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

The changes in beta-adrenoceptor-mediated cardiac function in experimental hypothyroidism: the possible contribution of cardiac beta3-adrenoceptors

Thyroid hormone deficiency has been reported to decrease expression and function of both beta(1)- and beta(2)-adrenoceptor in different tissues including heart. The purpose of this study was to examine the possible contribution of beta(3)-adrenoceptors to cardiac dysfunction in hypothyroidism. In addition, effect of this pathology on beta(1)- and beta(2)-adrenoceptor was investigated. Hypothyroidism was induced by adding methimazole (300 mg/l) to drinking water of rats for 8 weeks. Cardiac hemodynamic parameters were measured in anesthetised rats in vivo. Responses to beta-adrenoceptor agonists were examined in rat papillary muscle in vitro. We also studied the effect of hypotyroidism on mRNA expression of beta-adrenoceptors, Gialpha, GRK, and eNOS in rat heart. All of the hemodynamic parameters (systolic, diastolic and mean arterial pressure, left ventricular pressure, heart rate, +dp/dt, and -dp/dt) were significantly reduced by the methimazole treatment. The negative inotropic effect elicited by BRL 37344 (a beta(3)-adrenoceptor preferential agonist) and positive inotropic effects produced by isoprenaline and noradrenaline, respectively, were significantly decreased in papillary muscle of hypothyroid rats as compared to those of controls. On the other hand, hypothyroidism resulted in increased cardiac beta(2)- and beta(3)-adrenoceptor, Gialpha(2), Gialpha(3), GRK3, and eNOS mRNA expressions. However, beta(1)-adrenoceptor and GRK2 mRNA expressions were not changed significantly in this pathology. These results show that mRNA expression of beta(3)-adrenoceptors as well as the signalling pathway components mediated through beta(3)-adrenoceptors are significantly increased in hypothyroid rat heart. Since we could not correlate these alternates with the decreased negative inotropic response mediated by this receptor subtype, it is not clear whether these changes are important for hypothyroid induced reduction in cardiac function.

Thyroid disease and vascular function

Altered cardiac function in thyroid disease is well recognized and has been extensively investigated, vascular function has however been less well studied in those with thyroid dysfunction. Thyroid hormones, thyroxine (T(4)) and triiodothyronine (T(3)) are important regulators of cardiac function and cardiovascular hemodynamics. The cardiovascular system responds to minimal but persistent changes in circulating thyroid hormone levels producing changes in vascular reactivity and endothelial function. The detection of endothelial dysfunction and/or arterial stiffness allows early identification of individuals at risk as these occur in both patients with risk factors for coronary artery disease and in those with established disease. This may allow treatment to be targeted at high risk individuals with the aim of slowing the progression of vascular disease. The various methods used to assess arterial function are reviewed and the changes demonstrated in human and animal models of thyroid dysfunction.

Comparison of muscarinic receptor- and beta-adrenoceptor-mediated vasorelaxation between euthyroid and acute hyperthyroid rats

Hyperthyroidism was induced by subcutaneous injections of L-thyroxine (T4) (0.5 mg/kg/day) for 3 days in order to investigate the effects of acute hyperthyroidism on the vasorelaxing responses to isoprenaline and acetylcholine in isolated rat aortae. In the aortae, there was no significant difference in isoprenaline-induced relaxation between hyperthyroid and control rats, however acetylcholine-induced relaxation was significantly greater in hyperthyroid rats than in control rats. N(G)-nitro-L-arginine (L-NOARG), an inhibitor of nitric oxide (NO) synthase, reduced isoprenaline- and acetylcholine-induced relaxations in both hyperthyroid and control rats and in the presence of L-NOARG no significant difference in the acetylcholine-induced relaxation was seen between the two groups of rats. Indomethacin, a cyclo-oxygenase inhibitor, had no significant influence on both isoprenaline- and acetylcholine-induced relaxations in both control and hyperthyroid rats. 17-Octadecynoic acid (17-ODYA), a cytochrome P-450 mono-oxygenase inhibitor, reduced the both isoprenaline- and acetylcholine-induced relaxation in both hyperthyroid and control rats, and acetylcholine-induced relaxation was still greater in hyperthyroid rats than in control rats. These results indicate that an acute hyperthyroidism significantly enhances muscarinic receptor- but not adrenoceptor-mediated relaxations of the aortae and L-NOARG abolished an enhancement by acute hyperthyroidism of muscarinic receptor-mediated relaxation, suggesting that the effects may be due to an alteration in muscarinic receptor-mediated NO systems of the aortae at early stage of hyperthyroidism.

