Mechanical stretch and angiotensin II differentially upregulate the renin-angiotensin system in cardiac myocytes in vitro

Pressure overload in vivo results in left ventricular hypertrophy and activ ation of the renin-angiotensin system in the heart. Mechanical stretch of n eonatal rat cardiac myocytes in vitro causes secretion of angiotensin II (A ng II), which in turn plays a pivotal role in mechanical stretch-induced hy pertrophy. Although in vivo data suggest that the stimulus of hemodynamic o verload serves as an important modulator of cardiac renin-angiotensin syste m (RAS) activity, it is not clear whether observed upregulation of RAS gene s is a direct effect of hemodynamic stress or is secondary to neurohumoral effects in response to hemodynamic overload. Moreover, it is unclear whethe r activation of the local RAS in response to hemodynamic overload predomina ntly occurs in cardiac myocytes or fibroblasts or both. In the present stud y, we examined the effect of mechanical stretch on expression of angiotensi nogen, renin, angiotensin-converting enzyme (ACE), and Ang II receptor (AT( 1A), AT(1B), and AT(2)) genes in neonatal rat cardiac myocytes and cardiac fibroblasts in vitro. The level of expression of angiotensinogen, renin, AC E, and AT(1A) genes was low in unstretched cardiac myocytes, but stretch up regulated expression of these genes at 8 to 24 hours. Stimulation of cardia c myocytes with Ang II also upregulated expression of angiotensinogen, reni n, and ACE genes, whereas it downregulated AT(1A) and did not affect AT(1B) gene expression. Although losartan, a specific AT(1) antagonist, completel y inhibited Ang II-induced upregulation of angiotensinogen, renin, and ACE genes, as well as stretch-induced upregulation of AT(1A) expression, it did not block upregulation of angiotensinogen, renin, and ACE genes by stretch . Western blot analyses showed increased expression of angiotensinogen and renin protein at 16 to 24 hours of stretch. The ACE-like activity was also significantly elevated at 24 hours after stretch. Radioligand binding assay s revealed that stretch significantly upregulated the AT(1) density on card iac myocytes. Interestingly, stretch of cardiac fibroblasts did not result in any discernible increases in the expression of RAS genes. Our results in dicate that mechanical stretch in vitro upregulates both mRNA and protein e xpression of RAS components specifically in cardiac myocytes. Furthermore, components of the cardiac RAS are independently and differentially regulate d by mechanical stretch and Ang II in neonatal rat cardiac myocytes.