GENE-EXPRESSION OF NATRIURETIC PEPTIDE RECEPTORS IN MYOCARDIAL-CELLS

Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) a re cardiac hormones that serve to unload the heart through their effec ts 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 is olated myocytes, several studies have been unable to detect binding of the hormone to these cells. The present study demonstrates that the g enes for all three natriuretic peptide receptor (NPR) subtypes, NPR-A, NPR-B, and NPR-C, are expressed in the rat heart. For microlocalizati on of the receptor mRNAs in myocytes and nonmyocytic cells, a combinat ion of cell isolation and reverse transcription-polymerase chain react ion (RT-PCR) was used. Cardiac myocytes were isolated by enzymatic dis sociation of rat ventricular tissue, purified by density gradient cent rifugation, and collected as single cells under microscopic control. A nalysis by RT-PCR revealed the presence of transcripts for NPR-A as we ll as NPR-B and NPR-C. However, cGMP generation in purified myocytes w as stimulated only by ANP and BNP, which specifically bind to NPR-A, w hereas C-type natriuretic peptide (CNP, an NPR-B agonist) was ineffect ive. Therefore, rat ventricular myocytes appear to produce predominant ly 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 th e rat heart. In contrast to the myocytes, large increases in cGMP were observed in response not only to ANP but also to CNP. Cardiac fibrobl asts may therefore synthesize functionally relevant amounts of both NP R-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-1 25]ANP can be displaced by the NPR-C agonist C-ANP 4-23. These observa tions may help to resolve whether the heart muscle cells themselves ar e targets for natriuretic peptides. The finding that in addition to my ocytes, cardiac fibroblasts are capable of expressing the NPR genes ra ises the interesting possibility that natriuretic peptides are involve d in the structural remodeling of the heart.