The thyroid hormones (TH) play a role in the entire organism, regulating biological processes, such as metabolic rate, oxygen consumption, gene transcription, and protein synthesis.^{1 , 2} Hyperthyroidism (Hy) is characterized by an increase in the endogenous production of the triiodothyronine (T3) and/or thyroxine (T4) hormones, or by the exogenous administration of these hormones.³ The heart is the major target organ for TH actions.^{4 , 5}

In the heart, TH has an effect on the membrane ion channels,⁶ on the contractile apparatus, and on the sarcoplasmic reticulum (SR),⁷ which are linked to the excitation-contraction coupling mechanism⁸ and directly alter contractility.⁹

Common changes reported in the hyperthyroid heart are increased myocardial contractility and cardiac output,² improvement in diastolic relaxation,¹⁰ changes in electrical activity,⁷ increments in ventricular mass,⁵ and arrhythmias.¹

In short-term Hy, TH is a positive regulator of the cardiac function. By contrast, in the long-term, TH promotes deleterious effects in the heart,⁵ causing heart problems, the major cause of death in hyperthyroid patients.⁶ However, the influences of TH in molecular mechanisms of the cardiac function have not yet been fully understood. Thus, the present study aimed to evaluate functional changes in contractile parameters and the Na⁺/Ca²⁺ exchanger (NCX) function in the isolated papillary muscle 10 days after hyperthyroidism had been induced in rats. This investigation could bring new evidence to explain the actions of TH in the physiology of the heart.

Body and heart weight of all animals after 10 days of treatment are represented in Table 1 . As expected in HG, body weight decreased 7.35%, and heart weight increased by 18.98% when compared to CG.

In this study, contractility in isolated papillary muscles from induced hyperthyroid rats was assessed and demonstrated an increase in the speed of diastolic relaxation (–dF/dt) associated with a higher NCX function when compared to the CG. No changes were observed in contractile force (Tmax and Fmax) or in maximum speed of systolic contraction (+dF/dt).

In animals treated with T3, an increase in the cardiac mass/body mass ratio is commonly reported.^{1 , 5 , 14 , 17} This was also observed in this study. Such results are related to a rise in the basal metabolic rate, increased energy expenditure, and oxygen consumption associated with an increment in protein and lipid catabolism.^{10 , 18} In cardiac myocytes, an increase in total protein synthesis⁹ and an incremented expression of the alpha myosin heavy chain ( α -MHC)⁶ were observed, which resulted in an enhanced myocardial function.²

The papillary muscles of hyperthyroid rats showed no changes in contractile force when compared with the controls (Tmax, Table 2 ), which runs in line with results obtained by Szkudlarek et al.¹⁴ and Vieira et al.¹¹ Fmax in response to caffeine ( Table 3 ) also showed no differences, which demonstrated that the SR Ca²⁺content has not changed, which is in agreement with previous data published by Alba-Aguayo et al.¹

Regarding contractility, represented by +dF/dt and –dF/dt, the available information is conflicting, and there is paucity of data. Our data revealed no changes in +dF/dt and a significant increase in diastolic function in HG when compared to the CG ( Table 2 ); similar results were observed by Szkudlarek et al.¹⁴ and Palmieri et al.¹⁹ Vieira et al.¹¹ reported an increment in + and –dF/dt and a decrease in time to peak contraction. On the contrary, Wolska et al.⁸ reported an increase in time to peak contraction, while Rozanski et al.⁵ reported a decrease in +dF/dt and no changes in –dF/dt.

Szkudlarek et al.¹⁴ showed, in vivo, the effects of Hy in rats using echocardiography, and, in HG, demonstrated a decreased end diastolic volume, a total diastolic time and ejection time associated with the increased heart rate, an ejection fraction, and a cardiac output. In a similar experiment, Palmieri et al.¹⁹ observed in hyperthyroid patients that the increased left ventricle performance was sustained by increased preload with enhanced diastolic function.

The changes in cardiac function promoted by Hy are commonly associated with the upregulation of ryanodine receptors isoform 2 (RyR2)¹⁷²⁰²¹ and SERCA2 expression,⁷⁹¹⁰ as well as the downregulation of phospholamban (PLB).¹⁶⁷ Moreover, TH stimulates the expression of β-adrenergic receptors in the heart,⁶⁸⁹ which raises the sensitivity to sympathetic stimulation. The activation of the AMPc/PKA/CaMKII pathway leads to increments in RyR2 and PLB phosphorylation with a consequent rise in the Ca²⁺release to sarcoplasm and the Ca²⁺uptake to SR.²²

For the diastole, Ca²⁺is removed from the cytosol by four mechanisms: SERCA2, NCX, the plasma membrane Ca²⁺ATPase (PMCA), and the mitochondrial Ca²⁺uniporter.²² In rat ventricles, under normal conditions, approximately 92% of cytosolic Ca²⁺is removed by SERCA2, 7% by NCX, and 1% by PMCA and Ca²⁺uniporter.²³ The NCX activity assessment, represented in Table 4 , revealed no changes in the early phase of relaxation between the groups. However, a significant decline of 149% in the time of late-phase relaxation was observed in the HG when compared to the CG. Although there is a record of increment,²⁴ most papers report a decline in NCX expression in Hy.^{2 , 25} Cernohorský et al.²⁶ showed a 50% reduction in rat ventricular NCX expression. According to Davis & Davis,²⁷ the PMCA appears to be indirectly influenced by TH through increased PKC activation, although its function is unlikely to influence cytosolic Ca²⁺concentration.

Another mechanism by which T3 alters the cardiac function is by modulating the activity of Ca²⁺channels in the cell membrane² . Gotzsche²⁸ attributes the changes in myocardial Ca²⁺homeostasis to an augmented expression of the β-receptor and the activity of AMPc/PKA/CaMKII pathways, while Yu et al.²⁹ reports an increased activity of these pathways with increased L-type Ca²⁺channel phosphorylation, raising the density of Ca²⁺currents through these channels. According to Bers,²² the Ca²⁺that enters via ICa is approximately the same as that extruded by the NCX. A consequence of the increased Ca²⁺entry in the cell is an increased Ca²⁺ extrusion via NCX, which may have boosted the exchanger activity.

In this work, after the contraction of the papillary muscle had been induced by caffeine in the presence of cyclopiazonic acid, simultaneously with the reactivation of NCX, the late phase of relaxation was reduced. Since in this condition the activity of the SR Ca²⁺ pump was blocked, the relaxation of the muscle was mainly due to NCX activity. This suggests that Hy induced an increase in activity and/or expression of NCX.

Much information is missing concerning hyperthyroidism and the cardiac function, especially in isolated muscles. Certainly, the time at which the cardiac muscle is exposed to an increase in circulating thyroid hormones influences the magnitude of the consequences, whose primary changes seem to be related to an increase in the cardiac mass/body mass ratio and increased relaxation kinetics.