alpha -Tropomyosin mutations Asp super(175)Asn and Glu super(180)Gly affect cardiac function in transgenic rats in different ways

To study the mechanisms by which missense mutations in alpha -tropomyosin cause familial hypertrophic cardiomyopathy, we generated transgenic rats overexpressing alpha -tropomyosin with one of two disease-causing mutations, Asp super(175)Asn or Glu super(180)Gly, and analyzed phenotypic changes at molecular, morphological, and physiological levels. The transgenic proteins were stably integrated into the sarcomere, as shown by immunohistochemistry using a human- specific anti- alpha -tropomyosin antibody, ARG1. In transgenic rats with either alpha - tropomyosin mutation, molecular markers of cardiac hypertrophy were induced. Ca super(2+) sensitivity of cardiac skinned-fiber preparations from animals with mutation Asp super(175)Asn, but not Glu super(180)Gly, was decreased. Furthermore, elevated frequency and amplitude of spontaneous Ca super(2+) waves were detected only in cardiomyocytes from animals with mutation Asp super(175)Asn, suggesting an increase in intracellular Ca super(2+) concentration compensating for the reduced Ca super(2+) sensitivity of isometric force generation. Accordingly, in Langendorff- perfused heart preparations, myocardial contraction and relaxation were accelerated in animals with mutation Asp super(175)Asn. The results allow us to propose a hypothesis of the pathogenetic changes caused by alpha -tropomyosin mutation Asp super(175)Asn in familial hypertrophic cardiomyopathy on the basis of changes in Ca super(2+) handling as a sensitive mechanism to compensate for alterations in sarcomeric structure.