Experimental hyperthyroidism II mechanics of contraction and relaxation of isolated ventricular myocardium

Case report: coronary vasospasm--relation to the hyperthyroid state

While angina is not uncommonly seen in association with hyperthyroidism, only rare case reports have suggested that myocardial ischemia in this state may be due to coronary artery spasm. The authors review the literature and describe a case in which the repetitive occurrence of episodes of myocardial ischemia due to coronary spasm correlated with repeated transient elevations in thyroid hormone levels, thus clarifying this relationship. The importance of defining thyroid status in patients presenting with coronary vasospasm is emphasized and the effects of thyroid hormone on the heart are reviewed.

The effect of propranolol, verapamil and dantrolene treatment on cardiac hypertrophy, enhanced myocardial contractility and tachycardia in the hyperthyroid rat

Experimentally induced hyperthyroidism is associated with cardiac hypertrophy, tachycardia and elevated myocardial contractility. To investigate the possibility of ameliorating the cardiac changes pharmacologically, hyperthyroid rats were treated with propranolol, verapamil or dantrolene. Cardiac hypertrophy was assessed from the heart mass: body mass ratio and cardiac function was measured in vitro. Both verapamil and propranolol reversed the cardiac hypertrophy of the hyperthyroid animals from 0.92 +/- 0.02 mg.g-1 to 0.70 +/- 0.01 mg.g-1 (P less than 0.001) and 0.72 +/- 0.02 mg.g-1 (P less than 0.001) respectively. Verapamil was effective in reducing the spontaneous heart rate from 331 +/- 8 beats.min-1 to 273 +/- 7 beats.min-1 (P less than 0.001) while propranolol reduced the dP/dtmax of the hyperthyroid hearts from 4089 +/- 87 mmHg.s-1 to 3497 +/- 97 mmHg.s-1 (P less than 0.001). Dantrolene had no effect on any parameter. We conclude from our results that cardiac hypertrophy of the hyperthyroid heart can be reversed by treatment with propranolol and verapamil probably via their inotropic and chronotropic properties.

Enhanced myocardial contractility but not tachycardia persists in isolated working hyperthyroid rat hearts

It is generally believed that the increased contractility and tachycardia of the hyperthyroid heart are a result of thyroid hormone-induced alterations of the mechanical and electrical properties of the heart, respectively. We compared the contractility (dP/dtmax) and the spontaneous beating rate of hyperthyroid and euthyroid hearts perfused in vitro in either a non-working or a working mode. The dP/dtmax (4196 +/- 74 mm Hg s-1) and beating rate (322 +/- 8 beats/min) of the non-working hyperthyroid hearts were significantly higher (p less than 0.001) than those of the euthyroid hearts (3267 +/- 115 mm Hg s-1 and 260 +/- 6 beats/min at an external Ca2+ of 2.5 mM). At 2.5 mM Ca2+, the working hyperthyroid hearts again displayed enhanced contractility (5636 +/- 179 mm Hg s-1 vs 4508 +/- 172 mm Hg s-1; p less than 0.001) but the spontaneous beating rate (275 +/- 7 beats/min) was not significantly different from euthyroid (261 +/- 8 beats/min). When hearts were subjected to periods of alternate non-working and working perfusion, the beating rate of the hyperthyroid hearts was significantly higher than euthyroid during non-working (p less than 0.02) but not during working perfusion. Increasing the afterload on the non-working preparations in a stepwise fashion from 75 cm H2O to 120 cm H2O caused significant changes in left ventricular pressure and dP/dtmax in both heart types but the tachycardia in the hyperthyroid hearts persisted (at 120 cm H2O; hyperthyroid, 294 +/- 9 beats/min; euthyroid, 224 +/- 10 beats/min; p less than 0.001). Alteration of the preload (10 to 25 cm H2O) and afterload (75 to 105 cm H2O) on working hyperthyroid and euthyroid hearts caused changes in both left ventricular pressure and dP/dtmax but the beating rates of both heart types were never significantly different. We conclude from our results that (i) the increased contractility of the hyperthyroid rat heart is due to thyroid hormone-induced alteration of the mechanical properties of the heart; (ii) the tachycardia of hyperthyroidism is not due to thyroid hormone-induced changes in the electrical properties of the heart, but probably involves some as yet unidentified chronotropic agent.

