Intermedin (IMD) is an endogenous peptide upregulated in hypertrophic and ischemic myocardium. We recently demonstrated a negative inotropic action of IMD in normal rat left ventricular (LV) papillary muscles, dependent of endothelial nitric oxide (NO) production. However the pathophysiological implications of increased IMD remain uncertain. We investigated the direct effects of increasing concentrations of IMD (10–8 to 10-6M) on contractile and relaxation parameters in isotated left ventricular (LV) papillary muscles from normal rats (Sham) and rats with LV hypertrophy, induced by transverse aortic constriction (TAC). Additionally we evaluated expression of CLRL/RAMP receptor system by real-time PCR, and NO bioavailability by protein quantification of endothelial nitric oxide synthase (eNOS) and phosporilated eNOS (P-eNOS). We found that, IMD induced negative inotropic action characterized by a significant decrease of 12 ± 4% in peak twitch active tension (AT) in Sham rats, which was blunted in TAC rats (figure 1 n=5, P < 0.05,* vs. baseline; # vs. Sham). Furthermore, IMD induced a positive lusitropic effect characterized by a significant increase of 9 ± 4% in maximum velocity of tension decline (dT/dtmin) in TAC rats but absent in Sham rats. In addition, the expression of CLRL, RAMP1 and RAMP3 was down-regulated 60 ± 13%, 53 ± 9% and 64 ± 18%, respectively (n=6, P < 0.05) in TAC rats, as well as P-eNOS (P-eNOS, percentage of Sham, 28 ± 7%, P < 0.05, n=3), while total eNOS expression was unaltered, indicating a decrease in activity of eNOS in hypertrpphic rats. These findings suggest that the distinct myocardial action of IMD in papillary muscles from hypertrophic rats (TAC) results from altered expression of receptor system and decrease NO bioavailability.