Presynaptic action of uranyl nitrate on the phrenic nerve-diaphragm preparation of the mouse

The contraction of the diaphragm of the mouse, induced by nerve stimulation, can be potentiated by uranyl nitrate [UO2(NO3)2, 0.2-0.8 mM]. At concentrations greater than 0.8 mM, uranyl nitrate also directly enhanced the contractions induced by electrical stimulation of muscle. The effects of uranyl nitrate in potentiating the twitch were augmented by low calcium (0.25-0.5 mM) and antagonized by high levels of calcium (5-10 mM). Electrophysiological studies on the effects of uranyl nitrate on the diaphragm in the mouse have revealed that the frequency of miniature end-plate potentials (MEPP) but not the amplitude was increased; the amplitude and quantal content of end-plate potential (EPPs) were also markedly increased by uranyl nitrate. The most peculiar phenomenon induced by uranyl nitrate was that repetitive end-plate potentials, as well as repetitive action potentials, of the diaphragm of the mouse were triggered by single nerve stimulation in the presence of uranyl nitrate. The duration of muscle action potentials was significantly prolonged. Recordings of the evoked compound action potentials from nerve axons showed that uranyl nitrate not only prolonged the duration of the axonal compound action potentials but also induced antidromic activity with a single stimulation. This latter effect of uranyl nitrate was blocked by d-tubocurarine. All of these findings suggest that uranyl nitrate potentiated the contractions of the diaphragm of the mouse, possibly through the activation of stimulus-activated repetitive discharges which resulted in the repetitive end-plate potentials and muscle action potentials.