Influence of the thyroid state on the calcium transient in ventricular muscle

Thyroid hormone regulation of calcium cycling proteins

Alterations in thyroid hormone levels have a profound impact on myocardial contractility, speed of relaxation, cardiac output, and heart rate. The mechanisms for these changes include altered expression of several key proteins, involved in the regulation of intracellular calcium homeostasis. Most notably, increases in thyroid hormone and the coordinated increases in cardiac contractile parameters are marked by increases in the levels of the sarcoplasmic reticulum (SR) Ca2+-adenosine triphosphatase (ATPase) and decreases in its inhibitor, phospholamban. These changes at the protein level result in enhanced SR calcium transport and myocyte calcium cycling, leading to increases in the force and rates of contraction as well as relaxation rates at the organ level. However, decreases in thyroid hormone levels are associated with opposite alterations in these two proteins, leading to reduced myocyte calcium handling capacity and lower cardiac contractility. Furthermore, changes in the relative ratio of phospholamban/Ca2+-ATPase correlate with changes in the affinity of the SR Ca2+-transport system and relaxation rates in beating hearts. These findings suggest that thyroid hormone directly regulates SR protein levels and thus, cardiac function.

Prevention of abnormal sarcoplasmic reticulum calcium transport and protein expression in post-infarction heart failure using 3, 5-diiodothyropropionic acid (DITPA)

Heart failure of diverse causes is associated with abnormalities of sarcoplasmic reticulum (SR) Ca(2+)transport. The purpose of this study was to determine whether the thyroid hormone analogue, 3,5-diiodothyropropionic acid (DITPA), prevents abnormal Ca(2+)transport and expression of SR proteins associated with post-infarction heart failure. New Zealand White rabbits were randomly assigned to circumflex artery ligation or sham operation, and to DITPA administration (3.75 mg/kg/day) or no treatment in a two-by-two factorial design. After 3 weeks, echo-Doppler and LV hemodynamic measurements were performed. From ventricular tissue, single myocyte shortening and relaxation were determined, and Ca(2+)transport was measured in homogenates and SR-enriched microsomes. Levels of mRNA and protein content were determined for the SR Ca(2+)-ATPase (SERCA2a), phospholamban (PLB), cardiac ryanodine receptor (RyR-2) and calsequestrin. The administration of DITPA improved LV contraction and relaxation and improved myocyte shortening in infarcted animals. The improvements in LV and myocyte function were associated with increases in V(max)for SR Ca(2+)transport in both homogenates and microsomes. Also, DITPA prevented the decrease in LV protein density for SERCA2a, PLB and RyR-2 post-infarction, without measurable changes in mRNA levels. The thyroid hormone analogue, DITPA, improves LV, myocyte and SR function in infarcted hearts and prevents the downregulation of SR proteins associated with post-infarction heart failure. The specific effects of DITPA on post-infarction SR Ca(2+)transport and the expression of SR proteins make this compound a potentially useful therapeutic agent for LV systolic and/or diastolic dysfunction.

Thyroid hormone-induced overexpression of functional ryanodine receptors in the rabbit heart

Modifications in the Ca(2+)-uptake and -release functions of the sarcoplasmic reticulum (SR) may be a major component of the mechanisms underlying thyroid state-dependent alterations in heart rate, myocardial contractility, and metabolism. We investigated the influence of hyperthyroid state on the expression and functional properties of the ryanodine receptor (RyR), a major protein in the junctional SR (JSR), which mediates Ca(2+) release to trigger muscle contraction. Experiments were performed using homogenates and JSR vesicles derived from ventricular myocardium of euthyroid and hyperthyroid rabbits. Hyperthyroidism, with attendant cardiac hypertrophy, was induced by the injection of L-thyroxine (200 microg/kg body wt) daily for 7 days. Western blotting analysis using cardiac RyR-specific antibody revealed a significant increase (>50%) in the relative amount of RyR in the hyperthyroid compared with euthyroid rabbits. Ca(2+)-dependent, high-affinity [(3)H]ryanodine binding was also significantly greater ( approximately 40%) in JSR from hyperthyroid rabbits. The Ca(2+ )sensitivity of [(3)H]ryanodine binding and the dissociation constant for [(3)H]ryanodine did not differ significantly between euthyroid and hyperthyroid hearts. Measurement of Ca(2+)-release rates from passively Ca(2+)-preloaded JSR vesicles and assessment of the effect of RyR-Ca(2+)-release channel (CRC) blockade on active Ca(2+)-uptake rates revealed significantly enhanced (>2-fold) CRC activity in the hyperthyroid, compared with euthyroid, JSR. These results demonstrate overexpression of functional RyR in thyroid hormone-induced cardiac hypertrophy. Relative abundance of RyR may be responsible, in part, for the changes in SR Ca(2+) release, cytosolic Ca(2+) transient, and cardiac systolic function associated with thyroid hormone-induced cardiac hypertrophy.

