In contrast to proteolytic activation of inactive prorenin by cleavage of the N-terminal 43 residue peptide, we found that prorenin is activated without proteolysis by binding of the prorenin receptor to the pentameric "handle region" I(11P)LLKK(15P). We hypothesized that such activation occurs in hypertensive rats and causes cardiac renin-angiotensin system (RAS) activation and end-organ damage. To test this hypothesis, we devised methods of specifically inhibiting nonproteolytic activation by decapeptide spanning the pentameric handle region peptide as a decoy. In stroke-prone spontaneously hypertensive rats (SHRsp) fed a high-salt diet, arterial pressure started to rise significantly with a marked increase in the cardiac prorenin receptor mRNA level at 8 weeks of age, and cardiac fibrosis had developed by 12 weeks of age. By immunohistochemistry using antibodies to the active site of the renin molecule, we demonstrated increased proteolytic or nonproteolytic activation of prorenin in the heart but not in plasma of SHRsp. Continuous subcutaneous administration of the handle region peptide completely inhibited the increased staining by antibodies to the active site of the renin molecule, indicating the increased nonproteolytic but not proteolytic activation of prorenin in the heart of SHRsp. Administration of the handle region peptide also inactivated tissue RAS without affecting circulating RAS or arterial pressure and significantly attenuated the development and progression of cardiac fibrosis. These results clearly demonstrate the significant role of nonproteolytically activated tissue prorenin in tissue RAS activation leading to cardiac fibrosis and significant inhibition of the cardiac damage produced by chronic infusion of the handle region peptide.