Hyperthyroid Atria Research Articles

The effect of adenosine deaminase inhibition on the A1 adenosinergic and M2 muscarinergic control of contractility in eu- and hyperthyroid guinea pig atria.

The A1 adenosine and M2 muscarinic receptors exert protective (including energy consumption limiting) effects in the heart. We investigated the influence of adenosine deaminase (ADA) inhibition on a representative energy consumption limiting function, the direct negative inotropic effect elicited by the A1 adenosinergic and M2 muscarinergic systems, in eu- and hyperthyroid atria. Furthermore, we compared the change in the interstitial adenosine level caused by ADA inhibition and nucleoside transport blockade, two well-established processes to stimulate the cell surface A1 adenosine receptors, in both thyroid states. A classical isolated organ technique was applied supplemented with the receptorial responsiveness method (RRM), a concentration estimating procedure. Via measuring the contractile force, the direct negative inotropic capacity of N6-cyclopentyladenosine, a selective A1 receptor agonist, and methacholine, a muscarinic receptor agonist, was determined on the left atria isolated from 8-day solvent- and thyroxine-treated guinea pigs in the presence and absence of 2′-deoxycoformycin, a selective ADA inhibitor, and NBTI, a selective nucleoside transporter inhibitor. We found that ADA inhibition (but not nucleoside transport blockade) increased the signal amplification of the A1 adenosinergic (but not M2 muscarinergic) system. This action of ADA inhibition developed in both thyroid states, but it was greater in hyperthyroidism. Nevertheless, ADA inhibition produced a smaller rise in the interstitial adenosine concentration than nucleoside transport blockade did in both thyroid states. Our results indicate that ADA inhibition, besides increasing the interstitial adenosine level, intensifies the atrial A1 adenosinergic function in another (thyroid hormone-sensitive) way, suggesting a new mechanism of action of ADA inhibition.

Thyroid hormones decrease the affinity of 8-cyclopentyl-1,3-dipropylxanthine (CPX), a competitive antagonist, for the guinea pig atrial A1 adenosine receptor

The aim of the present study was to investigate whether or not thyroxine (T(4)) treatment affects K(B), the equilibrium dissociation constant of the antagonist-receptor complex, for the interaction between CPX, a selective and competitive orthosteric antagonist, and the guinea pig atrial A1 adenosine receptor A1 receptor). The inotropic response to adenosine, a nonselective adenosine receptor agonist, or CPA, a selective A1 receptor agonist, was investigated in the absence or presence of CPX in paced left atria isolated from 8-day solvent- or T(4)-treated guinea pigs. To obtain K(B) values, adenosine and CPA concentration-response curves were evaluated by Schild analysis. CPA but not adenosine obeyed the requirements of the Schild analysis to provide correct K(B) values for CPX. According to the CPA concentration-response curves, affinity of CPX for the hyperthyroid guinea pig atrial A1 receptor (K(B) = 44.16 nM) was lower than that for the euthyroid one (K(B) = 16.63 nM). Regarding the intense reduction in the negative inotropic effect of adenosine and CPA in hyperthyroid atria, it is reasonable to assume that the moderate decrease in affinity of the guinea pig atrial A1 receptor is only in part responsible for the diminished A1 receptor-mediated effect in hyperthyroidism.

Adenosine deaminase inhibition enhances the inotropic response mediated by A 1 adenosine receptor in hyperthyroid guinea pig atrium

