Influence of the Thyroid State on the Intrinsic Contractile Properties and Energy Stores of the Myocardium*

Abstract

Abstract. The intrinsic contractile properties of isolated cat papillary muscles and myocardial high energy phosphate stores were examined at three levels of thyroid activity and correlated with hemodynamic measurements in the intact animal. In addition, the relationship of thyroid state to endogenous norepinephrine stores and myocardial responsiveness to certain inotropic in-.terventions were studied. In muscles from hyperthyroid cats, the velocity of shortening and the rate of tension development were markedly augmented, while duration of active state was decreased, compared to euthyroid muscles. These findings occurred in the presence and absence of intact norepinephrine stores and over a wide range of temperature and contraction frequency. The opposite changes occurred in muscles from hypothyroid cats. Isometric tension was slightly higher in muscles from hyperthyroid and lower in muscles from hypothyroid cats. The inotropic response to both norepinephrine and strophanthidin varied inversely with the level of thyroid state and allowed all three groups of muscles to reach a common ceiling of isometric tension regardless of thyroid state. Creatine phosphate and adenosine triphosphate stores were intact at all three levels of thyroid state. Thus, the level of thyroid activity profoundly affects the intrinsic contractile state of cardiac muscle, independent of both norepinephrine stores and alterations in high energy phosphate stores, and, in addition, modifies the responsiveness of cardiac muscle to inotropic agents.

Thyroid hormone regulation of cardiac bioenergetics: role of intracellular creatine

“…There has been controversy concerning the relative efficiency of hyperthyroid compared with euthyroid hearts (6,15,24). Efficiency, defined as work output per unit O 2 consumed, was found to be similar in euthyroid and hyperthyroid hearts in one study (24) but lower in hyperthyroid hearts in another study (15).…”

“…This results from a dual action of the hormone: a direct one, which is due to stimulation of cardiomyocyte metabolism, and a second, indirect one, which occurs in response to increased demand of peripheral tissues for oxygen and substrates and for the removal of waste products (5,10). The combination of these effects, at least during the early phase of hyperthyroidism, leads to increased "contractility" of the hyperthyroid heart, which is manifested by positive changes in virtually all measurable parameters of cardiac muscle function (1,3,5,6,10,21,29,30,46). Excess thyroid hormone is associated with an increase in the number and volume of mitochondria in cardiac myocytes and skeletal muscle, and the resultant stimulation of oxidative phosphorylation is associated with increased numbers of respiratory units per milligram mitochondrial protein with little or no change in the phosphorylation-to-oxidation ratio (5,10,30,46).…”

Synkope bei Hyperthyreose?

“…Die übermässig ausgeschütteten Schilddrüsenhormone bei Hyperthyreose haben aber auch einen direkten Einfluss auf das Herz [6]. Im Tierversuch konnte gezeigt werden, dass die Schilddrüsenhormone direkt auf die Kontraktilität des Myokards wirken und es zu einer rascheren Verkürzung und höheren Spannungsentwicklung des Myokards kommt [5]. Generell wird eher bei einer Hypothyreose eine arterielle Hypertonie nachgewiesen, wobei gehäuft eine diastolische Hypertonie nachgewiesen wird, die bei einer Hyperthyreose eher ungewöhnlich ist [6].…”

Biochemical and histochemical evidence for stimulation of myosin ATPase activity in thyrotoxic rabbit heart

“…Beginning with Barany [ 1 ], it has been appreciated that myosin ATPase activity is directly related to the intrinsic speed of muscle contraction. Thyrotoxicosis induced by thyroid hormone administration has been found to increase the speed of contraction of heart muscle [2][3][4] and to elevate the Ca2÷-ATPase activity of cardiac myosin [5][6][7][8]. However, it is uncertain whether this increase in enzymatic activity accounts for the increased speed of contraction, since in living muscle Mg-ATP is hydrolyzed by myosin under the stimulating influence of actin.…”