GENE-EXPRESSION OF NATRIURETIC PEPTIDE RECEPTORS IN MYOCARDIAL-CELLS

Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are cardiac hormones that serve to unload the heart through their effects on the kidney and vasculature. Whether the heart itself represents a site of action for these peptides is currently the subject of debate. Although functional studies indicate that ANP has some effects on isolated myocytes, several studies have been unable to detect binding of the hormone to these cells. The present study demonstrates that the genes for all three natriuretic peptide receptor (NPR) subtypes, NPR-A, NPR-B, and NPR-C, are expressed in the rat heart. For microlocalization of the receptor mRNAs in myocytes and nonmyocytic cells, a combination of cell isolation and reverse transcription-polymerase chain reaction (RT-PCR) was used. Cardiac myocytes were isolated by enzymatic dissociation of rat ventricular tissue, purified by density gradient centrifugation, and collected as single cells under microscopic control. Analysis by RT-PCR revealed the presence of transcripts for NPR-A as well as NPR-B and NPR-C. However, cGMP generation in purified myocytes was stimulated only by ANP and BNP, which specifically bind to NPR-A, whereas C-type natriuretic peptide (CNP, an NPR-B agonist) was ineffective. Therefore, rat ventricular myocytes appear to produce predominantly NPR-A. The expression of MPR-B may be low or even absent. The mRNAs for all three NPRs were also found in cultures of fibroblasts from the rat heart. In contrast to the myocytes, large increases in cGMP were observed in response not only to ANP but also to CNP. Cardiac fibroblasts may therefore synthesize functionally relevant amounts of both NPR-A and NPR-B. The expression of NPR-C in these cells is suggested by binding studies demonstrating that >70% of the maximum binding of [I-125]ANP can be displaced by the NPR-C agonist C-ANP 4-23. These observations may help to resolve whether the heart muscle cells themselves are targets for natriuretic peptides. The finding that in addition to myocytes, cardiac fibroblasts are capable of expressing the NPR genes raises the interesting possibility that natriuretic peptides are involved in the structural remodeling of the heart.