Release of norepinephrine and dopamine-beta-hydroxylase by nerve stimulation. I. Role of neuronal and extraneuronal uptake and of alpha presynaptic receptors.

After a brief period of low frequency nerve stimulation, a proportional release of norepinephrine (NE), dopamine-β-hydroxylase activity (DBH) , total 3 H and 3 H-NE was obtained from the isolated perfused cat spleen over a wide range of transmitter release. Approximately 30% of the increase in the overflow of total 3 H was recovered as 3 H-metabolites, largely as 3 H-dihyclroxyphenlylethyleneglycol ( 3 H-DOPEG). Cocaine (0.03-3µM) enhanced the outflow of NE and 3 H-NE and increased pressure responses; however, it failed to increase the overflow of total 3 H and produced a decrease in the overflow of DBH and 3 H-DOPEG When the formation of 3 H-DOPEG was markedly inhibited there was a reduction in the formation of 3 H-O-methylated deaminated metabolites. These results Support the view that 3 H-DOPEG is formed presynaptically after recapture of the NE released by nerve stimulation and that part of the glycol can be further O-methylated by extraneuronal enzymes. The present results also suggest that, although neuronal uptake is an effective mechanism to decrease the concentration of NE in the synaptic space. it does not appear to he highly coupled with subsequent storage of the recaptured amine. Phenoxyhenzamine and phentolamine produced concentration-dependent inhibition of pressure responses and enhanced the nerve stimulation-mediated overflow of NE, 3 H-NE, total 3 H and DBH activity. Higher concentrations of phenoxybenzamine (0.3 µM) markedly inhibited the formation of 3 H-DOPEG and a still higher concentration (3 µM) completely prevented the metabolism of 3 H-NE released by nerve stimulation. Phenoxyhenzamine was at least 10 times more potent and also more effective than phentolamine in enhancing the outflow of NE and DBH by nerve stimulation. Both phenoxybenzamine and phentolamine were more effective in inhibiting alpha postsynaptic than alpha presynaptic receptors, as judged by their potencies in blocking pressure responses and in enhancing nerve stimulation release of NE and DBH, suggesting that either differences in pre-and postsynaptic alpha receptors may exist or there may be a greater preponderance of spare receptors at the presynaptic sites.