Regulation of cardiac sarcolemmal Ca2+ channels and Ca2+ transporters by thyroid hormone

In order to examine the regulatory role of thyroid hormone on sarcolemmal Ca(2+)-channels, Na(+)-Ca2+ exchange and Ca(2+)-pump as well as heart function, the effects of hypothyroidism and hyperthyroidism on rat heart performance and sarcolemmal Ca(2+)-handling were studied. Hyperthyroid rats showed higher values for heart rate (HR), maximal rates of ventricular pressure development +(dP/dt)max and pressure fall -(dP/dt)max, but shorter time to peak ventricular pressure (TPVP) and contraction time (CT) when compared with euthyroid rats. The left ventricular systolic pressure (LVSP) and left ventricular end-diastolic pressure (LVEDP), as well as aortic systolic and diastolic pressures (ASP and ADP, respectively) were not significantly altered. Hypothyroid rats exhibited decreased values of LVSP, HR, ASP, ADP, +(dP/dt)max and -(dP/dt)max but higher CT when compared with euthyroid rats; the values of LVEDP and TPVP were not changed. Studies with isolated-perfused hearts showed that while hypothyroidism did not modulate the inotropic response to extracellular Ca2+ and Ca2+ channel blocker verapamil, hyperthyroidism increased sensitivity to Ca2+ and decreased sensitivity to verapamil in comparison to euthyroid hearts. Studies of [3H]-nitrendipine binding with purified cardiac sarcolemmal membrane revealed decreased number of high affinity binding sites (Bmax) without any change in the dissociation constant for receptor-ligand complex (Kd) in the hyperthyroid group when compared with euthyroid sarcolemma; hypothyroidism had no effect on these parameters. The activities of sarcolemmal Ca(2+)-stimulated ATPase, ATP-dependent Ca2+ uptake and ouabain-sensitive Na(+)-K+ ATPase were decreased whereas the Mg(2+)-ATPase activity was increased in hypothyroid hearts. On the other hand, sarcolemmal membranes from hyperthyroid samples exhibited increased ouabain-sensitive Na(+)-K+ ATPase activity, whereas Ca(2+)-stimulated ATPase, ATP-dependent Ca2+ uptake, and Mg(2+)-ATPase activities were unchanged. The Vmax and Ka for Ca2+ of cardiac sarcolemmal Na(+)-Ca2+ exchange were not altered in both hyperthyroid and hypothyroid states. These results indicate that the status of sarcolemmal Ca(2+)-transport processes is regulated by thyroid hormones and the modification of Ca(2+)-fluxes across the sarcolemmal membrane may play a crucial role in the development of thyroid state-dependent contractile changes in the heart.

Combination treatment with captopril and the thyroid hormone analogue 3,5-diiodothyropropionic acid. A new approach to improving left ventricular performance in heart failure

BACKGROUND

An agent that improves left ventricular (LV) performance by non-cAMP-mediated mechanisms would be valuable in the treatment of chronic heart failure. We have shown earlier that the thyroid hormone analogue 3,5-diiodothyropropionic acid (DITPA) binds to nuclear receptors, alters transcription of T3-responsive genes, and increases +dP/dtmax in hypothyroid rats with substantially less effect on heart rate and metabolism than thyroid hormone, which makes it a selective cardiotonic agent.

