The activation of tyrosine hydroxylase in noradrenergic neurons during acute nerve stimulation

Abstract

Tyrosine hydroxylase present in the adrenergic neurons of the vas deferens preparation of the guinea pig appears to exist in two forms; a less active form with a relatively low affinity for pterin cofactor; and a more active state in which the affinity for pterin cofactor is enhanced. Incubation of the vas deferens in the absence of stimulation is associated with conversion of the more active, high affinity enzyme form to the less active form, which exhibits reduced affinity for cofactor. Electrical stimulation of the hypogastric nerve to the vas deferens preparation for periods as short as one min at 25 Hz results in almost complete activation of the enzyme to the high affinity form. Incubation of intact vasa deferentia in the presence of 8-methylthio cyclic AMP also is associated with enhanced tyrosine hydroxylase activity in situ and results in similar kinetic changes in the soluble enzyme which is prepared from these organs. Analogous changes in the kinetics of soluble tyrosine hydroxylase prepared from vasa deferentia can be achieved by incubation of the less active form of the enzyme in the presence of cyclic AMP, ATP and Mg ++. The effects of electrical stimulation on the activation of tyrosine hydroxylase can be potentiated by stimulating the hypogastric nerve vas deferens preparation in the presence of isobutylmethylxanthine, a phosphodiesterase inhibitor. These results are consistent with the notion that increased tyrosine hydroxylase activity in sity which is associated with nerve stimulation results from a cyclic AMP-dependent protein phosphorylating reaction. It has not yet been determined whether this cyclic AMP-dependent activation of tyrosine hydroxylase is a consequence of direct phosphorylation of the enzyme or phosphorylation of an activator or inhibitor of the enzyme. It is conceivable that the activation of tyrosine hydroxylase by nerve stimulation, by the cyclic AMP-dependent phosphorylating system, by polyanions, by anionic phospholipids and by limited proteolysis is mediated by a fundamentally similar mechanism.