Local diversity of myosin expression in mammalian atrial muscles. Variations depending on age and thyroid state in the rat and the rabbit.

Rat, rabbit, pig, and bovine atrial myocardia were investigated with anti-alpha and anti-beta myosin heavy chain monoclonal antibodies. Analysis of atrial fibers by indirect immunofluorescence and assay of myosin heavy chains in tissue micro samples by immunoaffinity chromatography revealed both heterogeneity and plasticity in the atrial myosin heavy chains, undetected by electrophoresis of native atrial myosins under nondenaturing conditions. We found both alpha- and beta-like myosin heavy chains to be expressed in rat and rabbit, as they are in pig and bovine, atrial myocardia. They were regionally distributed within atrial muscles. The beta-like myosin heavy chains were present at much lower levels in rat and rabbit atria than in pig and bovine atria. Young rat atrial myosin was composed of only alpha-like heavy chains. In the rat and the rabbit, hyperthyroidism induced a beta- to alpha-like myosin heavy chain transition, which was considerable in the right atria and complete in the left atria. In the rat, thyroidectomy induced a moderate alpha- to beta-like myosin heavy chain transition, visible in the left atria. The significance of this atrial myosin heavy chain polymorphism is discussed in relation to the existence of anatomical localizations of the two myosin variants.