METHODS AND RESULTS

To determine whether DITPA might be useful in treating heart failure, we compared chronic treatment with normal saline, captopril (2 g/L), or the combination of DITPA (375 micrograms/100 g) and captopril (2 g/L) in Sprague-Dawley rats beginning 3 weeks after coronary artery ligation. Both DITPA/captopril and captopril treatment decreased LV end-diastolic pressure compared with controls (21 +/- 2 and 26 +/- 2 mm Hg, respectively, vs 34 +/- 3 mm Hg, P < .05 for each). The addition of DITPA to captopril produced a 36% increase in resting cardiac index (P < .05) and shifted the cardiac function curve upward and to the left, indicative of enhanced myocardial performance. Also, DITPA/captopril compared with captopril treatment or control produced an increase in the rate of LV relaxation, as manifested by a decrease in tau, the time constant of LV pressure decline (17.5 +/- 1.0 vs 22.2 +/- 1.7 milliseconds, P < .05) and a larger absolute value for -dP/dtmax (-4561 +/- 361 vs -3346 +/- 232 mm Hg/s, P < .05). These changes occurred without changes in heart rate, LV mass, LV systolic pressure, or peripheral resistance relative to captopril treatment (P > .05).

CONCLUSIONS

The combination of DITPA and captopril improved cardiac output, increased -dP/dtmax, and increased the rate of LV relaxation to a greater extent than captopril treatment in the rat postinfarction model of heart failure. Use of a cardiotonic analogue of thyroid hormone represents a new approach to improving LV performance and may be a useful adjunct to afterload reduction for the treatment of heart failure.

Rat cardiac calcium channels and their relationships with beta-adrenergic and muscarinic receptors in hypothyroidism

Thyroid hormone deficiency is associated with changes in the cardiovascular system. No one has reported the measures of both atrial and ventricular calcium channel density and function, in association with measures of negative and positive channel modulators, in hypothyroid hearts. Hormonally-induced modulation of calcium channels has clinical significance in the development and application of therapeutic agents in dysthyroid states. We thyroidectomized male rats and sham-operated euthyroid controls, in order to measure radioligand binding to ventricular and atrial membrane Ca2+ channels ([3H]-isradipine), beta-adrenoceptors ([125I]-iodocyanopindolol) and atrial muscarinic receptors ([3H]-quinuclidinyl benzilate) and related these data to contraction and heart rate responses to isoproterenol, carbachol and calcium. When data from hypothyroid tissues were compared to those of controls, the densities of calcium channels increased 50% in ventricles, but no differences were seen in atrial homogenates. In both atria and ventricles, beta-adrenoceptors decreased modestly with no change in affinity. Atrial muscarinic receptor density was also unchanged. Dose response curves of left atrial contractions showed: decreased sensitivity (increased EC50 value) but equal maximal responsiveness to extracellular calcium; an increased carbachol sensitivity (decreased EC50 value); and no significant difference in isoproterenol response. Comparisons of within-individual preparation ratioed EC50 values confirm the changed calcium and carbachol sensitivities. Heart-rate dose response curves displayed: increased maximal heart rate responsiveness to calcium associated with increased EC50 values; isoproterenol sensitivity was decreased nearly 3-fold. There was no significant difference in heart rate response to carbachol; however, ratioed values of carbachol and calcium EC50s were significantly different. These data are consistent with a tissue-level state of enhanced negative chronotropism and inotropism occurring in hypothyroid myocardia. We also confirm an earlier controversial finding of increased calcium channel density in ventricles from hypothyroid rats.

Hyperthyroidism selectively modified a transient potassium current in rabbit ventricular and atrial myocytes

1. Transient outward potassium currents (I(t)) were compared in single cardiac myocytes obtained from normal and hyperthyroid rabbits. Currents were recorded using the suction electrode whole-cell voltage clamp technique. 2. In ventricular myocytes from hyperthyroid animals (at 22 degrees C and a stimulation rate of 0.2 Hz), I(t) was 4- to 5-fold larger than in normal myocytes, in a potential range of -20 to +60 mV. As in normal myocytes, I(t) in hyperthyroid myocytes was calcium insensitive, and was more than 90% suppressed by 2 mM 4-aminopyridine. 3. The increase in I(t) was observed over a wide range of stimulation rates, even at rates sufficiently slow to enable complete reactivation of the I(t) channels. However, there was a major change in the rate dependence of I(t) in hyperthyroid myocytes, with significant I(t) current still present at rates (e.g. 1-2 Hz) at which it is normally completely suppressed. 4. The augmentation of I(t) in the hyperthyroid myocytes could not be accounted for by changes in the voltage dependence or the kinetics of channel activation or inactivation. There was no change in the reversal potential of I(t), implying no change in the selectivity of the channel. 5. Single-channel activity was recorded using the cell-attached mode of recording. In myocytes from hyperthyroid rabbit we observed the following: (a) active patches (often containing two channels) were obtained more frequently in comparison to control; (b) the unitary conductance of the channel was the same; (c) single-channel openings persisted at high stimulation rates. 6. In contrast to hyperthyroid ventricular cells, I(t) in atrial cells from the same hearts was not substantially changed. 7. The rate dependence of I(t) in atrial cells was also unaffected by hyperthyroidism, in contrast to the large changes observed in ventricular cells. Thus, in atrial cells from hyperthyroid hearts the current was totally suppressed at rates of 1-2 Hz, as in euthyroid conditions. 8. Single-channel recordings in the cell-attached mode showed a unitary conductance similar to that found in normal atrial cells. Channel activity was suppressed at 2 Hz, in contrast to hyperthyroid ventricular cells. 9. In conclusion, I(t) is drastically changed in hyperthyroid rabbit ventricle cells. The changes are in the magnitude of the macroscopic current and its rate dependence. Since the unitary conductance is unchanged (and the peak open probabilities are normally high at positive membrane potential(s) the number of active channels in the membrane must be increased. In atrial cells from the same hyperthyroid hearts no changes are apparent.(ABSTRACT TRUNCATED AT 400 WORDS)

