Pharmacological action of the heptapeptide GFSKLYFamide in the muscle of the sea cucumber Holothuria glaberrima (Echinodermata).

The holothurian neuropeptide GFSKLYFamide (Gly-Phe-Ser-Lys-Leu-Tyr-Phe-NH2), GFSKLYFa, was characterized recently and shown to be present in nerve fibers that apparently innervate various muscle systems. We have studied the potential neurotransmitter role of this peptide by assaying its effects on the contractility of visceral and somatic muscles. GFSKLYFa in nanomolar concentrations induces a relaxation of the muscle tension in the intestine. A similar effect is observed on the longitudinal muscle bands of the body wall of the sea cucumber. The relaxing action of GFSKLYFa is dose dependent suggesting that its action is mediated by receptors present in the muscle cells. In addition, GFSKLYFa induces the relaxation of the acetylcholine contracted intestine. Our investigation provides additional evidence indicating that GFSKLYFa might be a neurotransmitter acting at the neuromuscular junctions of the sea cucumber Holothuria glaberrima.