Ex vivo working heart studies demonstrated that in contrast to that of wild‐type hearts, dysferlin‐deficient mouse hearts had blunted responses to 10 nm isoproterenol. A, lower increase in relaxation velocity (41.57±4.18% vs. 70.81±7.50%) with no difference in contractile velocity; B, lower increase in left ventricular peak systolic pressure (LVPmax, 21.14±2.76% vs. 28.08±1.45%) with no difference in left ventricular end diastolic pressure (LVPmin); and C, lower increase in stroke volume (17.03±10.29%vs. 43.03%±3.37%). *P<0.05 in Student's t test.