Acute hyperthyroidism alters adrenoceptor- and muscarinic receptor-mediated responses in isolated rat renal and femoral arteries

The effects of acute hyperthyroidism on the vasorelaxing responses to isoprenaline and acetylcholine were investigated in isolated rat renal and femoral arteries. In the renal artery, isoprenaline- and acetylcholine-induced relaxations were significantly greater in hyperthyroid rats than in control rats. In the femoral artery, only the acetylcholine-induced relaxation was significantly greater in hyperthyroid rats than in control rats. In the renal artery, NG-nitro-L-arginine (L-NOARG), an inhibitor of nitric oxide (NO) synthase, reduced isoprenaline- and acetylcholine-induced relaxations in both hyperthyroid and control rats and the isoprenaline-induced relaxation was still greater in hyperthyroid rats than in control rats, but no difference in the acetylcholine-induced relaxation was seen between the two groups of rats since L-NOARG almost abolished the acetylcholine-induced relaxation. In the femoral artery, L-NOAGR reduced the isoprenaline-induced relaxation in control rats but not in hyperthyroid rats, while it almost abolished the acetylcholine-induced relaxation in both groups of rats. 17-Octadecynoic acid (17-ODYA), a cytochrome P-450 monooxygenase inhibitor, reduced the isoprenaline-induced relaxation in renal and femoral arteries from hyperthyroid and control rats, but it did not change the acetylcholine-induced relaxation in both arteries. These results indicate that acute hyperthyroidism significantly enhances beta-adrenoceptor-mediated relaxation of the renal artery and muscarinic receptor-mediated relaxation of both renal and femoral arteries, suggesting that these effects may be due to an alteration in the NO and cytochrome P-450 systems of the artery.

Genomic and non-genomic regulation of L-type calcium channels in rat ventricle by thyroid hormone

Hyperthyroidism is associated with low exercise tolerance despite high cardiac output and sometimes with the development of heart failure. L-type calcium channels may play a role in the mechanism, but this has not been fully understood. We examined the effects of thyroid hormone on gene expression and function of L-type calcium channels in rat ventricles by the ribonuclease protection assay and whole-cell patch-clamp technique, respectively. The effects of bisoprolol, beta-blocking agent, on the regulation of calcium channel by thyroid hormone was also studied. In hyperthyroid animals, the mRNA of the calcium channel alpha1c subunit was reduced on day 4, compared with that in euthyroid animals, and remained low on day 8. Bisoprolol did not affect the thyroid hormone mediated decrease in alpha1c subunit mRNA. While L-type calcium current was greater in hyperthyroid than euthyroid myocytes on day 4, it was smaller on day 8. In addition, the isoproterenol-induced increase in calcium current in euthyroid rats was attenuated in hyperthyroid rats. Acetylcholine decreased calcium current in hyperthyroid myocytes, but not in euthyroid myocytes. In conclusion, L-type calcium current was increased by thyroid hormone in rat ventricular myocytes by the activation of the adenylate cyclase cascade, despite a decreased calcium channel gene expression. These genomic and non-genomic modifications may play an important role in the association of high cardiac output with low exercise tolerance, and in the development of heart failure in hyperthyroidism.

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.

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.

Electrophysiological mechanisms by which hypothyroidism delays repolarization in guinea pig hearts

Thyroid hormone is known to exert important effects on cardiac repolarization, but the underlying mechanisms are poorly understood. We investigated the electrophysiological mechanisms of differences in repolarization between control guinea pigs and hypothyroid animals (thyroidectomy plus 5-propyl-2-thiouracil). Hypothyroidism significantly prolonged the rate-corrected Q-T interval in vivo and action potential duration (APD) of isolated ventricular myocytes. Whole cell voltage-clamp studies showed no change in current density or kinetics of L-type Ca(2+) current, inward rectifier K(+) current, or Na(+) current in hypothyroid hearts. Dofetilide-resistant current (I(Ks)) step current densities were smaller by approximately 65%, and tail current densities were reduced by 80% in myocytes from hypothyroid animals compared with controls. The ratio of delayed rectifier step current at +50 mV to tail current at -40 mV was significantly larger in hypothyroid cells for test pulses from 60- to 4,200-ms duration, reflecting a smaller I(Ks). Dofetilide-sensitive current (I(Kr)) densities were not significantly changed. I(Ks) half-activation voltage shifted to more positive voltages in hypothyroidism (29.5 +/- 2.2 vs. 21.3 +/- 2.7 mV in control, P < 0.01), whereas I(Kr) voltage dependence was unchanged. We conclude that hypothyroidism delays repolarization in the guinea pig ventricle by decreasing I(Ks), a novel and potentially important mechanism for thyroid regulation of cardiac electrophysiology.

