Physiological effects of FMRFamide-related peptides and classical transmitters on dispersed muscle fibres of the turbellarian, Procerodes littoralis

The physiological effects of selected classical transmitters and FMRFamide- related peptides (FaRPs) on dispersed muscle fibres from the marine turbell arian, Procerodes littoralis have been examined. Confocal scanning laser mi croscopy coupled with fluorescein isothiocyanate (FITC) or tetramethylrhoda mine (TRITC)-labelled phalloidin revealed a highly developed body wall musc le system with circular, longitudinal and diagonal layers of muscle fibres. Dispersed muscle fibres contracted when depolarized by exposure to extrace llular media with elevated K+ (15-100 mM) in a concentration-dependent mann er, with a maximal response of 87%, achieved at greater than or equal to 75 mM. 5-Hydroxytryptamine (5-HT) induced concentration-dependent muscle cont raction between 0.01 and 1000 muM, with 10 muM producing a near maximal con traction response of 75%. Acetylcholine (ACh) had less pronounced excitator y effects (0.01-1000 muM), inducing contraction of only 32% of the fibres a t 100 muM. The flatworm FMRFamide-related peptides (FaRPs), GYIRFamide, YIR Famide and GNFFRFamide each had concentration-dependent myocontractile effe cts, indicating the occurrence of at least 1 FaRP receptor on P. littoralis muscle fibres. At 10 muM peptide, GNFFRFamide induced contractions in < 40 % of the muscle fibres examined, whereas YIRFamide and GYIRFamide induced c ontraction in 70 and 75% of muscle fibres, respectively. The order of poten cy of the peptides was: GYIRFamide > YIRFamide > GNFFRFamide. Pre-incubatio n of the muscle fibres in 5 muM 5-HT significantly reduced the responses to GYIRFamide, YIRFamide and 5-HT, while the responses to high K+ remained un altered. Muscle fibres pre-incubated in GYIRFamide (0.1 muM) were also less responsive to 5-HT but not to ACh and high-K+. The GYIRFamide analogue, GY IRDFamide, did not induce muscle contraction (0.01-100 muM) per se, but whe n co-applied with the myoactive peptides GYIRFamide, YIRFamide or GNFFRFami de, it significantly blocked their ability to elicit contractions. This sug gests that the peptides tested may act via a common muscle-based neuropepti de 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 t he ability of flatworm muscle fibres to contract.