A NOVEL MOLECULAR DETERMINANT FOR CAMP-DEPENDENT REGULATION OF THE FROG-HEART NA-CA2+ EXCHANGER()

Na+-Ca2+ exchanger is one of the major sarcolemmal Ca2+ transporters o f cardiac myocytes, In frog ventricular myocytes the exchanger is regu lated by isoproterenol via a ta-adrenoreceptor/adenylate-cyclase/cAMP- dependent signaling pathway providing a molecular mechanism for the re laxant effect of the hormone. Here, we report on the presence of a nov el exon of 27-base pair insertion, which generates a nucleotide bindin g motif (P-loop) in the frog cardiac Na+-Ca2+ exchanger. To examine th e functional role of this motif, we constructed a full-length frog hea rt Na+-Ca2+ exchanger cDNA (fNCX1a) containing this exon. The function al expression of fNCX1a in oocytes showed characteristic voltage depen dence, divalent (Ni2+, Cd2+) inhibition, and sensitivity to cAMP in a manner similar to that of native exchanger in frog myocytes. In oocyte s expressing the dog heart NCX1 or the frog mutant (Delta fNCX1a) lack ing the 9-amino acid exon, cAMP failed to regulate Na+-dependent Ca2uptake. We suggest that this motif is responsible for the observed cAM P-dependent functional differences between the frog and the mammalian hearts.