Effects of oral propylthiouracil treatment on nitric oxide production in rat aorta

The effects of oral propylthiouracil (PTU) treatment on vascular nitric oxide (NO) production were studied in the rat aorta. Rats were fed a standard low fat diet with or without 0.1% PTU, for 2 or 4 weeks, or for 2 weeks with additional thyroxine injections. Concentration response curves were then constructed to phenylephrine (PE) in both endothelium-intact and denuded aortic rings from these animals and after incubation with 0.1 mM L-N(G)nitroarginine (L-NOARG). In addition, expression of nitric oxide synthase (NOS) was analysed in sections of aorta from PTU-treated and control rats using rabbit polyclonal antibodies to both inducible NOS (iNOS) and endothelial NOS (eNOS). Oral PTU treatment resulted in a significant reduction in both the maximum response (control, 0.53+/-0.02; 2 week PTU, 0.20+/-0.07; 4 week PTU, 0.07+/-0.02 g mg(-1)) and vessel sensitivity (EC50 values: control, 9.10x10(-8)+/-0.67; 2 week PTU, 7.45x10(-7)+/-1.15; 4 week PTU, 9.73x10(-7)+/-0.45 M) to PE in endothelium-intact vessel rings, as compared to controls (P<0.05). Both endothelial removal and incubation with L-NOARG restored the maximum response after 2, but not 4 weeks, although, in general, vessel sensitivity was not altered by either treatment. Vessels from PTU-treated rats given thyroxine injections showed no significant differences between any of the dose response curve parameters. Immunohistochemical analysis suggested that labelling for eNOS may be increased after PTU treatment as compared to control animals, whereas iNOS antibody immunoreactivity was not different between the two groups. These results suggest that the hyporesponsiveness to PE observed after oral PTU treatment is, in part, due to enhanced nitric oxide (NO) production by the endothelium, and demonstrate for the first time that thyroid hormones may play a role in the regulation of eNOS activity in the rat aorta.

Antioxidant-sensitive shortening of ventricular action potential in hyperthyroid rats is independent of lipid peroxidation

The effects of substances able to reduce peroxidative processes on thyroid hormone-induced electrophysiological changes in ventricular muscle fibres were examined. For this study, 60 day old euthyroid and hyperthyroid rats were used. One group of hyperthyroid rats was untreated and the others were treated with vitamin E, N-acetylcysteine, and cholesterol, respectively. Hyperthyroidism was elicited by 10 day treatment with daily i.p. injections of triiodothyronine (10 microg/100 g body weight). Vitamin E and N-acetylcysteine were administered for 10 days by daily i.m. injections (20 mg/100 g body weight) and daily i.p. injections (100 mg/100 g body weight), respectively. Cholesterol was administered by cholesterol-supplemented diet (4%) from day 30. Hyperthyroidism induced a decrease in the whole antioxidant capacity and an increase in both lipid peroxidation and susceptibility to oxidative stress. Vitamin E and N-acetylcysteine administration to hyperthyroid rats led to reduction in lipid peroxidation and susceptibility to oxidative stress and to increase in antioxidant level, while the diet addition of cholesterol decreased lipid peroxidation but did not modify the other parameters. The hyperthyroid state was also associated with a decrease in the duration of the ventricular action potential recorded in vitro. The vitamin E and N-acetylcysteine administration attenuated the thyroid hormone-induced changes in action potential duration, which was however, significantly different from that of the euthyroid rats. In contrast, cholesterol supplementation did not modify the electrical activity of hyperthyroid heart. These results demonstrate that the triiodothyronine effects on ventricular electrophysiological properties are mediated, at least in part, through a membrane modification involving a free radical mechanism. Moreover, they indicate that the antioxidant-sensitive shortening of action potential duration induced by thyroid hormone is likely independent of enhanced peroxidative processes in sarcolemmal membrane.