Thyroid control of contractile function and calcium handling in neonatal rat heart

Newborn rats were rendered hyperthyroid (daily subcutaneous injections of L-triiodothyronine, 10 micrograms 100 g-1 body weight) or hypothyroid (0.05% 6-n-propyl-2-thiouracil in drinking water to nursing mothers) during the first 3 weeks of postnatal life. Compared with the euthyroid group, hyperthyroidism resulted in: (1) cardiac enlargement with right ventricular preponderance, (2) increased cardiac contractile function, (3) increased Ca2+ uptake by the sarcoplasmic reticulum (SR), (4) decreased sensitivity to the negative inotropic effect of verapamil and (5) greater inhibition of contractile function by ryanodine. Hypothyroidism generally resulted in opposite changes. The data suggest that the development of the heart and its contractile function during early postnatal life depends on the plasma level of thyroid hormones. In particular, the relative contribution of the SR and sarcolemmal Ca2+ transport to the control of cardiac contractility seems to be markedly affected by altered thyroid states. The postnatal maturation of the SR function is accelerated in hyperthyroidism but retarded in hypothyroidism. Consequently, hyperthyroid hearts appear to be less dependent and hypothyroid ones more dependent on trans-sarcolemmal Ca2+ fluxes when compared with age-matched euthyroid animals.

Mechanisms of intrinsic tone in ferret vascular smooth muscle

1. Circular strips from ferret aorta were used to investigate the mechanism of the intrinsic basal tone. 2. Determinations of stiffness using small sinusoidal length changes showed an abolition of both stiffness and force with cooling, but the temperature dependence of the change in active stiffness did not parallel that of force. At temperatures below 22 degrees C there appeared to be a relatively large population of attached, non-force-generating cross-bridges, indicating that separate mechanisms are involved in regulating cross-bridge attachment and the force per cross-bridge. 3. Active intrinsic tone was not affected by removal of extracellular Ca2+ or removal of endothelium. 4. Intracellular ionized Ca2+ concentrations ([Ca2+]i) as measured with the photoprotein aequorin, did not significantly change when intrinsic tone was abolished by cooling. 5. Myosin light chain phosphorylation, as measured by 2-dimensional polyacrylamide gel electrophoresis, significantly decreased on cooling, but the temperature dependence of phosphorylation did not parallel that of force. The change in phosphorylation in the absence of a change in [Ca2+]i suggests the presence of a constitutively active Ca(2+)-independent form of myosin light chain kinase. 6. Maximal concentrations of staurosporine inhibited but did not eliminate intrinsic tone. 7. Changes in myosin light chain kinase and protein kinase C activities may explain part but not all of the intrinsic tone.

Effect of thyroid hormone on the expression of mRNA encoding sarcoplasmic reticulum proteins

