We have measured myocyte cell shortening, troponin-I (Tn-I) phosphorylation, Ca²⁺ dependence of actomyosin adenosinetriphosphatase (ATPase) activity, adenosine 3′,5′-cyclic monophosphate (cAMP) levels, and myofibrillar isoform expression in the spontaneously hypertensive rat (SHR) during decompensated cardiac hypertrophy (76 wk old) and in age-matched Wistar-Kyoto rat (WKY) controls. The decreased inotropic response to β-adrenergic stimulation previously observed in myocytes from 26-wk-old SHR was further reduced at 76 wk of age. In response to β-adrenergic stimulation, Tn-I phosphorylation was greater in the 76-wk-old SHR than in the WKY, although cAMP-dependent protein kinase A (PKA)-dependent Tn-I phosphorylation in the SHR did not increase with progression from compensated (26 wk) to decompensated (76 wk) hypertrophy. We also observed a dissociation between the increased PKA-dependent Tn-I phosphorylation and decreased cAMP levels in the 76-wk-old SHR versus WKY during β-adrenergic stimulation. Baseline Tn-I phosphorylation was significantly reduced in 76-wk-old SHR versus WKY and was associated with decreased basal cAMP levels and increased Ca²⁺ sensitivity of actomyosin ATPase activity. The change in myofilament Ca²⁺ sensitivity during β-adrenergic stimulation in the 76-wk-old SHR (0.65 pCa units) was over twofold greater than in the 76-wk-old WKY (0.30 pCa units). We also determined whether embryonic troponin T isoforms were reexpressed in decompensated hypertrophy and observed significant reexpression of the embryonic cardiac troponin T isoforms in the 76-wk-old SHR. The significant decrease in Ca²⁺ sensitivity with β-adrenergic stimulation in 76-wk-old SHR may contribute to the severely impaired inotropic response during decompensated hypertrophy in the SHR.