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 mechanical 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. METHODS Myocytes and fibroblasts were isolated by enzymatic dissociation from hearts of 1-3-day-old Wistar rat strain pups, grown for 1 day in serum-supplemented medium and then cultured in a chemically defined, serum-free medium. Medium and cell lysate were collected 5 days later or after exposure of the cells to cyclic stretch for 24 h. Prorenin, renin and angiotensinogen were measured by enzyme-kinetic assay; Ang I and Ang II were measured by radioimmunoassay after SepPak extraction and HPLC separation. RESULTS Prorenin, 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 activity corresponding with these low prorenin levels could not be inhibited by the specific rat renin inhibitor CH-732, suggesting that it was most likely due to bovine and/or horse prorenin sequestered from the serum-containing 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 detectable quantities. Stretch increased myocyte protein synthesis by 20%, but was not accompanied by Ang II release into the medium. CONCLUSION Cardiac myocytes and fibroblasts do not synthesize renin, prorenin or angiotensinogen in concentrations that are detectable or, it not detectable, high enough to result in Ang II concentrations of physiological relevance. These cells do synthesize ACE, thereby allowing the synthesis of Ang II at cardiac tissue sites when renin and angiotensinogen are provided via the circulation. Ang II is not a prerequisite to observe a hypertrophic response of cardiomyocytes following stretch.