Thyroid status affects the rat cardiac beta-adrenoceptor system transiently and time-dependently

1. The aim of this study was to investigate the time-dependency of the influence of dysthyroid states on the beta-adrenoceptor system in rat heart left ventricle. Therefore, the influence of acute and chronic hyper- and hypothyroidism on beta-adrenoceptor-induced left ventricular responses, beta-adrenoceptor density, cardiac noradrenaline tissue concentrations, Gs alpha-proteins, and basal and stimulated adenylate cyclase activities was determined. 2. Hyperthyroid rats were obtained by feeding with thyroxine (T4)-containing rat-chow for 1, 4 and 8 weeks. Hypothyroidism was induced by adding 0.05% propylthiouracil (PTU) to the drinking water. Rats of varying ages were used in order to compensate for the differences in the duration of the treatments. Rats were aged 3 and 5 months at the end of the experiments. 3. Thyroxine treatment for 4 and 8 weeks increased the cardiac sensitivity to isoprenaline, but maximal induced inotropic responses were decreased. Cardiac ventricular beta-adrenoceptor density was increased only in rats treated with T4 for 1 week. This transient effect of hyperthyroidism on cardiac beta-adrenoceptor density was not observed in older rats. The PTU treatment resulted in a stable decrease of cardiac beta-adrenoceptor density. 4. Left ventricular tissue noradrenaline concentrations were unaffected by hyperthyroidism, where a decrease was observed in hypothyroid rats. Density of Gs alpha proteins was increased in hearts from chronic hyperthyroid rats. 5. These results indicate that the increased sensitivity to beta-adrenoceptor-mediated stimulation in chronic hyperthyroidism cannot be attributed to changes in cardiac beta-adrenoceptor density, but is probably caused by an enhanced content of Gs alpha. Accordingly, in hyperthyroidism, the beta-adrenoceptor system is influenced time-dependently, whereas hypothyroidism affects the beta-adrenoceptor system independent of time.

Increased responses to adenosine in isolated left atria from streptozotocin-diabetic rats: evidence for the involvement of hypothyroidism

Direct and indirect (antiadrenergic) inotropic effects of adenosine in the isolated left atrial preparations from streptozotocin-induced diabetic rats were examined by comparing with those from propylthiouracil-induced hypothyroid rats. Experimental diabetes was induced by a single i.v. injection of streptozotocin (45 mg/kg). Subsets of diabetic rats were treated daily with either insulin (6-8 units/kg) or triiodothyronine (T3; 8-10 micrograms/kg). After 10 weeks, negative inotropic and antiadrenergic effects of adenosine were assessed in the atria from nondiabetic, diabetic, insulin- or T3-treated diabetic and hypothyroid rats. Diabetic rats exhibited a significant increase in negative inotropic and antiadrenergic effects of adenosine, as well as signs of hypothyroidism such as decreased thyroid hormone levels. The same changes in the adenosine effects also were noticed in the hypothyroid rats. In our study, the influence of T3 treatment on the hyperreactivity of diabetic rat atria to adenosine also was examined by comparing with that of insulin treatment. Both insulin and T3 treatments restored the hyperreactivity to adenosine, with the exception of adenosine receptor affinity (pD2 value) calculated for the negative inotropic effect of adenosine. These findings suggest that thyroid hormone deficiency is likely to be involved in the inotropic and antiadrenergic effects of adenosine on the left atria from rats with experimental diabetes for 10 weeks.

Thyroid hormone modulates inotropic responses, alpha-adrenoceptor density and catecholamine concentrations in the rat heart

