AF3 (AVPGVLRFamide) and AF4 (GDVPGVLRFamide) are endogenous RFamide-like
peptides isolated from the
parasitic nematode Ascaris suum. Here the actions of these peptides
on the somatic musculature of Ascaris have been
investigated and compared to the action of acetylcholine (ACh), the
excitatory transmitter at the neuromuscular junction.
ACh, AF3 and AF4 contracted muscle with EC50s of
13±1 μM, 24±6 nM and 37±2 nM,
respectively (n=6). The muscle
cells were depolarized by ACh (3 μM; 5·2±0·4
mV,
n=42), AF3 (1 μM; 2·6±0·3 mV,
n=19) and AF4 (1 μM;
3·3±0·4 mV, n=19). EC50s were
681±329 nM (AF3) and 901±229 nM (AF4),
but an
estimate could not be made for ACh
due to muscle contraction at concentrations greater than 10 μM.
The
depolarization to 3 μM ACh was abolished by the
nicotinic receptor antagonist mecamylamine (10 μM;
n=5) but the responses to the peptides were not (111±7%
and
108±17% with respect to control; n=5). The depolarization
elicited by ACh was reduced to a greater extent by a 50%
reduction in extracellular Na+
concentration than the response to AF3 and AF4 (P<0·02).
Cobalt was more effective at
blocking the AF3 and AF4 depolarizations than those to ACh. These
observations suggest that AF3 and AF4 contract
Ascaris muscle without an action at the Ascaris nicotinic
receptor. Furthermore, the ionic mechanism through which AF3
and AF4 depolarize Ascaris muscle is different from that for ACh.
ACh,
AF3 and AF4 were also found to contract Ascaridia
galli somatic muscle with EC50s of 13±3 μM,
721±236 nM and 371±177 nM, respectively
(n=7). The muscle cells were
depolarized by ACh (EC50=14±5 μM, n=5),
AF3 (EC50=5±3 μM, n=4) and AF4
(EC50=10±5 μM, n=4). Therefore the
response
to these peptides is not unique to Ascaris and they may subserve
a
functional role in the motor nervous system of parasitic nematodes.