Recent studies have found that selective stimulation of troponin (Tn)I protein kinase A (PKA) phosphorylation enhances heart rate-dependent inotropy and blunts relaxation delay coupled to increased afterload. However, in failing hearts, TnI phosphorylation by PKA declines while protein kinase C (PKC) activity is enhanced, potentially augmenting TnI PKC phosphorylation. Accordingly, we hypothesized that these site-specific changes deleteriously affect both rate-responsive cardiac function and afterload dependence of relaxation, both prominent phenotypic features of the failing heart. A transgenic (TG) mouse model was generated in which PKA-TnI sites were mutated to mimic partial dephosphorylation (Ser22 to Ala; Ser23 to Asp) and dominant PKC sites were mutated to mimic constitutive phosphorylation (Ser42 and Ser44 to Asp). The two highest-expressing lines were further characterized. TG mice had reduced fractional shortening of 34.7 +/- 1.4% vs. 41.3 +/- 2.0% (P = 0.018) and slight chamber dilation on echocardiography. In vivo cardiac pressure-volume studies revealed near doubling of isovolumic relaxation prolongation with increasing afterload in TG animals (P < 0.001), and this remained elevated despite isoproterenol infusion (PKA stimulation). Increasing heart rate from 400 to 700 beats/min elevated contractility 13% in TG hearts, nearly half the response observed in nontransgenic animals (P = 0.005). This blunted frequency response was normalized by isoproterenol infusion. Abnormal TnI phosphorylation observed in cardiac failure may explain exacerbated relaxation delay in response to increased afterload and contribute to blunted chronotropic reserve.