Release of noradrenaline and dopamine by nerve stimulation in the guinea-pig and rat vas deferens

Abstract

Spontaneous and nerve stimulated release of noradrenaline and dopamine from rat and guinea-pig vas deferens have been measured electrochemically after separation by high performance liquid chromatography (h.p.l.c.). In the absence of nerve stimulation both noradrenaline (NA) and dopamine were released into the bathing fluid in the rat but in the guinea-pig only noradrenaline could be detected. Drugs which block neuronal and extraneuronal uptake of catecholamines had little effect on spontaneous overflow but both tetraethylammonium and phenoxybenzamine increased overflow. Transmural nerve stimulation (5-10 Hz) increased catecholamine overflow in both species and dopamine release was now measurable from the guinea-pig vas. In the rat, the proportion of dopamine to NA was unchanged from that released spontaneously. The release of both amines was little affected by drugs that block neuronal and extraneuronal uptake and a monoamine oxidase inhibitor, but was inhibited by tetrodotoxin 0.2 microgram ml-1. In the guinea-pig tetraethylammonium 10 mM doubled overflow and phenoxybenzamine 10(-5)M increased it by five times but the dopamine percentage remained constant and equal to the control. Following nerve stimulation the amount of dopamine released expressed as a percentage of total catecholamine release was 6% for the rat and 1.3% for the guinea-pig. These values were considerably higher than the comparable figures for dopamine: NA content of the two tissues (2% and 0.5% respectively). Repeated periods of stimulation depleted these tissue stores and the depletion of dopamine was significantly greater than that of NA. 6 Our interpretation of these results is that both dopamine and NA are released from a common store during normal noradrenergic transmission. While all or most of the axonal dopamine is contained in this releasable pool, most of the axonal NA lies in a second, less readily released pool.