A subpopulation of cardiomyocytes expressing alpha-skeletal actin is identified by a specific polyclonal antibody

The NH2-terminal decapeptide of alpha-skeletal actin that contains a primar y sequence specific for this isoform was used to raise a polyclonal antibod y in rabbits. Using sequential affinity chromatography, we recovered from s erum antibodies reacting exclusively with alpha-skeletal actin when tested by immunoblotting and immunofluorescence. Epitope mapping by means of compe tition assays with synthetic peptides indicated that the acetyl group and t he first 9 amino acids are essential for specificity. The monospecific anti body was then used to investigate the distribution of alpha-skeletal actin in the myocardium of newborn and normal or hypertensive (with or without fi brotic areas) adult rats. Immunostaining of normal heart revealed that alph a-skeletal actin is diffusely distributed within practically all myocardial fibers of the newborn rat, whereas it is restricted to a small proportion of adult rat cardiomyocytes, which appear intensely stained. A correlation, albeit not complete, was found between the distribution of alpha-skeletal actin and beta-myosin heavy chain. During cardiac hypertrophy induced by ao rtic ligature between the renal arteries, the expressions of alpha-skeletal actin mRNA and protein were increased. The distribution of immunostaining had a focal pattern similar to that of normal adult rats, reactive fibers b eing more numerous and more intensely stained compared with normal myocardi um. Positive fibers were particularly abundant at the periphery of fibrotic areas. Using this antibody, we have demonstrated for the first time the di fferential distribution of alpha-skeletal actin in heart tissues. Changes i n the distribution of this isoform in hypertrophic heart provide new insigh t into the mechanisms by which the heart adapts to work overload. This anti body will prove useful in exploring the mechanisms of expression of alpha-s keletal actin and in defining its role in physiological and pathological si tuations. The full text of this article is available at http://www.circresa ha.org.