The influence of hormonal and neuronal factors on rat heart adrenoceptors

Specific inhibition of mitochondrial oxidative stress suppresses inflammation and improves cardiac function in a rat pneumonia-related sepsis model

Using a mitochondria-targeted vitamin E (Mito-Vit-E) in a rat pneumonia-related sepsis model, we examined the role of mitochondrial reactive oxygen species in sepsis-mediated myocardial inflammation and subsequent cardiac contractile dysfunction. Sepsis was produced in adult male Sprague-Dawley rats via intratracheal injection of S. pneumonia (4 × 10(6) colony formation units per rat). A single dose of Mito-Vit-E, vitamin E, or control vehicle, at 21.5 μmol/kg, was administered 30 min postinoculation. Blood was collected, and heart tissue was harvested at various time points. Mito-Vit-E in vivo distribution was confirmed by mass spectrometry. In cardiac mitochondria, Mito-Vit-E improved total antioxidant capacity and suppressed H(2)O(2) generation, whereas vitamin E offered little effect. In cytosol, both antioxidants decreased H(2)O(2) levels, but only vitamin E strengthened antioxidant capacity. Mito-Vit-E protected mitochondrial structure and function in the heart during sepsis, demonstrated by reduction in lipid and protein oxidation, preservation of mitochondrial membrane integrity, and recovery of respiratory function. While both Mito-Vit-E and vitamin E suppressed sepsis-induced peripheral and myocardial production of proinflammatory cytokines (tumor necrosis factor-α, interleukin-1β, and interleukin-6), Mito-Vit-E exhibited significantly higher efficacy (P < 0.05). Stronger anti-inflammatory action of Mito-Vit-E was further shown by its near-complete inhibition of sepsis-induced myeloperoxidase accumulation in myocardium, suggesting its effect on neutrophil infiltration. Echocardiography analysis indicated that Mito-Vit-E ameliorated cardiac contractility of sepsis animals, shown by improved fractional shortening and ejection fraction. Together, our data suggest that targeted scavenging of mitochondrial reactive oxygen species protects mitochondrial function, attenuates tissue-level inflammation, and improves whole organ activities in the heart during sepsis.

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.

A fluctuation in adrenocepter- and muscarinic receptor-mediated blood pressure responses in acute hyperthyroid rats

Hyperthyroidism was induced by daily subcutaneous injections of L-thyroxine (T(4), 0.5 mg/kg/day) for 3 days, 1 week, or 2 weeks to study whether there is a fluctuation in adrenoceptor- and muscarinic receptor-mediated blood pressure responses at a hyperthyroid stage. T(4) treatment for 3 days or 1 week significantly suppressed the pressor response induced by norepinephrine (NE). The depressor responses induced by isoprenaline or acetylcholine (ACh) were increased by T(4) treatment for only 3 days. The pressor response induced by N(G)-nitro-L-arginine (L-NA) was increased by T(4) treatment for only 3 days. Results suggest that adrenoceptor- and muscarinic receptor-mediated blood pressure responses fluctuate in hyperthyroidism caused by T(4) in rats, that the basal nitric oxide (NO) production and/or release are increased in hyperthyroid rats at an early stage of the disease.

Changes in vascular reactivity induced by acute hyperthyroidism in isolated rat aortae

Hyperthyroidism was induced by subcutaneous injections of L-thyroxine (T(4)) (500 mg/kg/day) for 3 days in order to study whether adrenergic and muscarinic receptor-mediated vascular responses alter at an early stage of the disease. T(4) treatment was sufficient to induce a significant degree of thyroid weight loss, tachycardia, cardiac hypertrophy, and an elevation in serum T(4) levels. The tension of aortic ring preparations isolated from rats was measured isometrically to investigate the influence of acute hyperthyroidism. The contractions induced by norepinephrine (NE) were significantly suppressed in aortic rings from rats treated with T(4) compared with control rats. N(G)-nitro-L-arginine (L-NOARG), an inhibitor of nitric oxide synthase (NOS), significantly enhanced NE-induced contraction in aortic rings from both control and T(4)-treated rats, and the enhancement was greater in rats treated with T(4) than control rats. The relaxations induced by either acetylcholine (ACh) or sodium nitroprusside (SNP) were also significantly enhanced by T(4) treatment. L-NOARG abolished the relaxation induced by ACh in aortic rings from both control and T(4)-treated rats. L-NOARG shifted SNP-induced relaxation curves of aortic rings from those of control rats to the left, but not with rats treated with T(4). T(4) treatment showed no influence on the amount of endothelial NOS (eNOS) protein. These results suggest that vascular responses alter at an early stage of hyperthyroidism and that it may be due to a modification in the NO system which is independent from the amount of eNOS protein.

Cardiovascular α1-adrenoceptor subtypes: functions and signaling

α1-Adrenoceptors (α1AR) are G protein-coupled receptors and include α1A, α1B, and α1D subtypes corresponding to cloned α1a, α1b, and α1d, respectively. α1AR mediate several cardiovascular actions of sympathomimetic amines such as vasoconstriction and cardiac inotropy, hypertrophy, metabolism, and remodeling. α1AR subtypes are products of separate genes and differ in structure, G protein-coupling, tissue distribution, signaling, regulation, and functions. Both α1AAR and α1BAR mediate positive inotropic responses. On the other hand, cardiac hypertrophy is primarily mediated by α1AAR. The only demonstrated major function of α1DAR is vasoconstriction. α1AR are coupled to phospholipase C, phospholipase D, and phospholipase A2; they increase intracellular Ca2+ and myofibrillar sensitivity to Ca2+ and cause translocation of specific phosphokinase C isoforms to the particulate fraction. Cardiac hypertrophic responses to α1AR agonists might involve activation of phosphokinase C and mitogen-activated protein kinase via Gq. α1AR subtypes might interact with each other and with other receptors and signaling mechanisms.Key words: cardiac hypertrophy, inotropic responses, central α1-adrenoreceptors, arrythmias.

