1. 1.

    Long-term facilitation (LTF) and short-term facilitation (STF) at excitatory neuromuscular junctions were studied under conditions of varying [K+]0 and with the addition of strophanthidin.

  2. 2.

    The magnitude of LTF varied inversely with [K+]0, while STF was unaffected.

  3. 3.

    Strophanthidin concentrations greater than 10−5 M greatly increased LTF, but not STF.

  4. 4.

    Both LTF and STF were unaffected by large decreases in excitatory junctional potential amplitude produced by the addition of GABA or Mn+2.

  5. 5.

    The results support the hypothesis that LTF is a consequence of Na+ accumulation in presynaptic terminals and behaves independently of STF.

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