Abstract
The uptake and the release of [3H] epinephrine ([3H]E) and the release of endogenous E in slices of guinea pig hypothalamus were investigated. [3H]E rapidly accumulated in the slices incubated in Krebs-Ringer solution containing [3H]E. Kinetic analysis indicated two components of E accumulation, one representing a high (Km1, 7.7×10-8M and Vmax1, 0.13 pmoles/mg/10min) and the other a low (Km2, 1.8×10-6M and Vmax2, 1.4 pmoles/mg/10min) affinity uptake system. Endogenous E released in response to electrical stimulation was estimated using gas chromatography and mass spectrometry. The electrical stimulation produced a release of both [3H]norepinephrine ([3H]NE) and [3H]E from hypothalamic slices preloaded with [3H]NE. With electrical stimulation of the slices, there was an efflux of [3H]E from tissues preloaded with [3H]E, in a current and frequency-dependent manner. Electrically stimulated release of [3H]E from the slices was inhibited by tetrodotoxin (10-6M) and by a calcium-free medium containing EGTA (10-4M), in cases of up to 1 mA of intensity of electrical stimulation. The release of [3H]E induced by electrical stimulation was enhanced by yohimbine, and this effect was suppressed by clonidine. These results provide strong evidence for the neurotransmitter role of this catecholamine in the hypothalamus and suggest the possible existence of a presynaptic regulatory mechaism of E release, through the presynaptic α2 receptors on the E nerve terminals.