Alterations of cardiac alpha 1-adrenoceptor subtypes in hypothyroid rats

1. Alterations in the cardiac alpha 1-adrenoceptor and its subtypes in hypothyroid rats were studied by radioligand binding assays and reverse transcription-polymerase chain reaction (RT-PCR). Hypothyroidism was created by feeding rats with 0.2% 2-thiouracil solution instead of daily drinking water for 20 days. 2. The density of cardiac alpha 1-adrenoceptors (Bmax) was increased from 67.5 +/- 4.3 fmol/mg in control rats to 81.1 +/- 7.2 fmol/mg (P < 0.05) in hypothyroid rats. 3. Compared with control rats, in hypothyroid rats the percentages of high-affinity sites for (+)-niguldipine and 5-methylurapidil were increased from 13.8 +/- 5.6 and 31.9 +/- 6.3%, respectively, to 24.9 +/- 7.3 and 45.5 +/- 2.4%, respectively (both P < 0.05), while those for BMY7378 were decreased from 37.2 +/- 8.9 to 23.8 +/- 8.4% (P < 0.05), respectively. The percentage of high-affinity sites for WB4101 was not significantly different in control and hypothyroid rats (43.3 +/- 9.1 and 39.4 +/- 3.6%, respectively). 4. Reverse transcription-PCR experiments revealed that the steady state levels of mRNA for alpha 1A- and alpha 1B-adrenoceptors were increased, while those for alpha 1D-adrenoceptor were decreased in the hearts of hypothyroid rats. 5. The concentration-contraction response curves for noradrenaline in the presence of a beta-adrenoceptor antagonist in control and hypothyroid rats showed that the maximal response was reduced from 344 +/- 58 to 200 +/- 23 mg, respectively (P < 0.05). 6. The data suggest that in hypothyroid rats the total number of cardiac alpha 1-adrenoceptors is increased. The change is subtype-selective, with levels of alpha 1A- and alpha 1B-adrenoceptors being increased and levels of alpha 1D-adrenoceptors being reduced. Furthermore, the positive inotropic response mediated by alpha 1-adrenoceptors is reduced in hypothyroid rats.

The influence of experimental hyperthyroidism on responsiveness in rabbit aortic smooth muscle

This study investigated the responsiveness of hyperthyroid and control (euthyroid) rabbit aortic rings to vasoconstrictor agents. There was no significant difference between the responsiveness of hyperthyroid and control tissues to KCl. Maximum responses (E(m)) to noradrenaline and serotonin were reduced in hyperthyroid rabbit aortic rings compared to those euthyroid rings. The sensitivity (i.e., EC(50)) of hyperthyroid aortic rings to noradrenaline and serotonin compared to euthyroid controls was unchanged. The K(b) value of prazosin against 10(-6) M noradrenaline-induced contractions in eu- and hyperthyroid states was 1.97 ± 0.59 · 10(-9) M and 1.92 ± 0.79 · 10(-9) M, respectively. The K(b) value of methisergide against 10(-6) M serotonin-induced contractions in eu- and hyperthyroid states was 1.46 ± 0.80 · 10(-9) M and 1.10 ± 0.16 · 10(-9) M, respectively. These data indicate that vascular reactivity is altered in experimental hyperthyroidism and this alteration, if any, do not occur at the receptor level.

Mutations in the c-erbA beta 1 gene: do they underlie euthyroid fibromyalgia?

Fibromyalgia, a chronic condition of widespread pain, stiffness, and fatigue, has proven unresponsive to drugs, the use of which is based on the 'serotonin-deficiency hypothesis'. An alternative hypothesis-failed transcription regulation by thyroid hormone-can explain the serotonin deficiency and other objective findings and symptoms of euthyroid fibromyalgia. Virtually every feature of fibromyalgia corresponds to signs or symptoms associated with failed transcription regulation by thyroid hormone. In hypothyroid fibromyalgia, failed transcription regulation would result from thyroid-hormone deficiency. In euthyroid fibromyalgia, failed transcription regulation may result from low-affinity thyroid hormone receptors coded by a mutated c-erbA beta 1 gene, yielding partial peripheral resistance to thyroid hormone. The hypothesis of this paper is that, in euthyroid fibromyalgia, a mutant c-erbA beta 1 gene (or alternately, the c-erbA alpha 1 gene) results in low-affinity thyroid-hormone receptors that prevent normal thyroid hormone regulation of transcription. As in hypothyroidism, this would cause a shift toward alpha-adrenergic dominance and increases in both cyclic adenosine 3'-5'-phosphate phosphodiesterase and inhibitory Gi proteins. The result would be tissue-specific hypothyroid-like symptoms despite normal circulating thyroid-hormone levels.

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.

The influence of hyperthyroidism on pharmacologically induced contractions of isolated resistance arteries

We investigated the effect of hyperthyroidism on the responses of small mesenteric resistance arteries to several contractile and dilator agents. Hyperthyroidism was established by feeding rats for 28 days with 5 mg/kg L-thyroxine-containing rat chow. This treatment produced a stable hyperthyroid state, as indicated by the increased serum T4 levels (236 +/- 7 vs. 60 +/- 2; T4-treated vs. control). Preparations of small mesenteric arteries were mounted in an isometric wire myograph. Subsequently, concentration-effect curves were determined for K+, Ca2+, methoxamine, phenylephrine, 5-hydroxytryptamine (5-HT), 9,11-dideoxy-11 alpha, 9 alpha-epoxymethano-prostaglandin F2 alpha (U46619), methacholine and nitroprusside. Our results indicate that hyperthyroidism does not induce major changes in the sensitivity of isolated resistance vessels to K+, Ca+, the alpha-adrenoceptor agonist, methacholine and nitroprusside. Furthermore, neither the affinity of alpha-receptors for methoxamine, nor the alpha-receptor reserve was influenced by the hyperthyroid state of the animal. A clearly sensitizing influence of hyperthyroidism was found for the vasoconstrictor effects of both 5-HT (6.57 +/- 0.04 vs. 6.29 +/- 0.06; hyperthyroid vs. control) and the thromboxane A2 receptor agonist U46619 (6.78 +/- 0.13 vs. 6.30 +/- 0.09; hyperthyroid vs. control). Sensitization to both 5-HT and U46619 was abolished in the presence of N omega-nitro-L-arginine methylester HCl (L-NAME, 0.1 mM). 5-HT-induced contractions in vessels from hyperthyroid rats were diminished by prior incubation with indomethacin (10 microM). The present results indicate that during hyperthyroidism resistance vessels are sensitized to both 5-HT and U46619. This sensitization involves the nitric oxide/L-arginine pathway and probably also certain steps in the cyclooxygenase pathway.

