Catechol and noradrenaline facilitate the neuromuscular transmission via different mechanisms

Using curarized frog sartorius muscles, selective presynaptic facilitatory effects of two structurally related chemicals, noradrenaline (NA) and catechol (pyrocatechol, PC), were compared by calculating the fractional release (P) from the rundown of endplate potential during a train-of-three stimulation. Below 10 microM, both NA and PC increased P in a similar manner. At higher concentrations, however, the effects of NA were found to be quite different from those of PC. NA never increased further the value of P (0.11-0.13) whereas PC augmented P in a dose dependent fashion up to about 0.76. Thus, the present data suggested that NA and PC facilitate the transmitter release via different mechanisms. The effects of the latter were identical to the case of "chemical potentiation or accelerated evoked release" by 4-aminopyridine.