Purpose: Previous studies have shown that relatively low concentrations of ryanodine (Ry) enhance the short-term-facilitated excitatory junctional potential (EJP) of the dactyl opener muscle of the crayfish (Procambarus clarkii and P. simulans) walking limb neuromuscular junction (NMJ). Tryptamine (Trp) has been shown to enhance the EJP at this NMJ as well. The Ry effect is at least partially due to a presynaptic mechanism releasing calcium from intraterminal sources. Both substances appear to exert effects presynaptically. A competition study was carried out to determine whether Trp and Ry may affect the same calcium release mechanism to enhance the EJP. The effect of Trp on the EJP was examined at a Ry concentration that maximally enhanced EJPs. Materials and Methods: The excitatory axon in the meropodite of the first and second walking limbs and the dactyl opener muscle were isolated for stimulation and recording, respectively. The limb was bathed sequentially in Van Harreveld's (Van H) solution and in Van H solutions containing 5 μM Ry, 50 μM Trp, and 5 μM Ry + 50 μM Trp. The pH was adjusted to 7.2 ± 0.1. A 30 Hz stimulus was applied to the excitatory axon for 10 sec. prior to a 10 sec. stimulation for data collection in which the EJP was recorded intracellularly from a superficial opener muscle fiber by a 3.0 M KCl-filled microelectrode. The preparations were washed with VanH in three complete exchanges of the bath media between trials. All effects were reversible, with EJPs returning to control level upon wash. Qualitatively similar results were observed in two additional trials using two walking limb preparations. Results: The EJP amplitude was seen to increase 140% with respect to control with the 5μM Ry solution, 200% with the 50μM Trp solution, and 220% with the 5μM Ry + 50μM Trp solution. Conclusions: Trp increases the amplitude of the Ry-enhanced EJP in the crayfish model studied. These results suggest that Trp and Ry may act cooperatively or affect separate systems to increase free calcium in the pre-synaptic nerve terminals of the crayfish NMJ and further studies are warranted.
|Original language||English (US)|
|Journal||Journal of Investigative Medicine|
|State||Published - Feb 1999|
ASJC Scopus subject areas
- Biochemistry, Genetics and Molecular Biology(all)