Transgenic mice with cardiac-specific overexpression of Gαq exhibit a biochemical and physiological phenotype of load-independent cardiac hypertrophy with contractile dysfunction. To elucidate the cellular basis for altered contractility, we measured cellular contraction, Ca²⁺transients, andl-type Ca²⁺channel currents (I_Ca) in left ventricular (LV) myocytes isolated from non transgenic (NT) controls or Gαq hearts. Although baseline contractile function (% shortening) and the amplitude of Ca²⁺transients in Gαq myocytes were similar to NT myocytes, the rates of cellular shortening and relengthening and the duration of Ca²⁺transients were prolonged in Gαq myocytes. Myocytes from Gαq hearts had larger cell capacitance but no change in I_Cadensity, voltage-dependence of activation and inactivation. The responses of I_Cato dihydropyridine drugs and a membrane permeable cAMP analog, 8-(4-chlorophenylthio) cAMP, were not altered; however, the time course of I_Cainactivation was significantly slower in Gαq myocytes compared to NT myocytes. The kinetic difference in inactivation was abolished when Ba²⁺was used as the charge carrier or when the sarcoplasmic reticulum (SR) Ca²⁺was depleted by ryanodine, suggesting that Ca²⁺-dependent inactivation is reduced in Gαq myocytes due to altered SR Ca²⁺release. Consistent with this hypothesis, the function of SR as assessed by the maximal Ca²⁺uptake rates and the apparent affinity of SR Ca²⁺-ATPase for Ca²⁺was reduced in ventricles of Gαq heart. These results suggest that the reduced SR function contributes to the depressed contractility associated with this form of cardiac hypertrophy.