MN2-HYDROXYLASE PHOSPHORYLATION IN BOVINE ADRENAL CHROMAFFIN CELLS( CAN SUBSTITUTE FOR CA2+ IN CAUSING CATECHOLAMINE SECRETION BUT NOT FOR INCREASING TYROSINE)

The ability of the divalent cation manganese (Mn2+) to substitute for calcium (Ca2+) both in triggering catecholamine release and in stimula ting catecholamine synthesis, as indicated by an increase in tyrosine hydroxylase (TOH) phosphorylation, has been determined in bovine adren al medullary chromaffin cells maintained in tissue culture. Mn2+ was f ound to enter chromaffin cells through pathways activated by nicotinic receptor stimulation and potassium depolarisation, and via the Na,:Ca , exchange mechanism in Na+-loaded cells. Like Ca2+, entry of Mn2+ thr ough these pathways triggered immediate catecholamine release and, lik e Ca2+, maintained quantitatively comparable release at least up to 40 min. Unlike Ca2+, Mn2+ did not stimulate an increase in TCH phosphory lation in intact chromaffin cells, even over a prolonged time course, but Mn2+ did stimulate increased TOH phosphorylation in lysed cell pre parations showing that its lack of effect in the intact cells was not due to inhibition of the specific phosphorylation pathway. In lysed ce ll preparations, Mn2+ stimulated also phosphorylation of a different s pectrum of proteins to Ca2+, and of the same proteins to different ext ents. In particular, P80 (MARCKS protein) was more intensely phosphory lated in the presence of Mn2+ than in the presence of Ca2+. Since TOH phosphorylation always occurs when intracellular Ca2+ is increased, th e absence of an increase with Mn2+ indicates that none of its intracel lular effects could have occurred as a consequence of Mn2+ mobilisatio n of intracellular Ca2+. In summary, the data show that Mn2+ is a surr ogate for Ca2+ in triggering and maintaining catecholamine release, bu t does not substitute for Ca2+ in stimulating TOH phosphorylation.