Expression of the Proenkephalin A Gene and [Met(5)]enkephalin Secretion Induced by Prostaglandin E(2) in Bovine Adrenal Medullary Chromaffin Cells: Involvement of Second Messengers.

We have reported that prostaglandin E(2) (PGE(2)) increases the long-term secretion of [Met(5)]enkephalin (ME) and the expression of proenkephalin A (proENK) mRNA in bovine adrenal medullary chromaffin (BAMC) cells. In order to characterize the underlying mechanisms for the PGE(2)-induced responses, we have now studied the effects of PGE(2) on intracellular free calcium Ca(2+) levels. The interactions of PGE(2) with several second messenger systems were also studied. Treatment with PGE(2) (10 muM) produced a transient increase followed by a prolonged plateau in the levels of Ca(2+). Ionomycin (3 x 10(-6)M), which depletes intracellular calcium pools, did not inhibit the PGE(2)-induced responses. Nimodipine (1 x 10(-6)M ), an L-type calcium channel blocker, did not block the initial transient but blocked the plateau phase of the PGE(2)-induced Ca(2+) response. Long-term (24 h) treatment with PGE(2) (10 muM) increased both the secretion of ME and the expression of proENK mRNA. Pretreatment of BAMC cells with nimodipine but not with omega-conotoxin GVIA inhibited the secretion of ME and the expression of proENK mRNA induced by PGE(2). Calmidazolium, a calmodulin antagonist, also significantly inhibited PGE(2)-induced responses. However, a protein kinase C (PKC) inhibitor, sphingosine (36 muM), was ineffective in blocking PGE(2)-induced responses. In addition, the down-regulation of PKC by phorbol myristate acetate (PMA) (0.1 muM) for 48 h did not inhibit the PGE(2)-induced responses. Furthermore, PGE(2) did not affect AP-1 DNA binding activity, while PMA (1 muM) increased AP-1 DNA binding activity. Forskolin (an adenyl cyclase activator) alone increased ME secretion as well as proENK mRNA levels and when coincubated with PGE(2) showed less than an additive effect on the secretion of ME and the levels of the proENK mRNA. The results suggest that the Ca(2+)/ calmodulin pathway, but not the protein kinase A or PKC pathways, appear to be involved in mediating the PGE(2)-induced increases of the long-term secretion of ME and the levels of proENK mRNA.