Left ventricular myocardial contractile response to aortic constriction in the hyperthyroid guinea pig

Biochemical and morphological study of cardiac hypertrophy. Effects of thyroxine on enzyme activities in the rat myocardium

Experimental hyperthyroidism induced in rats by daily injections of 3,3',5,5'-tetraiode-L-thyroxine (0.5 mg/kg i.p.) for 14 days resulted in a significant increase in heart weight and heart weight/body weight ratio. Hemodynamic and morphological studies were performed in one group. Thyroxine-treated rats showed a characteristic cardiovascular hyperdynamic state, such as tachycardia and augmented rate of contraction, but no evidence of heart failure such as elevated end-diastolic pressures. The cardiac cells in hyperthyroid rats had a significantly larger diameter and more mitochondria than did those of the control rats. In another group the activities of cardiac enzymes involved in energy utilization and liberation were measured biochemically and compared with those of normal controls. Hyperthyroidism resulted in increased specific activity of cytochrome C oxidase and actomyosin ATPase in the myocardium. The specific activity of long-chain acyl-CoA synthetase, carnitine palmityl-transferase, carnitine acetyltransferase, malate dehydrogenase and citrate synthase showed a moderate to marked increment, whereas the specific activity of lactate dehydrogenase and pyruvate kinase remained at the control values. These results suggest that in hyperthyroid rat hearts the functions of both energy liberation and utilization systems are enhanced to meet the added workload. Moreover, the increased activity of the enzymes participating in fatty acid metabolism suggest that in thyroxine-induced hypertrophic and hyperdynamic rat hearts, fatty acids contribute more to the energy supply than do carbohydrates.

Effects of physical training and thyroxine on rodent cardiac functional and biochemical properties

The effects of 8 weeks of interval running and of 14 days of thyroxine treatment were studied on selected cardiac functional and biochemical parameters of contraction in both female guinea pigs and rats. Relative to their respective sendentary-control groups, both trained groups demonstrated typical exercise adaptations of resting bradycardia and of increases in skeletal muscle oxidative enzymes and relative heart mass (P less than 0.05). Myosin ATPase, Ca2+ regulated myofibril ATPase, and + dP/dt max were at least two-fold higher in rats compared to guinea pigs (P less than 0.001). Physical training did not augment any of these parameters in either species under the experimental conditions studied. However, biochemical and functional indices of intrinsic cardiac contractility in guinea pigs were increased by injections of thyroxine; whereas, the same treatment regimen was without effect on the rats. These findings suggest that chronic running does not alter the intrinsic cardiac contractility state in either species. Also, exercise doesn't change the thyroid state of the guinea pig to a level sufficient to affect cardiac function.

Evaluation of oxidative phosphorylation in hearts from euthyroid, hypothyroid, and hyperthyroid rats

The energy relationships between cytosolic and mitochondrial metabolism were studied in the hearts from euthyroid, hypothyroid, and hyperthyroid rats. Isolated mitochondria showed high respiratory control ratios and impermeability to exogenous NADH. Hypo- and hyperthyroidism, respectively, resulted in lower and higher contents of both cytochromes per mitochondrion and mitochondrial protein per gram of wet weight of heart without changes in the ratio of cytochrome c to cytochrome aa3. In isolated perfused heart, the hyperthyroid state led to an increase in work rate and thereby an elevation of Vo2, which resulted in an increase oxidation-reduction turnover number for the cytochromes. An agreement was found between [ATP]/[ADP][Pi] of cytosolic free adenine nucleotides and the value calculated from a mathematical model of mitochondrial respiration. This implies that mitochondrial respiration is controlled at the cytochrome oxidase reaction and that oxidative phosphorylation in intact tissue is tightly coupled irrespective of thyroid state. It is concluded that thyroid hormone causes an increase in the mitochondrial mass, mitochondrial cytochrome content, and respiratory rate, and consequently expands the capacity of oxidative metabolism without an uncoupling effect on oxidative phosphorylation.

Inotropic effects of thyroxine and related compounds on guinea-pig left atria in vitro

Triiodothyronine, diiodothyronine and diiodotyrosine have positive inotropic activity on normal guinea-pig left atria in vitro. The increases produced by triiodothyronine and diiodothyronine are small but their detection shows that the inotropic responses to these agents can be studied in vitro. Thyroxine, thyronine and monoiodotyrosine are inactive. Reserpine pre-treatment reduces the inotropic effect of diiodotyrosine but does not reduce those of triiodothyronine and diiodothyronine.

