Abstract
1 In the presence of tetrodotoxin, electrotonic depolarization of frog motor nerve terminals causes the appearance of stimulus-graded endplate potentials. When 4-aminopyridine is added, the graded endplate potential is converted into a triggered all-or-none response resulting in giant endplate potentials of about 70 mV amplitude and 50 ms duration. The triggered endplate potentials are abolished in Ca2+-free saline and are blocked by Mn2+ ions. Sr2+ but not Ba2+ can replace Ca2+ in supporting transmitter release. Mg2+ fails, even in concentrations as high as 32 mM, to affect the amplitude and the shape of the endplate potential but abolishes it when the Ca2+ concentration is reduced to 0.2 mM.
2 Despite the large amplitude of the triggered endplate potential in the presence of 4-aminopyridine and tetrodotoxin, repetitive stimulation up to 10 Hz causes only a small decline in amplitude of successive endplate potentials. However, in the presence of (+)-tubocurarine or gallamine, repetitive nerve stimulation produces a marked decline in successive endplate potential amplitude. The fall is counteracted when evoked transmitter release is reduced in the presence of 0.2 mM Ca2+. The results suggest that in the presence of 4-aminopyridine such large amounts of transmitter are released that even during repetitive stimulation (5 to 10 Hz) endplate potentials are of maximal amplitude.
3 4-Aminopyridine causes a prallel shift to the right of the dose-response curve to Mg2+ for blockade of nerve impulse-evoked transmitter release (in the absence of tetrodotoxin). A similar parallel shift occurs in the presence of tetraethylammonium and guanidine.
4 It is concluded that 4-aminopyridine increases transmitter release by enhancing the transport efficacy for Ca2+ across the nerve terminal membrane during nerve terminal depolarization.
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Selected References
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