Release, metabolism and intraneuronal disposition of exogenous, endogenous and newly synthesized norepinephrine in the rat vas deferens
Eisenhofer, G.; Ropchak, T.G.; Kopin, I.J.; Goldstein, D.S.
Journal of Pharmacology and Experimental Therapeutics 245(1): 81-88
1988
ISSN/ISBN: 0022-3565 PMID: 3361453 Document Number: 312476
In the isolated rat vas deferens the release and intraneuronal disposition of endogenous norepinephrine (NE) were compared with those of newly synthesized or exogenous radioactive NE by preloading tissues with trace amounts of tritiated dopamine ([3H]DA) or tritiated NE ([3H]NE) and measuring release of radioactive and endogenous NE and dihyhdroxyphenylglycol (DHPG). Tissues were examined before and during electrical simulation, exposure to tyramine or exposure to depolarizing concentrations of K+. In [3H]DA-preloaded tissues the [3H]DA was converted readily to [3H]NE. Newly synthesized radioactive NE formed from exogenous [3H]DA was distributed differently from endogenous NE within at least two intraneuronal pools. One pool contained a high concentration (high specific activity) of newly synthesized [3H]NE and less than 3% of the total NE content of the tissues, and it released NE more readily than the larger low specific activity pool which contained over 95% of the total tissue NE content. Exogenous [3H]NE in [3H]NE-preloaded vasa deferentia was distributed among at least three different tissue pools, one consisting of extraneuronally bound NE containing [3H]NE of high specific activity, and two intraneuronal pools containing [3H]NE of intermediate and low specificity activity in which NE was released from the intermediate specific activity pool more readily than from the low specific activity pool. Differences in the ratios of NE to DHPG between labeled and unlabeled compounds indicated that most of the endogenous DHPG released by tissues was derived from NE leaking into the neuronal cytoplasm from the large intraneuronal storage pool of NE, whereas most of the radioactive DHPG was derived from NE leaking into the cytoplasm from the small intraneuronal pool of readily released NE. DHPG released in response to all forms of stimulation was derived mainly from NE in the small intraneuronal pool of readily released newly synthesized NE. Preloading tissues with radioactive DA avoids artifact caused by extraneuronal binding of [3H] NE and, when combined with measurement of endogenous and radioactive NE and DHPG, allows examination of the behavior of two intraneuronal pools of NE, a fast release labile pool that preferentially contains newly synthesized NE and NE recaptured by nerves and a slow release pool that contains most of the tissue NE and which appears to serve mainly a storage function.