Abstract:
A FMRFamide-like neuropeptide with the sequence "DRNFLRF-NH2" was recently
isolated from pericardial organs of crayfish (Mercier et aI., Peptides, 14, 137-143, 1993). This
neuropeptide, referred to as "DF2'" has already been shown to elicit cardioexcitation and to
enhance synaptic transmission at neuromuscular junctions.
Possible effects ofDF2 on muscle were investigated using superficial extensor muscles of
the abdomen of the crayfish, Procambarus clar/ai. These muscles are of the tonic type and
generate slow contractions that affect posture. DF2, at concentrations of 10-8 M or higher,
increased muscle tonus and induced spontaneous, rhythmic contractions. These effects were
antagonized by 5 rnM Mn2+ but not by lO-7M tetrodotoxin (TTX). Thus, they represent direct
actions on muscle cells (rather than effects on motor neurons) and are likely to involve calcium
influx. In contrast, deep abdominal extensor muscles, responsible for rapid swimming
movements, and superficial flexor muscles do not generate contractions in response to the
peptide.
2
Spontaneous contractions were also induced in the superficial extensor muscles by
decreasing the temperature to II-13°C. Such contractions were also TTX-insensitive and they
were antagonized by adding calcium channel blockers (Mn2+, Cd2+ or Ni2+) or by removing
calcium from the bathing solution. This suggests that the spontaneous contractions depend on an
influx of calcium from the extracellular solution. N-type and L-type voltage dependent calcium
channel blockers did not reduce the effect of the peptide or the spontaneous contractions
suggesting that calcium influx is not through N- or L-type calcium channels.