The purpose of this study was to determine the expression of genes encoding various sarcoplasmic reticulum components that are functionally coupled with calcium release, uptake, and storage function during cardiac hypertrophy induced by thyroid hormone. Hyperthyroidism was induced in two groups of rabbits by the injection of 200 micrograms/kg L-thyroxine (T4) daily for 4 days (T4-4-day group) and 8 days (T4-8-day group). Hypothyroidism was induced in another group of rabbits by adding 0.8 mg/ml propylthiouracil to the drinking water for 4 weeks. The relative expression level of mRNA encoding different sarcoplasmic reticulum proteins was determined by RNA slot blot and Northern blot analysis. In hyperthyroid hearts, the steady-state level of cardiac ryanodine receptor mRNA and sarcoplasmic reticulum cardiac/slow-twitch Ca(2+)-ATPase mRNA were both increased to 147% (T4-4-day group) and 186% (T4-8-day group) of control, respectively, but decreased to 71% and 75%, respectively, in hypothyroid ventricles. The mRNA level for phospholamban was decreased in both hyperthyroidism (T4-8-day group, 72%) and hypothyroidism (77%) in these hearts. On the other hand, calsequestrin mRNA levels did not change in hyperthyroid and hypothyroid ventricles. In accord with the changes in Ca(2+)-ATPase mRNA levels, the Ca(2+)-ATPase protein was increased to 199% (T4-8-day group) in hyperthyroid ventricles and decreased to 86% of control in hypothyroid ventricles. The expression levels of ryanodine receptor, Ca(2+)-ATPase, phospholamban, and calsequestrin mRNAs were similarly altered in skeletal muscle tissues from hyperthyroid and hypothyroid rabbits. These results indicate that the mRNA levels of sarcoplasmic reticulum proteins responsible for calcium release and calcium uptake are coordinately regulated in response to changes in thyroid hormone level in both heart and skeletal muscle. These changes in mRNA level should lead to changes in protein levels and thus to altered calcium release and uptake in the chronic stages of hyperthyroidism and hypothyroidism.

The role of calcium in the enhanced myocardial contractility of the hyperthyroid rat heart

The hyperthyroid rat myocardium exhibits enhanced contractility. There is evidence that altered calcium handling by the myocardium may be responsible for this enhanced state. To investigate this, isolated hyperthyroid and euthyroid hearts were perfused in the working mode and exposed to alterations in external calcium concentration. Heart rate was not significantly different in either group of hearts, nor was it altered by the change in calcium. The concentration of calcium needed to elicit half-maximal contractility (dP/dtmax) was lower in the hyperthyroid (0.81 +/- 0.07 mM) than in the euthyroid hearts (1.12 +/- 0.09 mM, p less than 0.05). This increase in calcium sensitivity was unlikely to be at the site of the sarcolemma as verapamil exerted equal negative inotropic effects on both groups of hearts. Dantrolene, which blocks calcium release from the sarcoplasmic reticulum, exerted a significantly greater (p less than 0.01) depression in dP/dtmax after 12 min in the hyperthyroid (50 +/- 7%) than in the euthyroid heart (15 +/- 2%). We conclude from our results that the enhanced contractile state of the hyperthyroid rat heart is likely to involve an altered mechanical response to calcium which is possibly at the level of enhanced calcium release from the sarcoplasmic reticulum.

Effects of thyroid hormone on Ca2+ efflux and Ca2+ transport capacity in rat skeletal muscle

The present study was undertaken to investigate the effects of 3,5,3'-triiodothyronine (T3) treatment on passive Ca2+ efflux, Ca2(+)-dependent Mg2(+)-ATPase (Ca2(+)-ATPase) concentration and active Ca2+ transport in isolated rat skeletal muscle. In addition, the question was examined whether changes in Ca2+ efflux at rest and during electrical stimulation in the hyperthyroid state were accompanied by parallel changes in 3-O-methylglucose efflux. The resting Ca2+ efflux from rat soleus muscle was increased by 25% after 8 days of treatment with T3 (20 micrograms/100 g body weight). This was associated with a 78% increase in the basal efflux of 3-O-methylglucose. Electrical stimulation resulted in a rapid stimulation of Ca2+ efflux and 3-O-methylglucose efflux in the two groups of rats, and the levels obtained were significantly higher in the T3-treated group. The stimulating effect of the alkaloid veratridine on Ca2+ efflux was 60% larger in 8-day hyperthyroid rats. Within 24 h after the start of T3 treatment, a significant (21%) increase in Ca2(+)-ATPase concentration was detected. Significant increases in active Ca2+ uptake and passive Ca2+ efflux were not observed until after 2 and 3 days of T3 treatment, respectively. It is concluded that T3 stimulates the synthesis of Ca2+ ATPase and augments the intracellular Ca2+ pools (sarcoplasmic reticulum and mitochondria). The latter results in enhancement of the passive Ca2+ leak, which in turn, may lead to activation of substrate transport systems. The suggested increase in intracellular Ca2+ cycling after T3 treatment may, at least partly, explain the T3-induced stimulation of energy metabolism.

