Ascaris suum possesses a large number of FMRFamide-related peptides (FaRPs) of which KNEFIRFamide (AF1),
KHEYLRFamide (AF2) and KSAYMRFamide (AF8/PF3) have been shown to modulate the intrinsic, rhythmic activity
of the vagina vera of A. suum in vitro. In the present study, the effects of the nematode FaRPs, SDPNFLRFamide (PF1),
SADPNFLREamide (PF2) and KPNFIRFamide (PF4) (from Panagrellus redivivus) and AVPGVLRFamide (AF3) and
GDVPGVLRFamide (AF4) (from A. suum) on the in vitro activity of the vagina vera were examined. The effects of each
of the peptides were qualitatively and quantitatively distinct. All 3 FaRPs from P. redivivus were inhibitory, causing a
cessation of contractions. PF2 was 3 times more potent than PF1, with a threshold of 1 nM. Although PF4 was the least
potent (threshold, 10 nM), its effects at [ges ]10 nM were quantitatively the greatest. Both AF3 and AF4 (1 μM) induced
complex, multiphasic responses consisting of an initial contraction and spastic paralysis followed by a return of contractile
activity of increased amplitude. AF3 was 3 times more potent than AF4. The effects of these peptides had some similarities
to those observed on A. suum somatic body wall muscle in vitro, with PF1, PF2 and PF4 being inhibitory and AF3 and
AF4 being excitatory.