Influence of thyroid hormones on the sensitivity of cardiac and smooth muscle to biogenic amines and other drugs

1. Rats and guinea-pigs were injected with 1-thyroxine or thyroidectomized. Sensitivity of isolated tissues and of the intact cardiovascular system to drugs was investigated.2. Thyroxine increased the sensitivity of the isolated perfused heart to noradrenaline, adrenaline, isoprenaline, acetylcholine and histamine.3. Thyroxine increased the sensitivity of the rat isolated uterus to noradrenaline, adrenaline, isoprenaline, acetylcholine, histamine and 5-hydroxytryptamine.4. In contrast, thyroxine decreased the sensitivity of the isolated ileum to acetylcholine, histamine and 5-hydroxytryptamine, but not to adrenaline.5. Thyroxine also decreased the sensitivity of the isolated aorta to noradrenaline, adrenaline, histamine and 5-hydroxytryptamine.6. In pithed rats, thyroxine potentiated pressor responses to (+)-amphetamine but not to noradrenaline or synephrine.7. In anaesthetized thyroxine-treated rats, blood pressure responses to (+)-amphetamine were potentiated while those to noradrenaline were unaltered. Responses to acetylcholine were increased, and the response to isoprenaline became purely pressor.8. Thyroxine-induced changes in the sensitivity of the isolated uterus and ileum to modification of the concentration of calcium in the physiological salt solution paralleled the changes in drug sensitivity of these tissues.9. In general, thyroidectomy produced the opposite effects to those of thyroxine.10. It is concluded that there is no specific interaction between thyroxine and sympathomimetic amines, the effect of thyroxine on drug sensitivity being non-specific.

Anaerobic performance and metabolism of the hyperthyroid heart

Anaerobically periused hearts from rats with experimentally induced hyperthyroidism exhibited accelerated deterioration of pacemaker activity and ventricular performance. The diminished anaerobic performance of hyperthyroid hearts was associated with decreased adenosine triphosphate (ATP) levels and a reduced rate of anaerobic glycolysis as reflected in decreased lactic acid production during 30 min of anoxic perfusion.Studies on whole heart homogenates demonstrated inhibition at the phosphofructokinase (PFK) step of the glycolytic pathway. Such inhibition was not demonstrated in the hyperthyroid heart cytosol. It is postulated that an inhibitor of PFK which resides dominantly in the particulate fraction is probably responsible for the diminished anaerobic glycolysis and performance of the hyperthyroid heart.

Myocardial contractile response to norepinephrine, isoproterenol, and calcium chloride in hyperthyroid guinea pigs

Myocardial contractility, measured as the maximum left ventricular systolic pressure response to aortic constriction, is greater than normal in the hyperthyroid guinea pig. The effect of inotropic agents on this response was determined in anesthetized, open-chest, euthyroid, and hyperthyroid guinea pigs. Norepinephrine and isoproterenol in single doses, 1 µg/kg, and isoproterenol infused intravenously, 1, 4 and 10 µg/kg/min, caused a smaller increase in myocardial contractile response to aortic constriction in hyperthyroid than in euthyroid animals. Hyperthyroid guinea pigs also responded to an infusion of calcium chloride, 11 mg/kg/min, with an increase in maximum systolic pressure which was less than normal. With each drug the maximum systolic pressure level attained during aortic constriction was greater in the euthyroid than in the hyperthyroid animals. Cardiac pacing of euthyroid guinea pigs at 430 beats/min, a rate similar to the spontaneous rate of the hyperthyroid guinea pig, did not alter the inotropic response to infused calcium, though single injections and intravenous infusions of isoproterenol had less of an effect than in the spontaneously beating normal heart. These results suggest that the increased myocardial contractility of the hyperthyroid guinea pig may have reached a near maximum level. As a result, the added inotropic effects of isoproterenol and calcium in these animals are less than in normal controls.

Effect of thyroid hormone on the frequency-force relationship of atrial myocardium from the guinea pig

Measurements of the frequency-force relationship were made in left atrial muscle from euthyroid and hyperthyroid guinea pigs and the effects of calcium and norepinephrine on this relationship examined. Isometric contractile force was measured with the muscle suspended in a bath of Krebs-Henseleit solution oxygenated by a gas mixture of 95% O 2 -5% CO 2 at 28°C. Frequency of stimulation was varied from 0.5 to 200/min at suprathreshold voltage delivered by plate electrodes. Low frequency stimulation caused less of a negative inotropic effect in hyperthyroid than in normal controls. The maximum negative and positive inotropic effects in the thyroxine-treated guinea pigs occurred at lower frequencies than in normal controls. These changes altered the shape of the myocardial frequency-force curve of hyperthyroid animals from that of normal controls. Norepinephrine and calcium in the dose levels examined produced a smaller than normal increment in the developed tension of hyperthyroid animals, but the maximum developed tension was similar in the two groups of animals. The lack of effect of reserpine on the frequency-force curve of left atrial muscle from hyperthyroid animals suggests that endogenous norepinephrine stores are not necessary for the production of a change in the curve. The alteration in the frequency-force relationship of atrial muscle from hyperthyroid guinea pigs is further evidence of a direct effect of thyroid hormone on the myocardial contractile mechanism.