Junctional transmission in fast- and slow-twitch mammalian motor units.

The phenomena of facilitation and tetanic potentiation of end-plate potentials (EPPs) were investigated in the predominantly fast-twitch extensor digitorum longus (EDL) and the predominantly slow-twitch soleus (Sol) muscles of the rat, using extra- and intracellular recording methods. When the quantal content of the EPPs was reduced to average values of 2-3, facilitation and potentiation could be studied in both muscles without the masking effects of neuromuscular depression. Under these conditions, the facilitation and potentiation at the neuromuscular junctions of EDL were different from those at the junctions of Sol. The potentiation during sustained activation was higher in EDL than in Sol at stimulation frequencies varying between 10 and 100 Hz. The same tendency was found for the facilitation measured by paired stimuli; the facilitation in EDL was generally higher than in Sol with interpulse intervals between 300 and 10 ms. The dependence of facilitation and potentiation on the stimulation frequency was also different in EDL and Sol. As the interpulse interval decreased from 200 to 25 ms, the facilitation in Sol increased more steeply than in EDL, whereas with further decrease from 25 to 10 ms, the facilitation in Sol did not increase at all, whereas the facilitation in EDL exhibited a substantial increase. The same general pattern was observed for potentiation: the potentiation in Sol increased more steeply than in EDL between 10 and 40 Hz (100- to 25-ms intervals) and more moderately than in EDL between 40 and 100 Hz (25- to 10-ms intervals). Another paradigm, repetitive short trains, induced a progressive increase in potentiation at the neuromuscular junctions of both muscles. The cumulative effect of potentiation was more prominent in EDL than in Sol. The differences in potentiation between EDL and Sol were also evident from intracellular recordings. These recordings also revealed that the amplitude of miniature EPPs remained unaltered in both muscles before and during the stimulus trains. Comparisons of the EPPs elicited before and at the end of tetanic trains revealed that the onset time of the EPPs was delayed in Sol but not in EDL during the train. This latency shift was also observed in the focally recorded nerve terminal potentials and end-plate currents in Sol, but not in EDL. The differences in facilitation and potentiation between EDL and Sol may be due to heterogeneities of the transmitter release machinery of their nerve terminals or differences in the capability of their presynaptic nerves to conduct action potentials at high frequencies.(ABSTRACT TRUNCATED AT 400 WORDS)