Modification of transmitter release by ions which prolong the presynaptic action potential.

1. The action of ions which increase the nerve spike duration at Ranvier's node has been studied at the neuromuscular junction of the frog.The duration of the presynaptic action potential is increased by UO(2), Ni, Nz and TEA ions.2. The release of transmitter after a nerve impulse is delayed and prolonged in the presence of the four ions, but the amount of evoked transmitter release estimated from the mean quantal content of e.p.p. is increased only by UO(2) (2+) and TEA. Both Ni(2+) and Zn(2+) decrease it.3. The frequency of m.e.p.p.s is increased by UO(2) (2+) and decreased by Ni(2+) and Zn(2+) at KCl-depolarized junctions. It is not affected by TEA.4. It has been concluded that the increased presynaptic spike duration is responsible for the delayed and prolonged transmitter release in the presence of ions which increase the nerve spike duration at Ranvier's node. It is demonstrated that some ;specific' effects possibly resulting from ionic competition for presynaptic sites between UO(2) (2+), Ni(2+) or Zn(2+) and other ions present in the Ringer may increase, interfere with, or hide the increased transmitter release that may be predicted from the lengthened presynaptic depolarization.