Low-Na+Medium Increases the Activity and the Phosphorylation of Tyrosine Hydroxylase in the Superior Cervical Ganglion of the Rat

Abstract

Incubation of the rat superior cervical ganglion in Na+-free or low-Na+ medium increased the rate of synthesis of 3,4-dihydroxyphenylalanine (DOPA) in the ganglion 4-fold and caused a concomitant stable activation of tyrosine hydroxylase [TH]. DOPA synthesis was half-maximal in medium containing about 20 mM Na+. Low-Na+ medium also increased the incorporation of 32Pi into TH; the dependence of TH phosphorylation on the Na+ concentration resembled that of DOPA synthesis. The stimulatory effects of low-Na+ medium on DOPA production and on TH activity in vitro were dependent on extracellular Ca2+. The stimulation of DOPA synthesis in low-Na+ medium was inhibited by methoxyverapamil, an inhibitor of Ca2+ uptake, and was partially blocked by tetrodotoxin, but it was not affected by the cholinergic antagonists hexamethonium and atropine. Ionomycin, a calcium ionophore, stimulated DOPA synthesis to about the same extent as low-Na+ medium and also increased the incorporation of 32Pi into TH. 8-Bromo cAMP (1 mM) also stimulated DOPA production in the ganglion and this stimulation was more than additive with that produced by low-Na+ medium. Apparently, low-Na+ medium stimulates DOPA synthesis by raising intracellular Ca2+, which then promotes the phosphorylation of TH.