The dual effect of thyroid hormones on contractile properties of rat myocardium

This study was designed to investigate the changes in cardiac contractile properties induced by triiodothyronine (T3) administration in adult rats. Myofibrils and myosin were isolated from ventricular muscles from euthyroid and hyperthyroid animals and enzymatically and electrophoretically characterized. The time course of the isometric response, the force velocity curve, the force interval relation were studied in papillary muscles isolated from the right ventricles of euthyroid and hyperthyroid rats. T3 administration induced significant increases in Mg2+ activated myofibrillar ATPase activity (+11.4%) and in Ca2+ activated myosin ATPase activity (+20.1%). Significant increases in shortening velocity at low and zero loads (+20.4%) were found in papillary muscles from treated animals when compared with the control muscles. These variations in enzymatic activity and shortening velocity could be related to the increase in the amount of the fast isomyosin V1, as shown by pyrophosphate gel electrophoresis. The negative force-frequency relation at steady state, typical of rat cardiac preparations, was observed in treated and control animals; its slope was, however, halved in hyperthyroid papillary muscles when compared with control ones. In accordance with this finding, the potentiating effect of a prolonged diastolic interval was significantly reduced in hyperthyroid papillary muscles. In the frame of an interpretation of the force interval relation on the basis of the excitation contraction coupling processes, these latter observations might indicate an enhanced activity of the sarcoplasmic reticulum. We conclude that thyroid hormone administration has a dual effect on cardiac contractility, on one hand regulating the synthesis of the different isomyosin and, on the other hand, stimulating the activity of the sarcoplasmic reticulum.

Dynamic stiffness of barium-contractured cardiac muscles with different speeds of contraction

The dynamic stiffness of excised cardiac muscles that would be likely to have different intrinsic speeds of contraction, as judged by previous biochemical reports of their myosin ATPase rates, was compared. This study included muscles from thyrotoxic rabbits and newborn rabbits, rabbit atria, and normal papillary muscles at different temperatures. The usual excitation-contraction coupling process was bypassed by replacing bathing Ca2+ with Ba2+. The ensuring actively maintained contracture allowed us to focus more specifically on the contractile properties of the myofilaments. Dynamic stiffness was determined by sinusoidally oscillating muscle length at many different frequencies over the range 0.05-50 Hz while holding average muscle length at 95% of the systolic length, thus giving maximal developed force. The form of the stiffness modulus spectrum was similar for all muscles studied: stiffness was fairly constant at low frequencies, decreased to a minimum at an intermediate frequency, and then increased steeply, followed by a milder rate of increase over high frequencies. Differences in contraction speed were evident by shifts in the frequencies at which corresponding portions of the stiffness spectrum appeared. The clearest landmark was the frequency where stiffness became minimum (fmin). This varied strongly with temperature (Q10 = 2.9). Compared to normal adult papillary muscles (fmin = 1.2 Hz), fmin was 2.2 times faster in thyrotoxic myocardium, 1.9 times faster in 1-week-old rabbits, and 3.7 times faster in atrial trabeculae. These ratios of functional speed are similar to the corresponding ratios of myosin Ca2+-ATPase activities reported in the literature.

Effects of thyroid hormone on left ventricular function in patients treated for thyrotoxicosis

Systolic time intervals, echocardiographic indexes of left ventricular contractile function and serum triiodothyronine and thyroxine levels were measured before treatment in nine patients with hyperthyroidism, and again every 2 weeks for the first 2 months after therapy and then every 4 weeks until the subjects were clinically and chemically euthyroid. Six of the nine became transiently hypothyroid. Although the preejection period corrected for heart rate (preejection period index) increased as the patients became euthyroid, the change was not significant. Preejection period index increased dramatically in the patients becoming hypothyroid (p less than 0.005). Corrected left ventricular ejection time (left ventricular ejection time index) also increased as the patients became euthyroid (p less than 0.001), and increased again with the appearance of hypothyroidism (p less than 0.05). There was a linear correlation between velocity of circumferential fiber shortening and serum triiodothyronine level (r - 0.77) and between velocity of circumferential fiber shortening and serum thyroxine level (r = 0.70) at all stages of thyroid function. Thus thyroid hormone definitely enhances left ventricular function in human beings, and both excess and deficiency cause predictable reversible changes in myocardial contractile function. Thus thyroid hormone definitely enhances left ventricular function in human beings, and both excess and deficiency cause predictable reversible changes in myocardial contractile function. Furthermore echocardiographic measurements of velocity of circumferential fiber shortening provide rapid estimates of the chemical status of thyrotoxic patients before and after treatment.