We investigated the influence of hyper- and hypothyroidism on basal parameters of isolated perfused hearts of rats. In addition the effects of different extracellular calcium concentrations ([Ca2+]o), the calcium entry promoter Bay K8644 and the alpha 1-adrenoceptor agonist methoxamine were investigated. Since alterations in alpha-adrenoceptor density could explain the increased sensitivity to methoxamine in hearts from hypothyroid rats, alpha 1-adrenoceptor density in the left ventricle was also established. Different time-schedules of exposure to hyper- and hypothyroidism were used to investigate whether the influence of chronic dysthyroid states on alpha 1-adrenoceptor density is transient and time-dependent. Simultaneously myocardial noradrenaline and adrenaline tissue concentrations were determined, since they might correlate with the observed changes. Hyperthyroidism was induced by feeding rats for 1, 4 and 8 weeks with 5 mg/kg L-thyroxine (T4)-containing rat chow. Hypothyroid rats were obtained by adding 0.05% propylthiouracil (PTU) to the drinking water during 1, 4 and 8 weeks. For the functional experiments animals were treated during 4 weeks, to mimic the clinical situation of a chronic endocrine disease. Langendorff hearts from hyperthyroid hearts showed an increased maximally developed relaxation velocity, whereas Langendorff hearts from hypothyroid rats showed an increased left ventricular pressure (LVP). We observed an increased maximal inotropic response to [Ca2+]o in hearts from both hyperthyroid and hypothyroid rats, indicating that both dysthyroid states interfere with the handling of calcium ions by the contractile apparatus. Unchanged responses to Bay K8644 in hearts from hyperthyroid and depressed responses in hearts from hypothyroid rats suggest that the involvement of L-type calcium channels is rather unlikely. Furthermore, the reflex increase in coronary flow in response to enhanced contractile force appeared to fail in hearts from hypothyroid rats. Sensitivity of the response to methoxamine was increased in hearts from hypothyroid rats, which was accompanied by a decrease in the number of myocardial alpha 1-adrenoceptors. Both T4 and PTU treatment resulted in a non-transient decrease of alpha 1-adrenoceptor density in left ventricular tissue. Furthermore, hypothyroidism increased the percentage of alpha 1A-binding sites, whereas in hyperthyroidism the distribution of the alpha 1-adrenoceptor subtypes was not affected. Myocardial tissue concentrations of noradrenaline and adrenaline were unchanged in hyperthyroid rats and decreased in hypothyroid rats. The present study indicates that thyroid hormones have a direct rather than a sympathetically mediated effect on alpha 1-adrenoceptor mediated myocardial functions.

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.

Effect of an homologous series of aliphatic alcohols on neuronal and smooth muscle voltage-dependent Ca2+ channels

The acute inhibitory actions of alcohol on K(+)-stimulated 45Ca2+ uptake into synaptosomes shows regional variation in sensitivity throughout the brain, suggesting the possibility of a selective action on a specific Ca2+ channel subtype. This was examined by comparing the effects of a homologous series of aliphatic alcohols on synaptosomal Ca2+ channels with their actions on K(+)-stimulated Ca2+ channels in guinea-pig intestinal longitudinal muscle, which have been demonstrated to be of the L-type. K(+)-stimulated contraction of and [3H]nitrendipine binding to smooth muscle were both inhibited by the alcohols at similar concentrations, with the potency increasing with chain length. In synaptosomes, however, K(+)-stimulated 45Ca2+ uptake was 5-30 times more sensitive to the inhibitory actions of alcohol than were [3H]nitrendipine and [125I]omega-conotoxin binding. These observations suggest that K(+)-stimulated 45Ca2+ uptake is mediated by a non-L non-N type channel which is more sensitive to the acute effects of alcohols. This is supported by the observation that K(+)-stimulated 45Ca2+ uptake which is insensitive to L- and N-channel antagonists was inhibited by funnel web spider venom.

Suitability of CGP-12177 and CGP-26505 for quantitative imaging of beta-adrenoceptors

[3H]CGP-12177, a non-selective beta-adrenoceptor antagonist, and [3H]CGP-26505, a beta 1-selective beta-adrenoceptor antagonist, were intravenously administered to rats. 94-97% of the injected radioactivity disappeared from plasma with t1/2 0.2 and 0.5 min. Total/non-specific binding ratios of 5.4 and 6.9 (CGP-12177) or 2.0 and 2.8 (CGP-26505) were maintained in heart and lung from 10 to 40 min post-injection. Labelled plasma metabolites appeared after greater than 20 min (CGP-12177) or within 2 min (CGP-26505). No metabolites were found in the heart. CGP-12177 binds to blood cells, but CGP-26505 does not. CGP-12177 can be used for PET imaging of total (beta 1 and beta 2) adrenoceptors in the heart and lung of experimental animals, but CGP-26505 is less suitable for in vivo analysis of the beta 1-subpopulation.

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.

