The effects of the peptide KPNFIRFamide (PF4) on the somatic muscle cells of the parasitic nematode Ascaris suum

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Abstract
1 Commonly used anthelmintic agents act on the muscle cells of parasitic nematodes to cause paralysis of the parasite and its expulsion from the host. 2 The motonervous system of nematodes contains neuropeptides, many of which are myoactive and elicit prolonged worm paralysis. Here we describe the actions of a novel peptide, KPNFIRFamide (Lys‐Pro‐Asn‐Phe‐Ileu‐Arg‐Phe‐amide; PF4), which mediates relaxation of the somatic muscle of the parasitic nematode Ascaris suum. Its mechanism of action is compared to that of the inhibitory neuromuscular junction transmitter, γ‐aminobutyric acid (GABA), which gates a chloride channel on Ascaris muscle. 3 Both PF4 and GABA hyperpolarized the muscle cells (EC50 values 98 nM and 59 μM, respectively; n = 6) and this was accompanied by an increase in input conductance. 4 The increase in input conductance elicited by PF4 and a supramaximal concentration of GABA were additive (10 μm PF4, 7.78 ± 1.88 μS; 10 mM GABA, 4.68 ± 1.39μS; 10 mM GABA and 10μm PF4 12.05±2.6 μS, n = 6, Pr2 = 0.82). This is close to the slope of −26.5 for a chloride‐dependent event, as predicted by the Nernst equation. There was a significant correlation between the reversal potential for this event and the reversal potential for GABA (r = 0.94; Pn = 12). 6 The late response to PF4 (PF4‐late) appeared after 1 min and consisted of a slow reduction in the hyperpolarization to a plateau level, before the return of the membrane potential to the resting value. PF4‐late is not likely to be a chloride‐dependent event as during the hyperpolarization caused by a supramaximal concentration of GABA the muscle cells depolarized when a supramaximal concentration of PF4 was added to the perfusate. The membrane potential in the presence of 1 mM GABA was −61.8±4.8 mV and in the presence of 1 mM GABA with 10 μm PF4 was −47.5± 1.5 mV (Pn = 6). 7 The conductance increase elicited by 30 μm GABA was blocked by 10 μm ivermectin (before ivermectin 0.97 ±0.2 μS, after ivermectin 0.33 ±0.12 μS; n = 5; Pt test) but the conductance increase elicited by 1 μm PF4 was not (before ivermectin 0.96±0.14 μS, after ivermectin 1.07±0.19 μS; n = 0.34; Student's paired t test). 8 These data indicate that PF4 elicits a potent, inhibition of Ascaris muscle cells which is partially mediated by chloride and which is independent of the inhibitory GABA receptor.