Structural and functional aspects of the actomyosin complex from fast-twitch muscle of euthyroid and hypothyroid rats

The effects of hypothyroidism on structural and functional properties of the actomyosin-ATPase complex of rat fast-twitch gastrocnemius muscle were examined and related to energetic and mechanical parameters. Hypothyroidism resulted in the appearance of a small band of the myosin heavy chain subunit of the slow form (MHCs) 8% of total MHC) which was absent in the euthyroid group. This observation corresponded with lower activities of myofibrillar ATPase (-14%) and Ca-activated myosin ATPase (-9%) in the hypothyroid group, although these changes were not significant. No effect of hypothyroidism on the Ca2+-sensitivity of the myofibrillar-ATPase activity was observed and tetanic force was not changed. Twitch force, however, was significantly increased by hypothyroidism. The degree of myosin P-light chain phosphorylation (percentage of total amount of P-light chain) determined after 5 and 10 s of tetanic stimulation (130 Hz, 35 degrees C), respectively, proved to be significantly lower in the hypothyroid group (5 s: 57%; 10 s: 61%) vs the euthyroid group (5 s: 79%; 10 s: 82%). There was no difference in P-light chain phosphorylation at rest between eu- and hypothyroids. The results suggest that a decreased actomyosin-ATPase activity can only in part contribute to the 30% lower energy turnover during force development found for fast-twitch skeletal muscle of hypothyroid rats. Moreover, the increase in twitch force by hypothyroidism cannot be explained by a change in myosin P-light chain phosphorylation. Isometric twitch tension potentiation after a 2 s tetanus and during low-frequency repetitive stimulation was reduced (up to -60%) in muscles of hypothyroid rats, which may well be related to the lower extent of P-light chain phosphorylation in hypothyroids.