Effects of dopamine and dibutyryl cyclic adenosine monophosphate on delayed release of transmitter at the rat neuromuscular junction.

The effects of dopamine and dibutyryl cyclic adenosine monophosphate (db-cAMP) on delayed release of transmitter were studied in vitro in the phrenic nerve-diaphragm muscle preparation in the rat using intracellular recording techniques. Dopamine at 1 X 10(-4) mol 1(-1) prevented the initial facilitation of delayed release of transmitter. This inhibitory phase was transformed into a transient facilitation of delayed release. We observed that dopamine hyperpolarized muscle fibres by about 9%. Thus motor nerve terminals may also have been hyperpolarized by dopamine; however, it is unlikely that this hyperpolarization explains the observed effects on delayed release of transmitter. Db-cAMP at 1 X 10(-3) mol 1(-1) predominantly augmented delayed release of acetylcholine. These effects of dopamine and db-cAMP on delayed release of transmitter are discussed in terms of a modulation of calcium fluxes in the presynaptic nerve terminal.