RENAL EXPRESSION OF THE GENE FOR ATRIAL-NATRIURETIC-FACTOR

To date, atrial natriuretic factor (ANF) mRNA has eluded detection in the mammalian kidney, although we and others have identified ANF protein in the kidney using immunohistochemical and immunoassay techniques. Furthermore, we have demonstrated the synthesis and secretion of the ANF prohormone in the distal cortical nephron of the intact rat kidney and from rat primary cultured renal distal cortical tubular epithelial cells. In the present study, we show that the ANF gene is expressed in the kidney. Amplification of RNA isolated from rat distal cortical tubular epithelial cultures using ANF specific primers produced a 213-bp fragment that specifically hybridized to a P-32-labeled ANF cDNA. We had previously demonstrated these cultures to be enriched for the renal ANF synthetic and secretory cell type. However, we were unable to detect an ANF gene transcript in total rat kidney RNA using the above-mentioned polymerase chain reaction (PCR) conditions. Reanalysis of normal rat kidney PCR products by a second round of PCR amplification using nested primers successfully identified ANF mRNA. Similar to cultured kidney epithelial cells, normal rat kidney expresses ANF mRNA, but at a very low abundance, thus necessitating two rounds of PCR amplification. Further characterization of rat cortical distal tubular epithelia poly(A)+ RNA by Northern analysis revealed two ANF gene transcripts. A 1.0-kb message that comigrated with rat atrial ANF mRNA, and a second larger 1.4-kb transcript. These studies further substantiate the synthesis of ANF in the mammalian kidney. Unlike the mammalian heart, the kidney contains two ANF gene transcripts.