Influence of thyroid status on Ca2+ mobilization and amylase secretion in rat pancreatic acini

Thyroid hormones influence Ca2+ homeostasis in both skeletal and cardiac muscle. Since secretory cells, like muscle cells, store and use Ca2+ in stimulus-response coupling, we have studied the effects of thyroid status on Ca2+ mobilization and secretion in a model secretory tissue, the pancreatic acinar cell. Hyperthyroidism was induced by rats by daily, subcutaneous injections of triiodothyronine for 8 days and hypothyroidism by adding 6-n-propyl-2-thiouracil to the drinking water for 14 days. Pancreatic acini were prepared by collagenase digestion of pancreatic tissue from hyper- and hypo-thyroid animals and from euthyroid controls. Ca2(+)-mobilization was assessed using Quin-2 fluorescence and secretion by assaying amylase release. The data indicate that the amount of Ca2+ mobilized by the muscarinic agonist carbachol or by cholecystokinin octapeptide increases with increasing thyroid hormone concentrations. Only in hypothyroidism was this change in Ca2+ homeostasis reflected by a parallel change in amylase secretion. This implies the existence of some compensatory mechanism which stabilizes secretory rate in the face of stimulus-evoked increases in intracellular Ca2+ concentration.

Inotropic and lusitropic effects of MCI-154 (6-[4-(4- pyridyl)aminophenyl]-4,5-dihydro-3(2H)-pyridazinone) on human myocardium

We studied the inotropic and lusitropic responses to MCI-154 in 12 right or left ventricular trabeculae carneae isolated from 7 organ donors (non-cardiac) without known cardiovascular disease who met accepted criteria for brain death. Isometric tension was recorded from muscles superfused with a physiologic salt solution at 30 degrees C, and stimulated to contract at three-second intervals. Concentration-response curves were developed over a range of MCI-154 organs bath concentrations (10(-7) M to 3 x 10(-4) M; n = 9). Six experiments were conducted using 10(-6) M carbachol, a muscarinic agonist, in the presence of a maximally effective concentration of MCI-154 to test for dependence of tension development on cyclic adenosine monophosphate. Three experiments were conducted with MCI-154, 3 x 10(-5) M, in muscles loaded with the bioluminescent calcium indicator aequorin. MCI-154 produced a concentration-dependent rise in peak tension in the human muscle (positive inotropic effect), equivalent to 70% of the maximal response to calcium (P less than 0.001). Relaxation was enhanced (positive lusitropic effect), as evidenced by a fall in the time to 80% relaxation from 311 +/- 13 ms (baseline) to 248 +/- 15 ms at 10(-5) M (P less than 0.01). Aequorin studies showed the increase in tension to be accompanied by large increases in cystolic calcium, the principal mechanism of action. Carbachol caused MCI-154--induced maximum peak tension to decrease by 5 +/- 1%. While not excluding a cyclic adenosine monophosphate--mediated MCI action, this modest carbachol inhibition suggests the existence of additional mechanism(s) of action. MCI-154 had a negative lusitropic effect at high concentrations (greater than 10(-4)M) which may have been due to intracellular calcium overload, evidenced by the large amplitude aequorin signals. This does not exclude sensitization of the myofilaments to calcium as a possibility. Extrapolated to the in vivo setting, these experiments suggest that MCI-154 may be an effective positive inotropic agent in man.

Modulation by the thyroid state of intracellular calcium and contractility in ferret ventricular muscle

The purpose of this study was to determine whether the cardiac contractile abnormalities induced by hyperthyroidism or hypothyroidism are caused by changes in intracellular Ca2+ handling or by alterations in the distribution of isoenzymes of ventricular myosin. Right ventricular papillary muscles obtained from euthyroid ferrets and ferrets treated with L-thyroxine (hyperthyroid) or methimazole (hypothyroid) were loaded with the calcium indicator aequorin for recording intracellular Ca2+ levels during isometric contraction. In muscles from the hypothyroid ferrets, peak tension was reduced and the duration of contraction prolonged compared to the controls; these changes were associated with a Ca2+ transient of decreased amplitude and prolonged duration. Hyperthyroidism produced opposite changes in the time course of the Ca2+ transient and the associated isometric contraction. Native polyacrylamide gel electrophoresis was performed on myosin extracted from the right ventricular free wall of control and treated ferrets. The hyperthyroid state was associated with new myosin formation as indicated by the appearance of three myosin bands on the pyrophosphate gel. Gels of myosin from hypothyroid and euthyroid ferrets showed a single band that migrated with the slowest of the three bands from the hyperthyroid ferrets. These results suggest that changes in both Ca2+ handling and myosin isoenzymes may contribute to the contractile abnormalities observed in hyperthyroidism. Alterations in intracellular Ca2+ handling alone may account for the contractile changes induced by hypothyroidism.

