Effect of thyroid state on cardiac myosin P-light chain phosphorylation during exercise

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Abstract
This study ascertained the effects of thyroid deficiency (TD) and hyperthyroidism (H) on in vivo cardiovascular functional capacity in the context of cardiac myosin light chain 2 phosphorylation [P-LC(P)], a proposed modulator of myocardial function, at rest and during exercise. Compared with normal controls (NC), Ca2(+)-regulated myofibril adenosinetriphosphatase was reduced by 39% in TD and increased by 9% in H rats. This response was associated with a 20-fold increase in the V3 isoform and an 11% increase in the V1 isoform in TD and H rats, respectively. Submaximal treadmill exercise elicited significant elevations in all myocardial functional indexes examined in H rats compared with the NC group, whereas the opposite occurred for the TD group. Despite the marked contrast in cardiac function among the three groups, intrinsic levels of P-LC(P) were similar at rest among the groups and were significantly reduced in both TD and H groups relative to controls during exercise. These data suggest that although thyroid state exerts a profound impact on intrinsic myocardial functional state, it exerts little control over cellular processes regulating P-LC(P) during rest and exercise.

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