The aim of the present study was to test the hypothesis that inhibition of adenosine deaminase (ADA) enhances the efficiency of signal-transduction of myocardial A 1 adenosine receptors in hyperthyroidism. The inotropic response to N 6-cyclopentyladenosine (CPA), a selective A 1 adenosine receptor agonist resistant to ADA, was investigated in the absence or presence of erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), an ADA and cGMP-stimulated 3′,5′-cyclic nucleotide phosphodiesterase (PDE2) inhibitor, or of pentostatin (2′-deoxycoformycin; DCF), an exclusive ADA inhibitor, in left atria isolated from eu- or hyperthyroid guinea pigs. Both ADA inhibitors enhanced the effect of CPA only in hyperthyroid atria. EHNA significantly increased the E max (mean ± S.E.M.) from 83.8 ± 1.2% to 93.4 ± 1.2%, while DCF significantly decreased the log EC 50 from −7.5 ± 0.07 to −7.83 ± 0.07 in hyperthyroid samples. Conversely, EHNA also diminished the log EC 50 (from −7.5 ± 0.07 to −7.65 ± 0.07) and DCF also raised the E max (from 83.8 ± 1.2% to 85.7 ± 2%) in hyperthyroidism, but these changes were not significant. In conclusion, ADA inhibition moderately but significantly enhanced the efficiency of A 1 adenosine receptor signaling pathway in the hyperthyroid guinea pig atrium. This suggests that elevated intracellular adenosine level caused by ADA inhibition may improve the suppressed responsiveness to A 1 adenosine receptor agonists associated with the hyperthyroid state. Alternatively or in addition, the role of decreased concentration of adenosine degradation products cannot be excluded. Furthermore, in the case of EHNA, inhibition of PDE2 also appears to contribute to the enhanced A 1 adenosine receptor signaling in the hyperthyroid guinea pig atrium.

Altered role of C-type natriuretic peptide-activated pGC–cGMP–PDE3–cAMP signaling in hyperthyroid beating rabbit atria

The role of C-type natriuretic peptide (CNP) in the pathophysiology of atrial function in hyperthyroidism has not been defined. This study was to define the role of CNP-activated particulate (p) guanylyl cyclase (GC)-cGMP-phosphodiesterase (PDE)3 signaling in the regulation of cAMP levels and contractile and secretory functions in the atria from hyperthyroid rabbits. Experiments were performed in perfused beating rabbit atria. CNP was used to activate pGC. In euthyroid atria from sham-treated rabbits, CNP (100 nM) increased cGMP and cAMP efflux by 176.7 ± 17.7 and 55.3 ± 10.0%, respectively. CNP decreased stroke volume and pulse pressure and ANP release by 51 ± 7 and 41 ± 2 and 60.4 ± 3.2%, respectively. Pretreatment with milrinone blocked the CNP-induced increase of cAMP but without significant changes in decrease of atrial dynamics and ANP release. In hyperthyroid atria, CNP-induced increase of cGMP levels was accentuated, while CNP-induced increase of cAMP was attenuated. The gain of cAMP, i.e., change in cAMP efflux concentration in terms of cGMP was attenuated in the hyperthyroid compared to euthyroid atria. CNP rather increased atrial dynamics in hyperthyroid atria instead of decrease. CNP-induced decrease in atrial ANP release was attenuated. Pretreatment with milrinone blocked the CNP-induced increase of cAMP levels concomitantly with a decrease of atrial dynamics. The present study demonstrates that altered role of CNP-activated pGC–cGMP–PDE3–cAMP signaling is involved in the pathophysiology of hyperthyroid heart.

Influence of hyperthyroidism on the effect of adenosine transport blockade assessed by a novel method in guinea pig atria

The aim of the present study was to investigate the effect of hyperthyroidism on the trans-sarcolemmal adenosine (Ado) flux via equilibrative and nitrobenzylthioinosine (NBTI)-sensitive nucleoside transporters (ENT1) in guinea pig atria, by assessing the change in the Ado concentration of the interstitial fluid ([Ado]ISF) under nucleoside transport blockade with NBTI. For the assessment, we applied our novel method, which estimates the change in [Ado]ISF utilizing the altered inotropic response to N6-cyclopentyladenosine (CPA), a relative stable selective agonist of A1 Ado receptors, by providing a relative index, the equivalent concentration of CPA. Our results show an interstitial Ado accumulation upon ENT1 blockade, which was more extensive in the hyperthyroid samples (CPA concentrations equieffective with the surplus [Ado]ISF were two to three times higher in hyperthyroid atria than in euthyroid ones, with regard to the negative inotropic effect of CPA and Ado). This suggests an enhanced Ado influx via ENT1 in hyperthyroid atria. It is concluded that hyperthyroidism does not alter the prevailing direction of the Ado transport, moreover intensifies the Ado influx in the guinea pig atrium.

