Positive and negative controls by protein kinases of sodium-dependent Ca2+ efflux from cultured bovine adrenal chromaffin cells stimulated by lysophosphatidic acid

We previously found that lysophosphatidic acid (LPA), a bioactive phospholipid, induced Na+-dependent Ca2+ efflux from cultured bovine adrenal chromaffin cells, possibly by activating a Na+/Ca2+ exchanger. The present study on the structure-activity relationship of its action revealed that 1-acyl type LPAs were stronger stimulants than the corresponding 1-O-alkyl type LPAs having a long alkyl moiety with the same chain length. Lysophosphatidylglycerol, suramin and N-palmitoyl-tyrosine phosphoric acid have all been reported to inhibit the action of LPA in some animal cells and platelets, but only lysophosphatidylglycerol was found to inhibit selectively LPA-induced Ca2+ efflux from chromaffin cells. LPA-induced Ca2+ extrusion was suggested to be involved in both acceleration of return of intracellular Ca2+ in Fura 2-loaded bovine chromaffin cells after addition of carbachol, and inhibition of carbachol-induced Catecholamine release when the cells were co-incubated with LPA. The Ca2+ efflux from chromaffin cells stimulated by LPA was augmented by their pretreatment with staurosporine or calphostin C, inhibitors of protein kinase C, but reduced by their preincubation with phorbol 12-myristate 13-acetate. Furthermore, the response to LPA was potentiated by sodium vanadate, a protein tyrosine Phosphatase Inhibitor, but inhibited by genistein, an inhibitor of protein tyrosine kinase. These results suggest that protein kinase C and protein tyrosine kinase are involved negatively and positively, respectively, in the signal transduction triggered by LPA, leading to activation of the Na+/Ca2+ exchanger.