Heterogeneity of alpha-cardiac myosin heavy chains in a small marsupial, Antechinus flavipes, and the effect of hypothyroidism on its ventricular myosins

The effect of drug-induced hypothyroidism on ventricular myosin gene expression was explored in a small marsupial, Antechinus flavipes. Pyrophosphate gel electrophoresis, SDS-PAGE and western blotting were used to analyse changes in native myosin isoforms and myosin heavy chains (MyHCs) in response to hypothyroidism. In some animals, five instead of the normal three native myosin components were found: V1a, V1b, V1c, V2 and V3, in order of decreasing mobility. In western blots, V1a, V1b, and V1c reacted with anti-a-MyHC antibody, but not with anti-B-MyHC, whereas V2 and V3 reacted with anti-B-MyHC antibody. SDS-PAGE of the unusual ventricular myosins revealed three MyHC isoforms, two of which bound anti-a-MyHC antibody while the third bound anti-B-MyHC antibody. We conclude that V1a, V1b, V1c are triplets arising from the dimerization of two distinct a-MyHC isoforms. Hypothyroidism, veriWed by metabolic studies, decreased a-MyHC content significantly (t-test, P < 0.001) from 91.6 5.9% (SEM, n = 4) in control animals to 67.2 5.7% (SEM, n = 4) in hypothyroid animals, with a concomitant increase in B-MyHC content. We conclude that in adult marsupials, ventricular myosins are also responsive to changes in the thyroid state as found in eutherians, and suggest that evolution of the molecular mechanisms underlying this thyroid responsiveness predate the divergence of marsupials and eutherians