—A growing body of evidence has been accumulated recently suggesting that growth hormone (GH) and insulin-like growth factor-1 (IGF-1) affect cardiac function, but their mechanism(s) of action is unclear. In the present study, GH and IGF-1 were administered to isolated isovolumic aequorin-loaded rat whole hearts and ferret papillary muscles. Although GH had no effect on the indices of cardiac function, IGF-1 increased isovolumic developed pressure by 24% above baseline. The aequorin transients were abbreviated and demonstrated decreased amplitude. The positive inotropic effects of IGF-1 were not associated with increased intracellular Ca²⁺ availability to the contractile machinery but to a significant increase of myofilament Ca²⁺ sensitivity. Accordingly, the Ca²⁺-force relationship obtained under steady-state conditions in tetanized muscle was shifted significantly to the left (EC₅₀, 0.44±0.02 versus 0.52±0.03 μmol/L with and without IGF-1 in the perfusate, respectively; P<0.05); maximal Ca²⁺-activated tetanic pressure was increased significantly by 12% (211±3 versus 235±2 mm Hg in controls and IGF-1–treated hearts, respectively; P<0.01). The positive inotropic actions of IGF-1 were not associated with changes in either pH_i or high-energy phosphate content, as assessed by ³¹P nuclear magnetic resonance spectroscopy, and were blocked by the phosphatidylinositol 3-kinase inhibitor wortmannin. Concomitant administration of IGF binding protein-3 blocked IGF-1–positive inotropic action in ferret papillary muscles. In conclusion, IGF-1 is an endogenous peptide that through a wortmannin-sensitive pathway displays distinct positive inotropic properties by sensitizing the myofilaments to Ca²⁺ without increasing myocyte [Ca²⁺]_i.