The effects of altered thyroid state upon responses mediated by atrial muscarinic receptors in the rat

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 N(6)-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.

Special sensitization pattern in adenosine-induced myocardial responses after thyroxine-treatment

Chronic thyroxine treatment reduces the susceptibility of atrial myocardium to adenosine. While the possible role of membrane adenosine receptors in this action is supported by several studies, the involvement of intracellular adenosine mechanisms has not been defined. The present experiments were carried out in electrically driven euthyroid and hyperthyroid guinea pig atrial myocardium. The extracellular and intracellular actions of adenosine were analyzed pharmacologically by the use of specific blockers of membrane adenosine transport and intracellular adenosine deaminase (ADA). The involvement of phosphoprotein phosphatase, phospholamban, and sarcoplasmic reticulum Ca2+ ATPase (SERCA) in the adenosine-induced responses was also studied. The major findings were as follows: i) pD(2)- and E(max)-values for adenosine-induced decrease of mechanical activity were significantly reduced after an 8-day thyroxine treatment in atrial tissues; ii) in atria of thyroxine-treated animals, membrane purine transport inhibitors (dipyridamole, NBTI) induced similar leftward shifts in concentration-response curves for adenosine in both euthyroid and hyperthyroid atrial myocardium without altering the depressed E(max) values; iii) the leftward displacement evoked by inhibitors of intracellularly located ADA (coformycin, EHNA) was more striking in hyperthyroid than euthyroid myocardia. ADA inhibitors induced a complete reversal of the maximum adenosine actions; iv) inhibition by cantharidin of phosphoprotein phosphatases (after inhibition of ADA) reduced the adenosine-induced responses. This inhibition was stronger in hyperthyroid atria; v) pharmacological elimination of sarcoplasmic reticulum Ca2+ ATPase by cyclopiazonic acid did not alter the cardiac responses to adenosine and this was independent of thyroid status. It is suggested that distinct modulation of the extra- and intracellular adenosine actions is present in eu- and hyperthyroid hearts. In the latter, a predominance of intracellular adenosine mechanisms can be proposed.

Basal muscarinic activity does not impede beta-adrenergic activation in rabbit hearts in controls or thyroxine-induced cardiac hypertrophy

We tested the hypothesis that basal myocardial muscarinic receptor activity acts as a "brake" on beta-adrenergic activation and that this effect would be greater in hearts subjected to thyroxine (T4)-induced (0.5 mg/kg for 16 days) hypertrophy due to an increase in muscarinic receptor density. Twenty control and 20 T4-treated open-chest anesthetized New Zealand white rabbits were given isoproterenol (0.5 microg/kg/min, 10 min i.v.) and/or atropine (3 mg/kg bolus). Coronary blood flow (radioactive microspheres), aortic and left ventricular (LV) pressure, and wall thickening of the LV free wall were recorded. Hearts were quickly excised and stored in liquid nitrogen. Cyclic guanosine monophosphate (GMP) and cyclic adenosine monophosphate (AMP) were determined by radioimmunoassay. T4 increased heart weight/body weight ratio, blood pressures, and the first derivative of the maximal rate of increase of LV systolic pressure (dP/dt[max]). Isoproterenol increased heart rate in both groups. Atropine had no effects on hemodynamic parameters either alone or after stimulation with isoproterenol. At this dose, atropine completely blocked the depressant effects of acetylcholine (10 microg/kg). Isoproterenol increased the maximal time derivative of wall thickening (dWT/dt[max]) in control (from 11.0 +/- 1.0 to 16.4 +/- 1.5 mm/s) but not in T4 animals. T4 increased subepicardial (EPI) and subendocardial (ENDO) coronary blood flow. Isoproterenol increased coronary flow (control: EPI, from 173 +/- 11 to 346 +/- 28 ml/min/100 g; ENDO, from 197 +/- 15 to 364 +/- 30 ml/min/100 g; T4: EPI, from 314 +/- 45 to 459 +/- 43 ml/min/100 g; ENDO, from 339 +/- 48 to 458 +/- 43 ml/min/100 g). Cyclic AMP levels were higher in T4 animals. Isoproterenol increased cyclic AMP (control: EPI, from 540 +/- 82 pmol/g to 1,096 +/- 110; ENDO, 596 +/- 58 to 1,050 +/- 145 pmol/g; T4: EPI, from 882 +/- 107 pmol/g to 1,319 +/- 222; ENDO, from 954 +/- 134 to 1 ,409 +/- 261 pmol/g). Atropine, alone or after stimulation with isoproterenol, had no effect on coronary flow or cyclic AMP in either group. Cyclic GMP levels were unaffected by T4-induced hypertrophy or by any of the treatments in either group. Thus it appears that basal muscarinic activity does not significantly influence function or signal transduction either at baseline or during beta-adrenergic stimulation in controls or in T4-induced hypertrophy.

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.

Classification of the β-adrenoceptor subtype in the rat portal vein: effect of altered thyroid hormone levels

The potencies of the beta 1-adrenoceptor agonist, noradrenaline, and the beta 2-adrenoceptor agonist, fenoterol, at beta-adrenoceptors in portal vein were examined using preparations isolated from control, methimazole-treated or l-thyroxine-treated rats. Tissues were preincubated with phenoxybenzamine (1 mumol/l) to block alpha-adrenoceptors and neuronal and extraneuronal uptake. Fenoterol was approximately 400 times more potent than noradrenaline (-log IC50 7.85 vs. 5.26) in inhibiting the spontaneous contractions of the portal vein. The beta 2-adrenoceptor antagonist, ICI 118,551, was approximately 3000 fold more potent than the beta 1-adrenoceptor antagonist, atenolol, in blocking the effects of fenoterol (pA2 9.32 and 5.88 respectively) and 400 times more potent in antagonising noradrenaline (pA2 8.96 vs. 6.23). Treatment of rats with methimazole led to decreased myogenic tone, and treatment with thyroxine to increased tone. beta-Adrenoceptor binding densities and the relative potencies of the agonists and antagonists used were unaffected by methimazole treatment. Thyroxine administration was also without effect on the relative potencies of these compounds. Our data indicate that although the portal vein is a target tissue for thyroxine, as indicated by its influence on myogenic tone, the beta-adrenoceptor population in this preparation, confirmed to be of the beta 2-subtype, is unaffected.

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.

Induction of experimental thyroid dysfunction in rats with implantable pellets of thyroxine or propylthiouracil

Subcutaneously implanted pellets containing the thyroid hormone thyroxine or the thyrotoxic agent propylthiouracil were used to induce hyper- or hypothyroidism in rats. The results obtained were compared to those produced by daily subcutaneous injection of these substances. The thyroxine pellets caused substantial elevation of serum thyroxine concentrations for at least 25 days, whereas the propylthiouracil pellets caused a pronounced decrease of serum thyroxine concentrations. Changes in heart weight and rectal temperature were consistent with the observed alterations of serum thyroxine concentrations. Treatment with propylthiouracil was associated with small elevations of serum total protein, urea nitrogen, and creatine concentrations regardless of the method of administration of this agent. It is concluded that implantable pellets are an effective and convenient means of administering drugs for producing thyroid dysfunction in rats.