CLONING OF THE RAT AORTIC SMOOTH-MUSCLE NA+ CA-2+ EXCHANGER AND TISSUE-SPECIFIC EXPRESSION OF ISOFORMS/

The amino acid sequences of two isoforms of the rat aortic smooth musc le Na+/Ca2+ exchanger have been deduced by cloning and sequencing the cDNAs. These isoforms are identical in nucleotide sequence except that one has a 23-amino acid insertion at amino acid position 570. They ar e highly homologous to the canine cardiac exchanger except for the NH2 -terminal portion and part of the large central hydrophilic domain (am ino acid residues 570-631). They are 902 and 925 (with the insertion) amino acid long with calculated molecular masses of 100,676 and 103,20 0 (with the insertion), respectively, if the NH2- terminal 32-amino ac id residues are eliminated as a cleaved signal sequence. Amplification of the variable region (amino acids 570-631) of the exchanger by mean s of the reverse transcriptase-polymerase chain reaction and DNA seque ncing revealed that many isoforms of the exchanger are expressed in di fferent rat tissues. The two clones isolated in this study are the pre dominant isoforms expressed in aorta, stomach, liver, and kidney. In c ardiac and skeletal muscles, another isoform is dominant, which is equ ivalent to the canine cardiac exchanger. In brain, a third type is pre dominantly expressed. Alignment of the nucleotide sequences of these i soforms and Southern blot analysis of rat genomic DNA suggested that e ach isoform is generated through alternative splicing of the primary t ranscript.