Thyroid hormones increase the contractility but suppress the effects of beta-adrenergic agonist by decreasing phospholamban expression in rat atria.

The aim of the present study was to characterize the relationships between the thyroid-hormone-dependent changes in sarcoplasmic reticulum (SR) Ca2+ handling and contractile performance in atria. Hypothyroidism in rats was induced by adding 0.05% 6-n-propyl-2-thiouracil to their drinking water for 6 weeks. Hyperthyroidism was induced by daily subcutaneous injections of L-thyroxine (1 microgram/g body weight) to euthyroid rats for 1 week. Left atria from the hearts with different thyroid states were examined by means of contractile measurements, SR oxalate-supported Ca(2+)-uptake, and Western blot of SR proteins. The tissue level of SR Ca(2+)-pump protein decreased in hypothyroid (46 +/- 6%) atria, but remained unchanged in hyperthyroid (110 +/- 8%) atria as compared with euthyroid atria. Hypothyroidism was associated with increased phospholamban expression (141 +/- 25%), whereas it was drastically downregulated under hyperthyroidism (21 +/- 4%). The rate of SR Ca(2+)-uptake, measured in the presence of the protein kinase A inhibitor, H-89, was higher in hyperthyroid atria and lower in hypothyroid atria than in euthyroid atria (397 +/- 40, 55 +/- 6 and 194 +/- 17 nmol Ca2+/g protein/min, respectively). However, the stimulation of SR Ca(2+)-uptake by the catalytic subunit of protein kinase A was relatively weaker in hyperthyroid (130 +/- 20% over control level without catalytic subunit) and stronger in hypothyroid (640 +/- 60%) than in euthyroid atria (280 +/- 40%). The rates of inotropic contraction (+dT/dt) were higher in the hyperthyroid atria (133 +/- 10 mN/s), but lower in hypothyroid atria (15 +/- 3 mN/s) than in their euthyroid counterparts (95 +/- 13 mN/s). Inversely, hypothyroid atria responded to isoproterenol with much larger increases in contractility (883 +/- 164% over the control values for the same muscle before addition of isoproterenol) and hyperthyroid with smaller increases (25 +/- 9%) than euthyroid preparations (207 +/- 17%) Thyroid hormones increase the contractility, but decrease the inotropic response to isoproterenol through decreasing the phospholamban/SR Ca(2+)-pump ratio in rat atria.

Purinergic control of release of somatostatin in the rat stomach

The aim of the present study was to test the hypothesis that inhibition of adenosine deaminase (ADA) enhances the efficiency of signal-transduction of myocardial A1 adenosine receptors in hyperthyroidism.The inotropic response to N6-cyclopentyladenosine (CPA), a selective A1 adenosine receptor agonist resistant to ADA, was investigated in the absence or presence of erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), an ADA and cGMP-stimulated 3′,5′-cyclic nucleotide phosphodiesterase (PDE2) inhibitor, or of pentostatin (2′-deoxycoformycin; DCF), an exclusive ADA inhibitor, in left atria isolated from eu- or hyperthyroid guinea pigs.Both ADA inhibitors enhanced the effect of CPA only in hyperthyroid atria. EHNA significantly increased the Emax (mean ± S.E.M.) from 83.8 ± 1.2% to 93.4 ± 1.2%, while DCF significantly decreased the log EC50 from −7.5 ± 0.07 to −7.83 ± 0.07 in hyperthyroid samples. Conversely, EHNA also diminished the log EC50 (from −7.5 ± 0.07 to −7.65 ± 0.07) and DCF also raised the Emax (from 83.8 ± 1.2% to 85.7 ± 2%) in hyperthyroidism, but these changes were not significant.In conclusion, ADA inhibition moderately but significantly enhanced the efficiency of A1 adenosine receptor signaling pathway in the hyperthyroid guinea pig atrium. This suggests that elevated intracellular adenosine level caused by ADA inhibition may improve the suppressed responsiveness to A1 adenosine receptor agonists associated with the hyperthyroid state. Alternatively or in addition, the role of decreased concentration of adenosine degradation products cannot be excluded. Furthermore, in the case of EHNA, inhibition of PDE2 also appears to contribute to the enhanced A1 adenosine receptor signaling in the hyperthyroid guinea pig atrium.