Cardiovascular effects of 1.0, 1.5, and 2.0 minimum alveolar concentrations of isoflurane in experimentally induced hypothyroidism in dogs

This study was performed to determine the cardiovascular responses to isoflurane in euthyroid and hypothyroid dogs. Four healthy mixed-breed dogs were studied prior to thyroidectomy (PRE), 6 months after thyroidectomy (HYP), and after 2 months of oral supplementation with 1-thyroxine (SUP). Heart rate (HR), cardiac output (Q), stroke volume (SV), systolic, diastolic, mean arterial blood pressure (SAP, DAP, MAP), and total peripheral resistance (TPR) were determined in awake dogs and in the same dogs when end-tidal isoflurane concentration were 1.28%, 1.92%, and 2.56%. Ventilation was controlled in anesthetized dogs and PACO2 maintained between 38 to 42 mm Hg. Isoflurane caused significant (P < .05) dose-dependent reduction in Q, SV, SAP, DAP, and MAP in the PRE, HYP, and SUP dogs. Cardiac output was lower in the HYP dogs than in the PRE or SUP dogs during awake measurement. TPR was increased in the awake HYP dogs compared with the PRE or SUP dogs. During anesthesia, HYP dogs tended to have lower Q, SV, SAP, and MAP PRE or SUP groups, but the only significant reduction was SAP during 1.5 MAC. The cardiovascular responses to isoflurane in hypothyroid dogs are similar to euthyroid animals with a dose-dependent depression in Q, SV, and arterial pressure.

Alterations of alpha1-adrenoceptor subtypes in the hearts of thyroxine-treated rats

Alterations in the cardiac alpha1-adrenoceptor and its subtypes in thyroxine-treated rats were studied by means of radioligand binding assays, measurement of contractile response and reverse transcription-polymerase chain reaction (RT-PCR). The results showed that in thyroxine-treated rats the cardiac alpha1-adrenoceptor density (Bmax) was reduced from 51.6 +/- 6.0 fmol/mg in control to 40.9 +/- 3.7 fmol/mg (P<0.01); and the percentage of high affinity sites for 5-methyl-urapidil decreased from 23.3 +/- 2.0% in control to 10.8 +/- 2.0% in thyroxine-treated rats (P<0.05). The data indicated that the high-affinity sites for 5-methyl-urapidil (alpha1A-adrenoceptor) were reduced (from 12.0 to 4.4 fmol/mg), but the low-affinity sites for 5-methyl-urapidil (alpha1B- plus alpha1D-adrenoceptor) were not changed (from 39.6 to 36.5 fmol/mg). RT-PCR showed that steady-state levels of mRNA for alpha1A- and alpha1B-adrenoceptors were decreased, while that for alpha1D-adrenoceptor was raised in thyroxine-treated rats. In the isolated electrically driven left atria the phenylephrine-induced maximal contractions were reduced from 258 +/- 17 mg in control to 188 +/- 24 mg in thyroxine-treated rats (P<0.05). The pA2 values of 5-methyl-urapidil were reduced from 8.89 +/- 0.36 in control to the hyperthyroidism of 7.87 +/- 0.43 in thyroxine-treated rats (P<0.05). Chlorethylclonidine preincubation shifted concentration-response curves for phenylephrine to the right and reduced the maximal response to a lesser extent in thyroxine-treated rats than in control rats. Thus we concluded that the total number of cardiac alpha1-adrenoceptors is reduced in thyroxine-treated rats. The change is subtype selective, with alpha1A- and alpha1B-adrenoceptors being reduced in number and alpha1D-adrenoceptor being increased.

Effect of age on beta-receptors, Gs alpha- and Gi alpha- proteins in rat heart

beta-adrenoceptors, Gs alpha- and Gi alpha-proteins were investigated in a crude plasma membrane preparation from ventricles of young (2-4 months) and senescent (22-24 months) Wistar rats. Receptor density, ligand affinity and beta 1/beta 2-receptor ratio were independent of the age of the rats. The percentage of beta-receptors coupled to G-proteins increased with age. An age-related increase in the level of Gs alpha (124%) was paralleled by an increase in the ratio between the high and low molecular weight form of Gs alpha. The level of Gi alpha-protein almost doubled (170%) upon aging. We conclude that the age-related differences are small at the level of the beta-adrenoceptor molecule, but that the increase in Gi alpha-proteins could be responsible for the age-related reduction in myocardial inotropic and chronotropic responses. Moreover, we suggest that the changes in degree of high affinity coupling between beta-receptor and Gs-protein are possibly linked to alterations in the ratio between the Gs-molecular weight subtypes.

Modification of electrophysiological properties of rat heart with age

The authors have determined the serum thyroid hormone levels [total (TT3) and free (FT3) triiodothyronine], the heart weight/body weight ratio and the heart rate of differently aged male rats. The variations of these parameters show a modification of thyroid state as a function of ageing. The authors have also recorded, at about 26 degrees C, resting and action potentials from single cells of papillary muscles isolated from the same groups of rats. The animals in the higher thyroid state exhibited a repolarization speed higher than the other animals. The thyroidectomy, performed on 50 day old rats, and T3 treatment of the thyroidectomized rats give rise to modifications of repolarization speed and then of action potential duration analogous to ones obtained in previous study for animals thyroidectomized at 30 days of age. These data demonstrate that the modifications of heart electrophysiological properties with age, are due fundamentally to thyroid state modifications. The results suggest also that the cardiac chronotropism modifications which the rat undergoes as a function of ageing are due to the changes of levels of thyroid hormone which might exert its effect by modifying the ion channel kinetics as well as the cardiac receptors.