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.

Effect of thyroid status on beta-adrenoceptors and calcium channels in rat cardiac and vascular tissue

To determine the influence of thyroid hormone on beta-adrenoceptors and Ca2+ channels, rats were treated with thyroxine (75 micrograms/100 g sc daily for 5 days) or propylthiouracil (0.05% in drinking water for 30 days). beta-Adrenoceptor density in ventricular tissue, measured by [125I]iodocyanopindolol binding, was significantly increased and decreased respectively, following thyroxine or propylthiouracil treatment to 124.7 +/- 7.11 fmol/mg protein and 71.98 +/- 5.37 fmol/mg protein from euthyroid (control) levels of 93.7 +/- 4.58 fmol/mg protein. Ca2+ channel density, measured by [3H]nitrendipine binding, was altered in the opposite direction; it was significantly decreased and increased to 324 +/- 24 fmol/mg protein and 691 +/- 31 fmol/mg protein from 562 +/- 35 fmol/mg protein after thyroxine or propylthiouracil treatment, respectively. No changes in affinity of either ligand were observed. Responses of isolated papillary muscles from propylthiouracil-treated animals accorded with changes seen in the binding studies. The geometric mean EC50 of isoproterenol increased from 9.5 x 10(-9) mol/l to 5.5 x 10(-8) mol/l, and the EC50 for calcium decreased from 3.16 x 10(-3) mol/l to 1.36 x 10(-3) mol/l; moreover, the responsiveness to the Ca2+ channel activator Bay K 8644 was increased. The corresponding responses in thyroxine-treated animals could not be examined because of prominent arrhythmic activity. As with papillary muscles the sensitivity of left atria to isoproterenol was decreased after treatment with propylthiouracil, with geometric mean EC50 values increasing from 3.21 x 10(-9) mol/l to 89.4 x 10(-9) mol/l.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of thyroid state on cytosolic free calcium in resting and electrically stimulated cardiac myocytes

The effects of the thyroid state on the cytosolic free Ca2+ concentration, [Ca2+]i, of resting and K+-depolarized cardiomyocytes were studied using the fluorescent Ca2+ indicator fura2. The mean resting [Ca2+]i in euthyroid myocytes (89 +/- 8 nM) was not significantly different from that in hyperthyroid myocytes (100 +/- 14 nM). The resting O2-consumption rate was identical for both groups when expressed per mg protein, but a 35% higher value was observed in the hyperthyroid group when expressed per cell on account of the cellular hypertrophy induced by thyroid hormone. Potassium induced depolarization (50 mM [K+]0) raised the level of [Ca2+]i by 50% in both groups. When ATP-coupled respiration was blocked with oligomycin, the 50 mM K+-induced rise in [Ca2+]i was accompanied in both groups by a 40% rise in glycolytic activity as inferred from measurement of lactate production. Ca2+-fluorescence transients were recorded from electrically stimulated myocytes of euthyroid, hyperthyroid and hypothyroid rats. The time taken to reach peak fluorescence (TPL) and that to 50% decay of peak fluorescence (RL0.5) decreased in the direction hypothyroid----hyperthyroid, indicating an increase in Ca2+ fluxes in the same direction. Isoproterenol (1 microM) enhanced the peak Ca2+ fluorescence in electrically stimulated hypothyroid and euthyroid myocytes but not in hyperthyroid myocytes. Both the TPL and RL0.5 were decreased by isoproterenol in euthyroid, but more so in hypothyroid myocytes. None of these parameters were influenced by isoproterenol in the hyperthyroid group. We conclude that (1) thyroid hormone increases neither the O2-consumption rate nor the level of [Ca2+]i of resting cardiomyocytes and (2) the effects of the beta-receptor-agonist isoproterenol on Ca2+ transients of electrically stimulated myocytes, are inversely related to the documented changes in beta-receptor density in heart tissue occurring with alterations in the thyroid state.