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Characteristics of Tyrosine Hydroxylase Activation by K + ‐Induced Depolarization and/or Forskolin in Rat Striatal Slices
Author(s) -
El Mestikawy S.,
Gozlan H.,
Glowinski J.,
Hamon M.
Publication year - 1985
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1985.tb05490.x
Subject(s) - veratridine , depolarization , forskolin , chemistry , tyrosine hydroxylase , egta , biophysics , in vitro , medicine , biochemistry , enzyme , biology , calcium , sodium , sodium channel , organic chemistry
The mechanisms of tyrosine hydroxylase (TH) activation by depolarization or exposure of dopaminergic terminals to cyclic AMP have been compared using rat striatal slices. Tissues were incubated with veratridine or 60 m M K + (depolarizing conditions), on the one hand, and forskolin or dibutyryl cyclic AMP, on the other. K + ‐(or veratridine‐)induced depolarization triggered an activation of TH (+ 75%) that persisted in soluble extracts of incubated tissues. This effect disappeared when drugs (EGTA, N ‐(6‐aminohexyl)‐5‐chloro‐l‐naphthalenesulfon‐amide, Gallopamil) preventing Ca 2+ ‐ and calmodulin‐de‐pendent processes were included in the incubating medium. In contrast, prior in vivo reserpine treatment or in vitro addition of benztropine did not affect the depolarization‐induced activation of TH. In vitro studies of soluble TH extracted from depolarized tissues indicated that activation was associated with a marked increase in the enzyme V max but with no change in its apparent affinity for the pteridin cofactor 6‐methyl‐5,6,7,8‐tetrahydrop‐terin (6‐MPH 4 ) or tyrosine. Furthermore, the activated enzyme from depolarized tissues exhibited the same optimal pH (5.8) as native TH extracted from control striatalslices. In contrast, TH activation resulting from tissue incubation in the presence of forskolin or dibutyryl cyclic AMP was associated with a selective increase in the apparent affinity for 6‐MPH 4 and a shift in the optimal pH from 5.8 to 7.0–7.2. Clear distinction between the two activating processes was further confirmed by the facts that heparin‐ and cyclic AMP‐dependent phosphorylation stimulated TH activity from K + ‐exposed (and control) tissues but not that from striatal slices incubated with forskolin (or dibutyryl cyclic AMP). In contrast, the latter enzyme but not that from depolarized tissues could be activated by Ca 2+ ‐dependent phosphorylation. These data strongly support the concept that Ca 2+ – but not cyclic AMP‐dependent phosphorylation is responsible for TH activation in depolarized dopaminergic terminals.