Steroid hormones as mediators of neural plasticity

Steroid and thyroid hormone receptors are expressed in the developing brain and persist throughout adult life. They mediate a variety of effects on the brain, ranging from developmental effects of thyroid hormone and the process of sexual differentiation to the cyclic changes during reproductive cycles in adult female animals. This review summarizes data from the author's laboratory on three topics: (1) actions of extradiol and progesterone on the ventromedial nucleus of the hypothalamus in adult female and male rats, showing both the cyclicity and the consequences of brain sexual differentiation; (2) actions of estradiol on the cholinergic neurons of the basal forebrain of the female and male rat, reflecting the plasticity of the adult cholinergic system as well as sex differences which are developmentally programmed; and (3) diverse actions of estrogens, thyroid hormone and glucocorticoids on the morphology of hippocampal neurons. The review concludes by discussing the interactions between "organizational" (i.e. developmental) effects and the "activational" effects of steroids on the mature nervous system in relation to the environmental control of brain gene expression.

Acute effects of thyroid hormone on sodium currents in neonatal myocytes

Sodium currents and action potentials were recorded from myocytes of neonatal rats during acute exposure to thyroid hormone (5-20 nM). One to 5 minutes after addition of thyroid hormone to the bath, decay from peak Na current was slowed, with the fractional current flowing 20 ms after onset (relative to peak current) increasing from 6 +/- 5% to 17 +/- 13% (p less than 0.01, n = 12). Action potential durations were increased from 55 +/- 14 to 86 +/- 36 msec (p less than 0.05, n = 6). The effects of thyroid hormone were partially reversed by lidocaine (60 microM, n = 5), a specific blocker of a slow sub-population of Na channels. Thus thyroid hormone interacts directly with myocyte membrane, probably by slowing of inactivation of Na channels.

The influence of thyroid states upon responses of the rat aorta to catecholamines

1. Mechanical responses to various stimulants and the distribution of adrenoceptor subtypes were investigated in smooth muscle cells of the rat aorta in eu- (control), hyper- and hypo-thyroid states. 2. Concentration-response relationships for K showed that the KCl EC50 value was slightly higher (28.4 mM K) in hypothyroid than in euthyroid and hyperthyroid states (22.5 mM and 22.8 mM K, respectively). The order of maximum amplitudes of KCl contraction was control greater than hypothyroid greater than hyperthyroid. 3. The EC50 values for the noradrenaline (NA)-induced contraction for eu-, hypo- and hyper-thyroid states were 7.4, 12.5 and 5.5 nM, respectively. The density of alpha 1-adrenoceptors was increased to 1.4 times the control in the hypothyroid state and reduced to 0.26 times the control in the hyperthyroid state. 4. The EC50 values for the phenylephrine-induced contraction in eu-, hypo- and hyper-thyroid states were 43.4, 247 and 41.5 nM, respectively, and those for clonidine 104, 1360 and 56.5 nM, respectively. The IC50 values for prazosin against 0.1 microM NA-induced contractions in eu-, hypo- and hyper-thyroid states were 5.8, 3.5 and 0.7 nM, respectively, and those for yohimbine 8.9, 5.4 and 4.1 microM, respectively. 5. The IC50 values for isoprenaline against phenylephrine-induced contractions were 63 nM and 2.4 microM in hyper- and eu-thyroid states, respectively, and the corresponding IC25 values for eu- and hypo-thyroid states were 72 and 423 nM, respectively. The density of beta-adrenoceptors was increased to 3.91 times the control in the hyperthyroid and reduced to 0.68 times the control in the hypothyroid states. 6. NA consistently increased the amount of inositol 1,4,5-trisphosphate (1P3) within 30s in the three thyroid states. In the hypothyroid state, the amount of IP3 was slightly increased and in the hyperthyroid condition, it occurred to a lesser extent than in the euthyroid state. 7. The results indicate that in hypothyroidism, the density of alpha,-adrenoceptors is increased and that of ,Beta-adrenoceptors reduced in vascular beds. The latter but not the former is accompanied by corresponding mechanical responses. In hyperthyroidism, the density of alpha 1-adrenoceptors is decreased and beta-adrenoceptors increased with parallel changes in mechanical responses.

Effect of hypo- and hyperthyroidism on binding of [3H]-nitrendipine to myocardial and brain membranes

The density of calcium channel binding sites as determined by [3H]-nitrendipine binding was found to be decreased in hearts of hyperthyroid rats but hardly altered by hypothyroidism. In contrast, dihydropyridine binding sites in the cerebral cortex were unaffected by dysthyroid states. Myocardial [3H]-nitrendipine binding sites were also decreased after treatment of the animals with isoprenaline but not in spontaneously hypertensive rats. These findings suggest that myocardial hypertrophy is not necessarily accompanied by a loss of calcium channels and that thyroid hormone regulates the density of [3H]-nitrendipine binding sites in a tissue-specific manner.