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C.G. Moneypenny, N. Kreshchencko, T.A. Day, C. Moffett, D.W. Halton and A.G. Maule.
Parasitology 122:447-455
The physiological effects of selected classical transmitters
and FMRFamide-related peptides (FaRPs) on dispersed muscle fibres
from the marine turbellarian, Procerodes littoralis have been
examined. Confocal scanning laser microscopy coupled with fluorescein
isothiocyanate (FITC) or tetramethylrhodamine (TRITC)-labelled
phalloidin revealed a highly developed body wall muscle system
with circular, longitudinal and diagonal layers of muscle fibres.
Dispersed muscle fibres contracted when depolarized by exposure
to extracellular media with elevated K+ (15-100 mM) in a concentration-dependent
manner, with a maximal response of 87% achieved at 75 mM. Hydroxytryptamine
(5-HT) induced concentration-dependent muscle contraction between
0.01 and 1000 uM, with 10 uM producing a near maximal contraction
response of 75%. Acetylcholine (ACh) had less pronounced excitatory
effects (0.01-1000 uM), inducing contraction of only 32% of the
fibres at 100 uM. The flatworm FMRFamide-related peptides (FaRPs),
GYIRFamide, YIRFamide and GNFFRFamide each had concentration-dependent
myocontractile effects, indicating the occurrence of at least
1 FaRP receptor on P. littoralis muscle fibres. At 10 uM peptide,
GNFFRFamide induced contractions in 40% of the muscle fibres examined,
whereas YIRFamide and GYIRFamide induced contraction in 70% and
75% of muscle fibres, respectively.
The order of potency of the peptides was: GYIRFamide > YIRFamide
> GNFFRFamide. Pre-incubation of the muscle fibres in 5 uM
5-HT significantly reduced the responses to GYIRFamide, YIRFamide
and 5-HT, while the responses to high K+ remained unaltered. Muscle
fibres pre-incubated in GYIRFamide (0-1 uM) were also less responsive
to 5-HT but not to ACh and high-K+. The GYIRFamide analogue, GYIRdFamide,
did not induce muscle contraction per se, but when co-applied
with the myoactive peptides GYIRFamide, YIRFamide or GNFFRFamide,
it significantly blocked their ability to elicit contractions.
This suggests that the peptides tested may act via a common muscle-based
neuropeptide receptor. GYIRdFamide did not alter the contractile
effects of high K+, 5-HT or ACh. Collectively, these results indicate
that FaRPs, 5-HT and ACh all have the potential to cause muscle
contraction in flatworms and that 5-HT and FaRPs alter muscle
sensitivity to each other, but do not influence the ability of
flatworm muscle fibres to contract.