Force-velocity relations of the portal vein of hyperthyroid rats

The effect of thyroxine on the elementary process of contraction of vascular smooth muscle was tested on 38 hyperthyroid and in 38 control rats. Hyperthyroidism was induced by i.p. injections of triiodthyronine or 1-htyroxine for 2 weeks. By means of afterloaded isotonic contractions of the tetanized portal vein force velocity relations were calculated. There was a slight increase in both the extrapolated velocity of shortening at zero load [from 0.93 +/- 0.04 ML/s (control rats) to 1.03 +/- 0.04 ML/s (hyperthyroid rats; P less than 0.05)], and the peak force generation (from 14.8 +/- 0.4 mN to 16.1 +/- 0.4 mN; P less than 0.005). The maximum of mechanical power development at a distinct extent of afterload was augmented from 1.35 +/- 0.06 muW/ML to 1.68 +/- 0.06 muW/ML (P less than 0.005). The influence of thyroxine on the elementary process of contraction in vascular smooth muscle is discussed in connection with the much stronger effect of hyperthyroidism on cardiac muscle contraction.

Experimental hypothyroidism: depression of myocardial contractile function and hemodynamics and their reversibility by substitution with thyroid hormones

Hemodynamics and myocardial muscle mechanics have been studied in 22 euthyroid and 60 hypothyroid cats in which experimental hypothyroidism has been produced by thyroidectomy 61 days prior to the examination. Left ventricular to body weight ratio was altered due to a decrease in left ventricular weight and an increase in body weight. Heart rate, cardiac output and cardiac index were decreased (by 12-15 per cent), whereas stroke volume remained unchanges. Peak systolic pressure of the left ventricle was moderately decreased, the other pressures were in the normal range. There was a marked and significant reduction of isovolumic contractility indices indicating a depression of myocardial contractility in situ by 20-27 per cent. The isolated ventricular myocardium exhibited decreases of isotonic muscle shortening, of maximum isometric tension development and of the rates of both, isotonic shortening and isometric tension development by 12-35 per cent. Force-velocity relationships of contraction and relaxation were depressed to lower values of contraction and relaxation velocity as well as of maximum isometric muscle tension. The alterations in myocardial muscle mechanics and hemodynamics were completely reversible following substitution of the hypothyroid group with physiological doses of L-thyroxine (5 mug/kg/day for 8-18 days). Excess increases of parameters of myocardial performance were found following substitution of the hypothyroid group with L-thyroxine (500 mug/kg/day) in accordance with the induction of experimental hyperthyroidism in these animals. The results demonstrate impaired myocardial contractility and hemodynamics in experimental hypothyroidism. These changes are completely reversible by substitution with L-thyroxine in accordance with a reversible thyroid cardiomyopathy. The cellular mechanisms responsible for the altered cardiac activity in experimental hypothyroidism are discussed.

Experimental hyperthyroidism IV. Myocardial muscle mechanics and oxygen consumption in euand hyperthyroidism

Myocardial mechanics and oxygen consumption were studied in right ventricular papillary muscles taken from cats pretreated with cristalline L-thyroxine (1 mg/kg/day, i.p.) 8-18 days prior to the examination. Isotonic afterloaded and isometric contractions were employed. Oxygen consumption was determined polarographically. Data obtained were compared with control studies on papillary muscles taken from euthyroid cats. In isotonic afterloaded contractions the extent of shortening was nearly identical in both groups. However, maximum rate of isometric tension development and velocity of isotonic shortening were considerably increased in hyperthyroid myocardium. Myocardial oxygen consumption was significantly increased in hyperthyroidism, primarily due to an increased maximum rate of isometric tension development and-to a lesser extent-to increased isotonic contraction velocity. In isometric contractions maximum tension development (preload near Lmax) was similar in both groups. However, maximum rate of isometric tension development was markedly increased in hyperthyroidism. A close and linear relationship was found between maximum rate of isometric tension development (isometric contraction) and myocardial oxygen consumption. The results demonstrate increases of velocity factors of myocardial performance in experimental hyperthyroidism. Myocardial oxygen consumption is significantly increased. This increase in oxygen consumption quantitatively has its mechanical equivalent in increased isometric contraction velocity and, to a small amount, in increased isotonic contraction velocity.