Adrenergic regulation of the skeletal alpha-actin gene promoter during myocardial cell hypertrophy

The skeletal alpha-actin gene is expressed in fetal rat heart and is induced during norepinephrine (NE)-stimulated hypertrophy in cultures of neonatal rat cardiac myocytes. Here we report that NE positively regulates the human skeletal alpha-actin gene promoter in transiently transfected neonatal rat cardiac myocytes. NE increased expression from the full-length promoter by 2.4-fold. A DNA region required for NE responsiveness but not for tissue-specific expression was located between base pair -2000 and base pair -1300. Distinct regions required for cardiac myocyte expression were located between -1300 to -710 and -153 to -87. None of these elements separately conferred tissue specificity or adrenergic responsiveness on a heterologous promoter, although the intact promoter from -2000 to -36 conferred both when cloned in its correct position and orientation. Additional elements in the basal promoter (-87 to +187) were required for maximal NE responsiveness. The NE induction was mediated by the beta-adrenergic receptor in high-density cultures (3-4 x 10(6) cells per 60-mm dish), as was induction of hypertrophy, contractility, and endogenous skeletal alpha-actin gene expression. The beta-adrenergic agonist isoproterenol was as potent as NE in inducing expression. Furthermore, beta-adrenergic antagonists inhibited the effects on skeletal alpha-actin gene expression but alpha 1-adrenergic antagonists did not. The alpha 1-adrenergic system was intact in these high-density cultures, since the effects of NE on the expression of another contractile protein gene, alpha-myosin heavy chain, were blocked by alpha 1- but not by beta-adrenergic antagonists. In these high-density cultures, cell contact and intermyocardiocyte bridging were prevalent. When cardiac myocytes were plated at a low density, minimizing cell contact, NE induction of skeletal alpha-actin gene expression and hypertrophy was mediated by the alpha 1-adrenoceptor. Factors related to cell communication may influence the pathways mediating NE-regulated gene transcription during cardiac myocyte hypertrophy.

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.

Decreased adrenergic sensitivity in patients with hypothyroidism

Cardiovascular sensitivity to catecholamines was assessed in 15 patients with hypothyroidism (mean [+/- SEM] thyroxine [T4] index 2.7 +/- 0.5 micrograms/100 ml, thyroid stimulating hormone [TSH] 136.9 +/- 48.3 microU/ml), aged 45 +/- 4 years and in 8 healthy control subjects. The study was repeated in 10 patients with hypothyroidism 4.0 +/- 0.5 months after thyroid replacement therapy (T4 index 9.9 +/- 2.1 micrograms/100 ml, TSH 3.5 +/- 1.3 microU/ml). In addition, basal, average and maximal heart rates were measured using 24 h ambulatory electrocardiographic (ECG) monitoring, and plasma levels of epinephrine and norepinephrine were determined before and after thyroid replacement. Heart rate increased less after bolus injection of 0.8, 1.6 and 3.2 micrograms of isoproterenol in the hypothyroid (10 +/- 2, 15 +/- 2 and 21 +/- 4 beats/min, respectively) than in the euthyroid (16 +/- 3, 22 +/- 3 and 30 +/- 4 beats/min, respectively) state (p less than 0.05). Control subjects reacted similarly to patients receiving thyroid replacement. Basal heart rate (64 +/- 3 versus 68 +/- 3 beats/min, p less than 0.05) and maximal heart rate (116 +/- 5 versus 133 +/- 5 beats/min, p less than 0.05) were lower on 24 h ambulatory ECG monitoring in the hypothyroid than euthyroid state despite higher basal plasma norepinephrine levels (394 +/- 45 versus 315 +/- 45 pg/ml, p less than 0.05). Thus, patients with hypothyroidism display a decreased cardiac chronotropic response to beta-adrenergic stimulation. This may contribute in part to the decreased basal and maximal daily heart rates seen in patients with hypothyroidism, which occurs despite elevated plasma norepinephrine levels.

Myocardial chronotropic and inotropic responsiveness in vitro after chronic alpha 1-adrenoreceptor blockade in the rat

1. Positive chronotropic and inotropic myocardial responses to different adrenoreceptor agonists were studied using isolated right atria and left ventricular papillary muscles from rats treated orally for 5 weeks with prazosin, an alpha 1-antagonist, or vehicle (distilled water). 2. Chronotropic responses to the beta-adrenoreceptor agonists isoprenaline and salbutamol were similar in atria from both groups of rats, with no differences in their basal rates, maximum rate increases or pD2 values for either agonist. 3. Basal contractile force of field-stimulated papillary muscles was similar in both prazosin-treated (0.089 +/- 0.014 g mg-1) and control groups (0.104 +/- 0.035 g mg-1). In response to noradrenaline, force increased maximally by 145 +/- 30% and 131 +/- 30% above resting levels respectively, and pD2 values for this beta- and alpha-agonist showed no changes after chronic prazosin treatment. Inotropic responses to isoprenaline were also not different with maximum increases in force of 94.5 +/- 20.2% for prazosin-treated and 84.5 +/- 18.5% for controls, and similar pD2 values. 4. However, in response to the alpha 1-agonist phenylephrine (in the presence of propranolol), maximum increases in force were greater in relation to the noradrenaline maxima after prazosin treatment (48.8 +/- 4.2%) than in controls (32.0 +/- 4.3%, P less than 0.02). pD2 values for phenylephrine were also significantly higher after long-term alpha 1-blockade (5.71 +/- 0.10 vs 5.30 +/- 0.19 for controls, P less than 0.05). 5. Long-term alpha 1-blockade in the rat therefore led to supersensitivity of alpha 1-mediated inotropism in the heart, but both beta-mediated inotropic and chronotropic responses were unaffected. These results show selectivity of action of chronic prazosin treatment on alpha 1-receptors in the rat heart.

