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The aim of the present study was to use a highly sensitive, polymerase chain reaction (PCR) technique to examine the controversial question of renin gene expression in the heart and to determine whether expression in cardiac tissue can be stimulated.The study involved female Wistar rats fed either a normal-sodium diet or a low-sodium diet, together with an angiotensin I converting enzyme inhibitor, enalapril, orally for 1 week.RNA was extracted from atrium, ventricle, kidney (positive control) and submandibular gland (negative control) and, after reverse transcription into complementary DNA, was used in a 35-cycle PCR. Reaction products were visualized after electrophoresis by ethidium bromide staining and hybridization probing with [32P]-labelled target oligonucleotide.Atrial RNA from sodium-replete rats gave little or no renin messenger (m)RNA PCR product on ethidium bromide-stained gels. After DNA transfer, followed by hybridization probing and extended autoradiography, a faint band was seen. In sharp contrast, atrial extracts from sodium-depleted enalapril-treated rats displayed a pronounced band of hybridization, corresponding in size to that expected for renin mRNA. No band was seen for ventricle.The present PCR study has shown that in the normal sodium-replete rat, atrial tissue has only very low renin gene expression, but that after a low-sodium diet + treatment with enalapril, expression is switched on in atrium. Ventricular tissue does not express the renin gene in either state.