A selective inhibitor of cAMP-specific phosphodiesterase, Ro 20-1724, has no effect on the quantal release of acetylcholine from the mouse phrenic nerve.

The indirect twitch response of the mouse isolated phrenic nerve-diaphragm preparation, partially paralysed by tubocurarine, was restored only by about 10% by Ro 20-1724 at 2 to 280 microM. The solvent vehicle, dimethylsulphoxide, also showed the same effect to a similar extent. Intracellular recordings with glass microelectrodes revealed that Ro 20-1724 (40 microM) affected neither the resting membrane potential, the amplitude and frequency of miniature endplate potentials nor the amplitude of nerve-impulse evoked endplate potentials recorded in curarized preparations. The result indicates that Ro 20-1724 at a concentration four times the IC50 of phosphodiesterase inhibition has no effect on the quantal release of acetylcholine from a mammalian motor nerve and suggests that cAMP has no modulatory effect on the transmitter release.