The direct effect of the thyroid hormone on cardiac chronotropism

To establish whether thyroid hormone modifies the heart rate directly or through an action on other neuroendocrine modulators, the authors have examined several animals models differing in the plasma levels of such compounds. Induction of the hypothyroid state in rats produced a slow onset of bradycardia, which may be removed by a prolonged triiodothyronine treatment. The involvement of TSH was excluded as, by comparing thyroidectomized, hypophysectomized and cold exposed rats, the heart rate was found to vary according to the thyroid levels and not to the TSH levels. Moreover growth hormone, corticotropin and gonadotropins do not influence the heart rate, as the bradycardia induced by hypophysectomy was fully removed by triiodothyronine treatment. The lack of influence by ACTH and GnH was confirmed by treatment of thyroidectomized rats with corticosteroids or testosterone, respectively. Finally, thyroid hormone did not act on the heart rate by changing the norepinephrine output at the sympathetic nerve endings in the heart. In fact, thyroidectomy produced a more intense bradycardia than sympathectomy, and such bradycardia was equally removed by triiodothyronine treatment in thyroidectomized rats and in thyroidectomized and then sympathectomized ones. The authors suggest that the direct effect of the thyroid hormone on cardiac chronotropism is due to an early enhancement of beta-adrenoceptors, followed by a late modification of the electrophysiological properties of the myocardium.

Effect of thyroid replacement therapy on the frequency of benign atrial and ventricular arrhythmias

Whether thyroid replacement therapy can trigger cardiac arrhythmias in patients with hypothyroidism is not known. In this prospective study, 24 h ambulatory electrocardiographic (ECG) monitoring was used to assess the frequency of atrial and ventricular premature beats in 25 patients with hypothyroidism (5 men and 20 women, aged 56 +/- 3 years) before and 3.5 +/- 0.5 months (mean +/- SEM) after thyroid replacement therapy. Plasma thyroid-stimulating hormone was 73.6 +/- 12.3 and 3.1 +/- 0.6 microU/ml and free thyroxine index was 2.4 +/- 0.4 and 9.8 +/- 0.9 micrograms/100 ml at baseline and after thyroid replacement therapy, respectively. The frequency of ventricular premature beats was not affected by thyroid replacement therapy (from 273 +/- 221 at baseline to 352 +/- 235 beats/24 h after therapy), even in patients with frequent baseline arrhythmias. In contrast, the frequency of atrial premature beats was slightly increased after thyroid replacement therapy (from 47 +/- 17 to 279 +/- 197 beats/24 h), largely as a result of changes seen in three patients. No patient developed new onset of sustained ventricular or supraventricular arrhythmias. Average, basal and maximal heart rates during ECG monitoring increased significantly after thyroid replacement therapy (average 72 +/- 2 to 80 +/- 2; basal 64 +/- 2 to 70 +/- 2; maximal 114 +/- 3 to 130 +/- 3 beats/min, respectively, p less than 0.001). In conclusion, thyroid replacement therapy is safe in patients with common benign cardiac arrhythmias, and does not trigger an increase in arrhythmia frequency except in rare patients with baseline atrial premature beats. It is, however, associated with an increase in basal, average and maximal heart rates.

Impaired response of experimental diabetic mice to tricyclics: a possible beta-adrenergic mechanism

Diabetes is reportedly associated with alterations in peripheral and central noradrenergic systems. The latter might be involved in the antidepressant effects of imipramine-like drugs in both humans and animals. Therefore, it is possible that diabetics show an impaired responsiveness to tricyclics. To test this possibility the effects of streptozotocin (STZ)-induced experimental diabetes in mice were assessed in two psychopharmacological tests: 1) the reversal of apomorphine- (16 mg/kg) induced hypothermia and 2) the hypoactivity induced by a direct beta-agonist (clenbuterol 0.06 mg/kg). At day 15 after STZ or vehicle treatment, imipramine (4 mg/kg) antagonized the apomorphine-induced hypothermia in diabetic (D) and nondiabetic (ND) mice and clenbuterol produced hypoactivity in both groups. At day 30 and 45, the ability of imipramine (1, 2, 4, 8, 16 mg/kg), clomipramine (8 mg/kg) and desipramine (2 mg/kg) to reverse apomorphine-induced hypothermia disappeared at the same time that clenbuterol lost its ability to induce hypomotility in D mice. These impaired responses on both tests were corrected by a short period of insulin therapy. These two tests may reflect central beta-adrenergic functions. Therefore, these data suggest that the impaired responsiveness of diabetic mice might be due at least in part to a noradrenergic dysfunction. Possibly, in diabetes, a beta-adrenoceptor desensitization identical to that observed at the peripheral level occurs in the central nervous system. The possibility that a thyroid hormone deficiency may be involved was also tested. Decreased T3 plasma levels were found in D mice concomitant with the impaired pharmacological responses and T3 supplementation turned these responses to normal.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid hormone control of preganglionic innervation of the adrenal medulla and chromaffin cell development in the rat. An ultrastructural, morphometric and biochemical evaluation

In the rat, functional connections between the splanchnic nerve and the adrenal medulla are immature at birth and do not become fully competent until the first postnatal week. Neonatal administration of triiodothyronine (T3) accelerates this process, and the present study was undertaken to elucidate the underlying mechanisms. Rats were given T3 (0.1 mg/kg, s.c.) daily for 9 days beginning 1 day after birth. Preganglionic innervation of the adrenal medulla was examined by retrograde axonal transport of horseradish peroxidase (HRP). At 10 days of age, there was an increased number of labeled perikarya in the spinal cord of the hyperthyroid pups. Ultrastructural examination revealed a corresponding increase in synaptic density in the adrenal medulla and in the activity of choline acetyltransferase, a marker for preganglionic cholinergic nerve terminals. These effects were attenuated by 25 days of age, whereupon deficits in HRP-labeled neurons and adrenomedullary synapses were noted. Similarly, replication of chromaffin cells was enhanced transiently in the T3 group during the initial stage of hyperthyroidism, but subsequent long-lasting deficits in cell numbers were noted, along with a corresponding retardation of ontogeny of adrenal catecholamine biosynthesis and storage. Thus, neonatal hyperthyroidism accelerates synaptic development in the sympatho-adrenal axis but suppresses maturation of the target chromaffin cells, ultimately leading to impaired adrenomedullary function.

