Dual action of ouabain on transmitter release at neuromuscular junctions of the frog

Ouabain increased both spontaneous and evoked transmitter release in Mg++-treated frog neuromuscular junctions. This action developed as a two-step process which affected both miniature end-plate potential (m.e.p.p.) frequency and the binomial distribution of e.p.p.s. During the first part of its action, which lasts for approximately 60 min, ouabain (10(-5) M) increased the m.e.p.p. frequency following a saturable process. The increase in m.e.p.p. frequency was blocked by tetrodotoxin (15 nM). The quantal parameters of release, m and n, showed a significant increase but the parameter p was unaffected. Since the same changes in the binomial parameters were observed in Mg++-treated junctions exposed to low [Na+]0 in the absence of ouabain, it can be concluded that Na+ concentration played an important role in the increase of transmitter release. After 60 min in ouabain (10(-5) M) m.e.p.p. frequency increased by an exponential process. The binomial parameters of transmitter release, m and p, increased while n remained unchanged. This action was not influenced by TTX pretreatment nor was it reproduced by decreasing [Na+]0. The mechanism responsible for this action seems to be the Ca++- releasing effect of ouabain from the cytoplasmic sequestering sites.