Inherited restrictive cardiomyopathy (RCM) is a debilitating disease characterized by a stiff heart with impaired ventricular relaxation. Mutations in cardiac troponin I (cTnI) were identified as causal for RCM. Acute genetic engineering of adult cardiac myocytes was used to identify primary structure/function effects of mutant cTnI. Studies focused on R193H cTnI owing to the poor prognosis of this allele. Compared with wild-type cTnI, R193H mutant cTnI more effectively incorporated into the

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... comere, where it exerted dose-dependent effects on basal and dynamic contractile function. Under loaded conditions, permeabilized myocyte Ca 2ϩ sensitivity of tension was increased, whereas the passive tension-extension relationship was not altered by R193H cTnI. Normal rod-shaped myocyte morphology acutely transitioned to a "short-squat" phenotype in concert with progressive stoichiometric incorporation of R193H in the absence of altered diastolic Ca 2ϩ . The specific myosin inhibitor blebbistatin fully blocked this transition. Heightened Ca 2ϩ buffering by the R193H myofilaments, and not alterations in Ca 2ϩ handling by the sarcoplasmic reticulum, slowed the decay rate of the Ca 2ϩ transient. Incomplete mechanical relaxation conferred by R193H was exacerbated at increasing pacing frequencies independent of elevated diastolic Ca 2ϩ . R193H cTnI-dependent mechanical tone caused acute remodeling to a quasicontracted state not elicited by other Ca 2ϩ -sensitizing proteins and is a direct correlate of the stiff heart characteristic of RCM in vivo. These results point toward targets downstream of Ca 2ϩ handling, notably thin filament regulation and actin-myosin interaction, in designing therapeutic strategies to redress the primary cell morphological and mechanical underpinnings of RCM.