The modulating effects of various muscle factors on the function of motor nerve terminals.

Studies reported here showed that the incubation medium used for frog muscle contains factors able to modulate the secretion of mediator from motor nerve terminals, with increases in release from low-efficiency synapses and decreases in release from high-efficiency synapses. Both the stimulatory and the inhibitory effects of the incubation medium are mediated by the corresponding changes in the number of available mediator quanta stored. Factors with stimulatory presynaptic actions were present mainly in the low-molecular-weight (<10 kDal) fraction of the incubation medium. The extent of the stimulatory action of this fraction on low-efficiency synapses correlated with the concentration of histidine-containing substances.