Alpha 1-adrenoceptors of rat myocardium: comparison of agonist binding and positive inotropic response

Binding of agonists to alpha 1-adrenoceptors labelled by 3H-prazosin was investigated in membranes of rat myocardium and compared to the inotropic response elicited by alpha 1-adrenoceptor activation on isolated right ventricles. 1. At 30 degrees C the full agonists, adrenaline and phenylephrine, displaced 3H-prazosin with a shallow inhibition curve. The data are compatible with the assumption that 32% of the binding sites were in a state of high affinity for the agonist adrenaline (KI 85 nmol/l) and 68% in a low affinity state (KI 1738 nmol/l). GTP transformed all binding sites into the low affinity form suggesting that at least some of the cardiac alpha 1-adrenoceptors are coupled to N-proteins. 2. At 0 degree C most of the binding sites (86%) were in a state of high affinity for agonists (KI for adrenaline: 91 nmol/l). 3. For several partial agonists and antagonists (cirazoline, methoxamine, indanidine (Sgd 101-75), oxymetazoline and phentolamine) no such distinct temperature- and GTP-shifts could be demonstrated suggesting a different kind of interaction with alpha 1-binding sites. 4. When temperature was changed during incubation with adrenaline, a rise of temperature (from 0 degrees C to 30 degrees C) converted high affinity sites into the low affinity form, whereas a decrease in temperature (from 30 degrees C to 0 degrees C) failed to induce the high affinity state for agonists. Short term incubation (0.5 min) with adrenaline at 30 degrees C resulted in significantly lower IC50 values as compared to equilibrium conditions at the same temperature.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha 2- and beta-adrenergic receptor binding and action in gluteal adipocytes from patients with hypothyroidism and hyperthyroidism

The alpha 2- and beta-adrenergic receptor activities have been investigated in human adipocytes in relation to thyroid status. Adipocytes from 11 hypothyroid and 18 hyperthyroid were compared with 19 euthyroid (normal) subjects. The lipolytic and cAMP responses to isoproterenol and epinephrine were greatly enhanced in adipocytes from hyperthyroid subjects (P less than .01) and impaired in adipocytes from hypothyroid subjects (P less than .05). However, the antilipolytic effect of clonidine (alpha 2-agonist) and the effect of clonidine on cAMP were similar in all three groups. The alpha 2- and beta-receptor numbers were both slightly increased in hyperthyroidism, but the ratio between the alpha 2- and beta-receptors was unchanged in relation to normal subjects. On the other hand, the adrenergic binding to adipocyte membranes from hypothyroid subjects was reduced, and the beta-receptor binding was reduced even much more (50%) than the alpha 2-receptor binding (20%, P less than .01). Thus, the alpha 2- and beta-receptor ratio was almost doubled in hypothyroid adipocytes (P less than .01). The antagonist affinity against the adrenergic receptors (determined by propranolol and yohimbine, respectively) was unchanged in the three groups. Agonist binding determined in intact adipocytes revealed unaltered affinity of clonidine for the alpha 2-receptor in the three groups. In hyperthyroidism, though, there was enhanced affinity of isoproterenol in competition with the beta-receptor (P less than .05). It is concluded that the inhibitory alpha 2-receptor pathway functions normally in all groups, indicating that the alpha 2-receptors and the alpha 2-receptor-mediated pathway in human adipocytes are relatively unaffected by thyroid hormones.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative studies with the enantiomers of the glycol metabolite of propranolol and their effects on the cardiac beta-adrenoceptor

The two enantiomers ((R)- and (S)-) of propranolol glycol, a metabolite of propranolol, have been synthesized, and their effects upon the beta-adrenoceptor studied by two methods. The ability of these compounds to antagonize the inotropic actions of isoprenaline was examined on spontaneously beating rat atrial preparations. Also, the effects of these enantiomers upon the binding of [3H]dihydroalprenolol to beta-receptors in rat cardiac ventricular membranes was studied. Experiments with the atria indicated that the (S)-glycol was a reversible competitive antagonist of isoprenaline with a potency approximately one thousand times lower than that of (+/-)-propranolol. In contrast, the (R)-glycol appeared to act as an irreversible antagonist, producing complex dose-response curves. The effects of these compounds to cause displacement of alprenolol binding were consistent with the organ bath data. The interaction of the (S)-glycol with the beta-receptor binding site was reversible (Ki of 27.6 +/- 4.2 microM) but less potent than that of (+/-)-propranolol (Ki of 0.99 +/- 0.07 nM). On the other hand, pretreatment of ventricular membranes with the (R)-glycol, followed by extensive washing techniques, resulted in alprenolol binding which did not regain control values, providing further evidence for an irreversible effect upon the beta-receptor. The possible significance of these pharmacological actions of the two enantiomers is discussed in terms of the in vivo metabolic pathways for propranolol.

Beta-adrenergic responsiveness and cardiac autonomic receptors after implantation of the MtTW15 pituitary adenoma in the rat

The effects of chronic MtTW15 pituitary adenoma implantation on beta-adrenergic responsiveness, cardiac beta-adrenoreceptors, and muscarinic receptors were studied in the rat. Five weeks after s.c. administration of tissue fragments of the MtTW15 adenoma, there was a 51 and 20% increase in the heart weight and body weight, respectively, and a 49-fold increase in the serum prolactin level as compared to the controls. At this time there was also an attenuation in the adenoma-bearing group of the ability of isoproterenol to produce a dipsogenic response and to increase the heart rate. In contrast, isoproterenol stimulated cardiac ornithine decarboxylase (ODC) activity 4.2-fold in both the control and adenoma-bearing groups. There was no change between the two groups in the cardiac ventricular beta-adrenoreceptor or muscarinic receptor concentration as measured by specific (-)-[125I]iodocyanopindolol (CYP) and (-)-[3H]quinuclidinyl benzilate (QNB) respectively. In addition, the concentrations of isoproterenol and carbachol required to inhibit by 50% (IC50) [125I]CYP and [3H]QNB binding, respectively, in the absence of 5'-guanylylimidodiphosphate (Gpp(NH)p) were not different between the two groups. In the presence of Gpp(NH)p, the isoproterenol IC50 value was not different between the two groups, whereas the carbachol IC50 value was increased slightly in the adenoma-bearing group. The data indicate that chronic MtTW15 adenoma implantation attenuated beta-mediated dipsogenic and heart rate responses but had little or no effect on cardiac ODC activity or cardiac autonomic receptor concentrations and agonist binding properties.

