Willson, J., Holden-Dye, L., Harder, A. and Walker, R.J.
A possible mechanism for the action of the novel anthelmintic emodepside, using Ascaris suum body wall muscle preparations
Journal of Physiology, 536P, (S103), .
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The increasing resistance of parasitic nematodes, to existing anthelmintics has encouraged the search for novel compounds. A potential anthelmintic PF1022A, a 24 membered cyclic depsipeptide, has been shown to act as a potent broad-spectrum anthelmintic (Von Samson-Himmelstjerna et al. 2000). PF1022A causes a fast onset of paralysis of the nematodes, favouring the view that it is neuropharmalogically active. This study investigates the in vitro effects of a semi-synthetic, structurally similar derivative, emodepside, on the pig intestinal parasite Ascaris suum. Ascaris dorsal muscle strips (DMS) were bathed in artificial perienteric fluid (APF) (Trim et al. 1997). Statistical comparison was by Student's paired or unpaired t test, as stated. As a control, the DMS was contracted using acetylcholine (30 µM, 10 min) and maximum contraction was normalised to 100 %. A slow relaxation of the DMS was then observed over a period of 10 min. Addition of 10 µM emodepside at the point of maximum contraction, caused a significantly (P < 0.005, unpaired) faster relaxation of the DMS, 9.0 ± 0.3 % min-1 (mean ± S.E.M., n = 5), compared with the ACh control (6.1 ± 0.6 % min-1, n = 5).
The effects of emodepside on Ascaris somatic muscle cells were investigated using a 2-microelectrode set-up. A 2 min perfusion of emodepside over muscle cells caused a significant (P < 0.001, paired) slow hyperpolarisation of 5.5 ± 0.96 mV (n = 8), with no change in input conductance after 30 min. In the absence of calcium, 10 µM emodepside caused no significant change in membrane potential. Re-introduction of calcium resulted in a slow significant (P < 0.002, paired) hyperpolarisation of 5.5 ± 1.1 mV (n = 8).
Perfusion of the muscle cells with the potassium channel blockers 4-aminopyridine (4AP, 250 µM) and tetra-ethylammonium (TEA, 5 mM) resulted in a slight depolarisation of muscle cells 2.8 ± 0.5 mV (n = 5). Emodepside (10 µM) was then applied, and no significant change in the membrane potential occurred. Following wash-out of 4AP and TEA, the membrane potential hyperpolarized by 5.2 ± 2 mV (n = 5) over a 30 min period, although this effect was not significant. These results indicate that potassium is required for emodepside hyperpolarisation and this, combined with the results with calcium-free APF, suggests that this compound's relaxation effect may be mediated via a calcium-activated potassium channel.
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