title: External Ca-independent release of norepinephrine by sympathomimetics and its role in negative feedback modulation.
abstract: The release of [3H]norepinephrine from isolated mouse vas deferens has been measured. 1-Phenylephrine and 1-norepinephrine significantly enhanced the spontaneous release of radioactivity. As shown by a combination of HPLC and scintillation spectrometry, the release of total radioactivity in response to 1-phenylephrine or 1-norepinephrine consisted mainly of [3H]norepinephrine. Evidence has been obtained that the release of endogenous norepinephrine by exogenous norepinephrine and 1-phenylephrine is independent of the external Ca2+ concentration. The released endogenous norepinephrine in turn inhibits the release of norepinephrine in response to electrical field stimulation. In the presence of yohimbine, the enhancement of spontaneous release due to 1-phenylephrine (or to 1-norepinephrine) was not affected, whereas there was a significant superimposed release of [3H]norepinephrine in response to field stimulation, indicating that the inhibition of stimulation-induced norepinephrine release is an alpha 2-adrenoceptor-mediated process. An important consequence of these findings is to question previous interpretations that the effects of administration of 1-norepinephrine or 1-phenylephrine are due exclusively to their direct effects on the effector cells. The Ca-independent release of endogenous norepinephrine might partly initiate their pharmacological responses. It is concluded that this Ca-independent release is of functional importance, since norepinephrine may accumulate in a concentration sufficient to modulate the release of norepinephrine from varicosities in response to electrical stimulation.
authors.forename: E S
authors.initials: ES
authors.lastname: Vizi
authors.forename: G T
authors.initials: GT
authors.lastname: Somogyi
authors.forename: L G
authors.initials: LG
authors.lastname: Harsing
authors.forename: I
authors.initials: I
authors.lastname: Zimanyi
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000502:physiology; D004558:Electric Stimulation; D008297:Male; D051379:Mice; D008996:Monoamine Oxidase Inhibitors / Q000494:pharmacology; D009638:Norepinephrine / Q000378:metabolism*; D010656:Phenylephrine / Q000494:pharmacology; D012110:Reserpine / Q000494:pharmacology; D014649:Vas Deferens / Q000294:innervation / Q000378:metabolism; D015016:Yohimbine / Q000494:pharmacology
chemical_list: D000316:Adrenergic alpha-Agonists; D008996:Monoamine Oxidase Inhibitors; D010656:Phenylephrine; D015016:Yohimbine; D012110:Reserpine; D002118:Calcium; D009638:Norepinephrine