Activation of protein kinase A by atrial natriuretic peptide in neonatal rat cardiac fibroblasts: Role in regulation of the local renin-angiotensin system

There is an inverse relationship between renin and atrial natriuretic peptide (ANP) levels in the plasma. Since both the ANP and renin-angiotensin system (RAS) are upregulated in development and cardiac hypertrophy, we tested whether ANP differentially regulates RAS in cardiac cells. Cardiac fibroblasts isolated from neonatal rats were treated with ANP1-28, a biologically active fragment of ANP. Renin and angiotensinogen (Ao) mRNA levels were measured by quantitative multiplex RT-PCR and protein levels determined by Western blot analysis. ANP1-28 increased renin and Ao mRNA levels (737 ± 131% and 178 ± 51.3%) with EC50 values of 4.12 ± 0.3 and 8.67 ± 0.22 nmol/L, respectively. At the protein level, secretion of renin and Ao was significantly enhanced resulting in ∼4-fold increase in ANG II level in the medium. The effect of ANP1-28 on renin and Ao mRNA expression were reproduced by 8-bromo-cyclic GMP. Inhibition of protein kinase G (PKG) with KT5823 blunted ANP1-28-induced upregulation of renin, but not Ao mRNA, while inhibition of protein kinase A (PKA) with KT5720 attenuated the upregulation of both renin and Ao mRNA. These findings suggest that unlike in plasma, ANP positively regulates the RAS in cardiac fibroblasts, which may have a significant role in development of the fetal heart.