Cultured neonatal rat cardiac myocytes and fibroblasts do not synthesize renin or angiotensinogen: evidence for stretch-induced cardiomyocyte hypertrophy independent of angiotensin II

Objective: The hypertrophic response of cardiomyocytes exposed to mechanica l stretch is assumed to depend on the release of angiotensin (Ang) II from these cells. Here we studied the synthesis of renin-angiotensin system (RAS ) components by cardiac cells under basal conditions and after stretch, Met hods: Myocytes and fibroblasts were isolated by enzymatic dissociation from hearts of 1-3-day-old Wister rat strain pups, grown for 1 day in serum-sup plemented medium and then cultured in a chemically defined, serum-foe mediu m. Medium and cell lysate were collected 5 days later of after exposure of the cells to cyclic stretch for 24 h. Prorenin, renin and angiotensinogen w ere measured by enzyme-kinetic assay; Ang I and Ang II were measured by rad ioimmunonssay after SepPak extraction and HPLC separation. Results: Proreni n, but none of the other RAS components, could be detected in the medium of both cell types. However, its levels were low and the Ang I-generating act ivity corresponding with these low prorenin levels could not be inhibited b y the specific rat renin inhibitor CH-732, suggesting that it was most like ly due to bovine and/or horst prorenin sequestered from the serum-containin g medium to which the cells had been exposed prior to the serum-free period . When incubated with Ang I, both myocytes and fibroblasts generated Ang II in a captopril-inhibitable manner, Myocyte and fibroblast cell lysates did not contain prorenin, renin, angiotensinogen, Ang I or Ang II in detectabl e quantities. Stretch increased myocyte protein synthesis by 20%, but was n ot accompanied by Ang II release into the medium. Conclusion: Cardiac myocy tes and fibroblasts do not synthesize renin, prorenin or angiotensinogen in concentrations that are detectable or, if not detectable, high enough to r esult in Ang II concentrations of physiological relevance. These cells do s ynthesize ACE, thereby allowing the synthesis of Ang II at cardiac tissue s ites when renin and angiotensinogen are provided via the circulation. Ang I I is not a prerequisite to observe a hypertrophic response of cardiomyocyte s following stretch. (C) 1999 Elsevier Science B.V. All rights reserved.