Rationale: We designed this study to integrate in vivo assessment of left ventricular (LV) morphology and function and in vitro assays of isolated adult cardiac fibroblasts to determine whether blockade of either PAR-2 using the PAR-inactivating peptide, FSRLLY, or the Cox-2 selective inhibitor, Nimesulide (Nime), will prevent changes in LV structure and function secondary to an aortocaval fistula (ACF) volume overload.Further, we hypothesize that binding of tryptase to the PAR-2 receptor on cardiac fibroblasts will lead to increased expression of Cox-2 and subsequent formation of the arachodonic acid metabolite 15-d-Prostaglandin J2 (15-d-PGJ2).Methods/Results: LV volume and function were assessed by echocardiography and pressure/volume catheter at 7 and 14 days post surgery in four groups (n≥6 per group): 1) sham-operated (Sham); 2) untreated ACF; 3) ACF + Nime (10 mg/kg/day); and 4) ACF + PAR-2 antagonist (FSRLLY, 10μg/kg/day).The effects of tryptase (100mU) and co-incubation with FSRLLY (10-6M) on proliferation, hydroxyproline concentration, 15-d-PGJ2 formation and PAR-2/COX-2 expression were investigated in fibroblasts isolated from 9 wk old SD rats.The significant increase seen cardiac output, stroke volume, end diastolic volume, end systolic volume and end diastolic pressure in the untreated ACF group was significantly attenuated by Nimesulide treatment.In isolated adult cardiac fibroblasts tryptase induced a significant increase in fibroproliferation, hydroxyproline, 15-d-PGJ2 formation and PAR-2 expression which were markedly attenuated by FSRLLY.These results indicate that Cox-2 inhibition was able to prevent the initial changes in LV chamber dilatation and tryptase-induced changes in cardiac fibroblast phenotype utilizes a PAR-2 dependent mechanism.