Regulation of alpha and beta adrenergic receptors by triiodothyronine in cultured rat myocardial cells

Previously, in this laboratory, we established the presence of alpha and beta adrenergic receptors in primary cultures of neonatal rat heart cells. We now report that exposure to 1-triiodothyronine (10 nM) regulates the binding characteristics of these receptors. As measured by [125I]-I-2-[beta-(4-hydroxyphenyl)ethylaminomethyl]tetralone ([125I]IBE 2254), alpha receptor number (Bmax) decreased by 50% from 37,000 +/- 4,500 to 18,000 +/- 4,300 sites per cell and the equilibrium dissociation constant (KD) decreased from 420 +/- 24 to 150 +/- 37 pM. As measured by [125I]-iodocyanopindolol ([125I]ICYP), beta receptor number increased by 42% from 12,000 +/- 2,600 to 17,000 +/- 4,000 sites per cell with no accompanying change in affinity. An increase in maximal stimulation of adenylate cyclase activity by isoproterenol was also detected under conditions of excess triiodothyronine. No significant changes in agonist or antagonist affinities for either the alpha or the beta adrenergic receptor were detected in thyroid hormone treated cultures. It can be concluded that in cultured myocardial cells thyroid hormone regulates the characteristics of both alpha and beta adrenergic receptors, but in a strikingly different manner.

6-Hydroxydopamine treatment increases beta-adrenoceptors and Ca2+ channels in rat heart

Rats received 6-hydroxydopamine (50 mg/kg i.v.) on two successive days. After 14 days cardiac beta-adrenoceptors and Ca2+ channels were measured by radioligand binding. Ca2+ channel binding [( 3H]nitrendipine) was increased 31% with no change in affinity, and beta-adrenoceptor binding [( 3H]dihydroalprenolol) was increased 28% with no change in affinity. Innervation may be important in the regulation of Ca2+ channel expression.

Catecholamine, vasoactive intestinal peptide and thyrotrophin-dependent cAMP levels display a different sensitivity to iodothyronines in both normal and pathological human thyroid cells in culture

As the interactions of iodothyronines on adrenergic and vipergic receptors are not clear, the effect of exogenous T3 and T4 on catecholamine- and VIP-induced cAMP accumulation in human normal thyroid cells after eight days of primary culture has been investigated. To evaluate the effect of endogenous iodothyronines, the response of the adenylate cyclase system to isoprenaline, adrenaline, VIP, and TSH was studied during a 10 d period. T3 and T4 were unable to modify the catecholamine- and VIP-induced cAMP accumulation in human normal thyroid cells after 6-8 days of culture, while the response to TSH was significantly inhibited. In cells cultured from thyrotoxic tissue, the response of the adenylate cyclase system to catecholamines and VIP, during a 10 d primary culture, showed a behaviour similar to controls. TSH responsiveness was negligible up to the fourth day of culture, while in normal cells a response to all the agonists was present from the beginning. In view of the lack of effect of iodothyronines on catecholamine- and VIP-induced cAMP accumulation, and of the superimposable behaviour of the response to catecholamines and VIP in normal and hyperthyroid cells during the first days of culture, we can conclude that iodothyronines do not directly modify the response of the adenylate cyclase system to adrenergic and vipergic stimulation in human thyroid follicular cells. The lack of responsiveness to TSH of cells obtained from hyperthyroid tissue during the first 4 d of culture, associated with normal responsiveness to catecholamines and VIP, points to a possible involvement of biogenic amines and neuropeptides in sustaining such hyperthyroid states.

Adrenergic receptors: possible mechanism of inverse regulation of alpha- and beta-receptors

Many physiologic and pathologic conditions, including bronchial asthma, are associated with inverse changes in alpha- and beta-receptor-mediated responses in various tissues. The direction of the change elicited by a given stimulus is tissue specific, as exemplified by the actions of thyroid hormones: In the rat heart, hypothyroidism reduces beta- and increases alpha-receptor responses, whereas in the rat liver it has the opposite effects. A similar increase in beta- and decrease in alpha-receptor responses in the rat liver is triggered by a number of different conditions, including glucocorticoid deficiency, that appear to represent lower levels of cellular differentiation. Among these, incubation of isolated hepatocytes in a serum-free buffer triggers the conversion of the receptor response in vitro within 4 hours, without parallel changes in the density or affinity of receptor binding sites. This change can be acutely reversed by an endogenous inhibitor of membrane phospholipase A2, or accelerated by an activator of phospholipase A2, suggesting that changes in the activity of this enzyme are involved in the conversion of the hepatic adrenoceptor response. The glucocorticoid-induced increase in beta-receptors in cultured human lung adenocarcinoma cells also appears to be mediated indirectly through the induction of an endogenous inhibitor (lipomodulin) of membrane phospholipase A2. The possible relevance of altered membrane phospholipid metabolism in the pathomechanism of asthma and in the associated glucocorticoid-sensitive changes in adrenergic receptor mechanisms is discussed.

Increased myocardial alpha 1 adrenoceptor density in rats chronically treated with propranolol

Chronic treatment with propranolol for 6 weeks resulted in a 16.6% increase, over the level in control rats, in myocardial alpha 1-adrenoceptor density as measured by [3H]prazosin binding. The apparent affinity remained unaffected. No changes were observed in beta-adrenoceptor density and apparent affinity as measured by [3H]dihydroalprenolol binding. It is concluded that myocardial alpha 1-adrenoceptors might have a compensatory role under conditions where beta-adrenergic effects are attenuated.