Enhanced Negative Inotropic Effect of an Adenosine A 1-Receptor Agonist in Rat Left Atria in Hypothyroidism

Left atria were isolated from rats made hypothyroid by adding propylthiouracil to their drinking water, such rats after saturating doses of thyroid hormones, and from control rats. Isoproterenol (ISO: 1 μM) increased the values of developed tension (DT), maximal rate of tension development (+dT/d t) and tension fall (-dT/d t). The effect was largest in hypothyroid and lowest in hyperthyroid atria. The adenosine A 1-receptor agonist N 6-(phenylisopropyl)-adenosine (PIA) has a powerful negative inotropic effect in ISO-stimulated atria. The effects of PIA on +dT/d t. -dT/d t and DT were enhanced in hypothyroidism. Adenosine receptor number was not decreased. The amount of total G i-like proteins was estimated by pertussis toxin labeling. The amounts of G i2 and G i3 were estimated in Western blots such antisera raised in rabbits against peptides corresponding to parts of their sequences, using purified recombinant α subunits as standards. The amounts of low and high molecular weight forms of G s were estimated by cholera toxin labeling. G i2, G i3 and pertussis toxin substrate concentrations were slightly lower in the hypothyroid animals, while the amounts of both forms of G s per mg of protein were only half of those in euthyroid rat atria. The levels of G i2 and G i3 were greatly elevated as compared to G s as membrane marker. These changes were reversed by treatment of the hypothyroid rats with thyroid hormones. In conclusion, the present results show an enhanced negative inotropic effect of an adenosine A 1-receptor agonist in rat left atria in hypothyroidism, reflected both in sensitivity and responsiveness, and suggest that this is not caused by a change in receptor number but by a relative in G i proteins.

The effect of altered thyroid state on atrial intracellular potentials.

1. A group of rabbits was made hypothyroid by thyroidectomy, and another group was injected daily with L-thyroxine. After an appropriate interval respective alterations in thyroid state were confirmed by measurement of heart weight and of plasma iodine, and the animals' atria were isolated for recording.2. Measurements were made of atrial contractions, conduction velocity, spontaneous heart rate and maximum driven frequency, and action potentials were recorded with intracellular micro-electrodes.3. The resting potential and action potential heights were not affected by differences of thyroid state.4. Atrial arrhythmias are common in hyperthyroidism, rare in myxoedema. The possibility that hypothyroidism might reduce the rate of rise of the action potential, as do anti-arrhythmic drugs, and hyperthyroidism increase it, was investigated. Although the rate of rise was slower in hypothyroid atria at some driving frequencies, this could not alone account for an anti-arrhythmic effect, because at frequencies near the spontaneous heart rate the rate of rise of the action potential was not reduced.5. The duration of the repolarization phase of the action potential was greatly prolonged in atria from thyroidectomized rabbits, and was shortened in hyperthyroid atria. These changes could account for a reduced probability of arrhythmias in hypothyroidism, and the converse in hyperthyroidism.