Abstract
The mechanical properties of papillary muscles isolated from hyperthyroid, euthyroid and hypothyroid cats were compared and the relationship between hyperthyroidism and endogenous as well as exogenous catecholamines was studied. Changes in the level of thyroid activity profoundly affected the intrinsic contractile state of cardiac muscle primarily byaltering the speed of shortening of the contractile elements, while total force production was not modified markedly. Muscles from hyperthyroid animals were not supersensitive to exogenous norepinephrine and the increased speed of shortening associated with hyperthyroidism was independent of endogenous norepinephrine stores. {Endocrinology 82: 191, 1968)
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