I(ss)). We found that 1microM PF1 inhibited the two calcium currents. The I(peak) decreased from -146nA to -99nA (P=0.0007) and the I(ss) decreased from -45nA to -12nA (P=0.002). We also found that PF1 in the presence of calcium increased the voltage-activated outward potassium current (from 521nA to 628nA (P=0.004)). The effect on the potassium current was abolished when calcium was removed and replaced with cobalt; it was also reduced at a higher concentration of PF1 (10microM). These studies demonstrate a mechanism by which PF1 decreases the excitability of the neuromuscular system by modulating calcium currents in nematodes. PF1 inhibits voltage-activated calcium currents and potentiates the voltage-activated calcium-dependent potassium current. The effect on a calcium-activated-potassium channel appears to be common to both PF1 and emodepside (Guest et al., 2007). It will be of interest to investigate the actions of emodepside on calcium currents to further elucidate the mechanism of action.

Topics: Animals; Ascariasis; Ascaris suum; Calcium; Calcium Channels; Cobalt; Depsipeptides; Dose-Response Relationship, Drug; FMRFamide; Humans; Membrane Potentials; Muscles; Oligopeptides; Patch-Clamp Techniques; Potassium Channels, Calcium-Activated

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 applied externally to intact nematodes. The apparent lack of activity for these peptides when administered externally in whole-organism assays is likely a function of both absorption and metabolism. To delineate the factors that govern transport of peptides across the cuticle/hypodermis complex of nematodes, we measured the rates of absorption of a series of structurally related model peptides using isolated cuticle/hypodermis segments from A. suum and two-chamber diffusion cells. [14C]-Labeled peptides were prepared from D-phenylalanine, with the amide nitrogens sequentially methylated to give AcfNH2, Acf3NH2, Acf(NMef)2NH2, and Ac(NMef)3NHMe. These model peptides were designed to allow systematic analysis of the influence of peptide size, hydrogen bonding and lipophilicity on transport. Results of these studies show that, within this series, permeability across the cuticle increases with addition of each methyl group. The permeability coefficient of Ac(NMef)3NHMe, with four methyl groups, was 10-fold greater than that of the smaller peptide, AcfNH2, even though both peptides contain five hydrogen bonds. When compared with vertebrate membranes, transport of the model peptides across A. suum cuticle was about 10-fold slower. A biophysical model for transcuticular transport of peptides predicted that nematode FaRPs, which are larger, less methylated and less lipophilic than the model peptides tested, would not be absorbed across the cuticle of nematodes. This prediction was confirmed for the excitatory FaRP, AF2 (KHEYLRFamide), which did not diffuse across the cuticle/hypodermis complex, but diffused rapidly across lipid-extracted cuticle preparations.

Topics: Animals; Ascariasis; Ascaris suum; Biological Transport; Chromatography, High Pressure Liquid; FMRFamide; Kinetics; Peptides; Permeability