Thyroid-dependent alterations of myocardial adrenoceptors and adrenoceptor-mediated responses in the rat

Cardiovascular alterations in hypo- and hyperthyroidism have been ascribed to changes of noradrenergic neurotransmission. In the present study the influence of thyroid hormones on adrenoceptors in the rat heart was further characterized. The effect of artificial hypothyroidism (induced by feeding 6-propyl-2-thiouracil, PTU) and hyperthyroidism (induced by daily injections of triiodothyronine, T3) on myocardial adrenoceptor binding, catecholamines, some physiological responses, and their interdependence was examined. The density of myocardial beta-adrenergic binding sites (3H-dihydroalprenolol, 3H-DHA) was reduced after PTU (by 38%) and enhanced after T3 treatment (by up to 82%). The increase was dose- and time-dependent and reversible within 4 days. No changes of the affinity of 3H-DHA to its binding sites were observed. Only L-T3 and L-T4 proved to be active, D-T3 and reverse T3 had no effect. The rise in beta-adrenoceptor density caused by T3 was prevented by concomitant administration of cycloheximide, indicating its dependence on protein synthesis. The density of myocardial alpha 1-adrenergic binding sites (3H-prazosin) was significantly reduced in the PTU group (by up to 28%) and even more distinctly by T3 treatment (by up to 50%). KD values remained unaltered. The noradrenaline content and turnover of rat hearts was significantly reduced by T3-induced hyperthyroidism. PTU treatment had no influence on content and turnover of noradrenaline. Plasma noradrenaline as well as adrenaline levels in freely moving rats were increased by PTU treatment 9- and 5-fold, respectively. In T3-injected animals no significant changes were measured. The density of adrenoceptors is known to be inversely correlated with catecholamine levels in several organs. Neither alpha- nor beta-adrenoceptor changes in the myocardium of dysthyroid rats could be attributed to such a homologous regulation, since they still occurred after chemical sympathectomy with 6-hydroxydopamine and adrenalectomy.(ABSTRACT TRUNCATED AT 400 WORDS)

Time-dependent conversion of alpha 1- to beta-adrenoceptor-mediated glycogenolysis in isolated rat liver cells: role of membrane phospholipase A2

Incubation of isolated rat liver cells in a serum-free buffer leads to the reduction of the glycogenolytic effect of phenylephrine and the simultaneous emergence of a glycogenolytic response to isoproterenol within 4 hr. This conversion of the adrenergic activation of phosphorylase from an alpha 1- to a beta-adrenoceptor-mediated response is associated with no change in the glycogenolytic response to the calcium-linked activator vasopressin, and a reduction of the glycogenolytic response to the cAMP-linked activator glucagon. In vitro incubation of hepatocytes does not influence the density of affinity of [3H]prazosin-labeled alpha 1-receptors and [3H]CGP-12177-labeled beta-receptors. In cells preincubated for 4 hr, a further 30-min incubation with 50 nM lipomodulin, an endogenous inhibitor of membrane phospholipase A2 (EC 3.1.1.4), reverses the adrenergic activation of phosphorylase from a beta- to an alpha 1-receptor-mediated event, whereas in freshly isolated cells lipomodulin does not affect the predominant alpha-receptor response. Conversely, exposure of freshly isolated cells to a monoclonal antibody to lipomodulin in the presence of 10 microM phenylephrine, or to melittin, an activator of phospholipase A2, at 2 micrograms/ml, results in the suppression of the effect of phenylephrine and the emergence of a response to isoproterenol within 30 min. It is proposed that coupling of hepatic alpha 1- and beta-adrenoceptors to postreceptor pathways is regulated in an inverse reciprocal manner by changes in membrane phospholipase A2 activity.

Seasonal variation in responses of the toad renal vasculature to adrenaline

Adrenaline and noradrenaline produced both constriction and dilatation of the toad renal vasculature: constrictor effects were mediated by alpha-adrenoceptors; dilator effects were mediated by beta-adrenoceptors. Vasoconstriction was the predominant response to these amines. Dilatation was only revealed after blockade of alpha-adrenoceptors and constriction of the vasculature. There was a marked seasonal variation in the constrictor responses to adrenaline, but not to noradrenaline. The maximal increase in renal vascular resistance produced by adrenaline in summer was greater, by a factor of three, than the maximum constrictor response in winter. The response to adrenaline in summer was also significantly greater than the responses to noradrenaline in both summer and winter. However, after treatment with propranolol there was no difference between the maximum vasoconstrictions to these two amines, in summer or in winter. Determination of dissociation constants for phentolamine indicated that adrenaline and noradrenaline acted on the same population of alpha-adrenoceptors in both summer and winter. The enhanced vasoconstriction to adrenaline in summer appears to be due to a reduced potency of adrenaline on renal vascular beta-adrenoceptors. The reduction in potency may be caused by a subtle configurational change in the beta-adrenoceptors, perhaps induced by hormonal changes associated with the onset of breeding.

Adrenoreceptor-mediated responses in the isolated portal vein of the hypothyroid rat

1 The effect of hypothyroidism, induced by methimazole, upon the response of the isolated portal vein of the rat to adrenoceptor agonists and antagonists, to angiotensin II and to papaverine has been investigated. 2 The positions and maxima of log doses-response curves, to the vasoconstrictor agonists, noradrenaline and angiotensin II, and to the vasodilator agonists isoprenaline and papaverine, were unaffected by methimazole treatment. 3 The alpha-adrenoreceptor antagonist phentolamine competitively inhibited the effects of noradrenaline to a similar extent in preparations from control and hypothyroid animals. The competitive antagonism of isoprenaline by the beta-adrenoreceptor antagonist propranolol was similarly unaffected by hypothyroidism. 4 These results taken together indicate that hypothyroidism is without significant effect upon either the properties of postjunctional alpha- and beta-adrenoreceptors in the rat portal vein, or upon the reactivity of this blood vessel to the vasoactive agonists studied.

Temperature dependence of [3H]dihydroalprenolol binding in rat myocardium

The beta-adrenergic antagonist, [3H]dihydroalprenolol, was used to label binding sites in crude rat myocardial plasma membranes. The specificity of binding was dependent on the temperature of the assay. Specific binding at 22 and 37 degree C and at concentrations of radioligand less than 5 nM was consistent with binding to the myocardial beta-receptor. Binding sites labeled at 4 degree C possessed quite different properties. Binding was non-stereoselective and of lower affinity. Agonist compounds were much less effective at competing for the labeled myocardial sites at 4 degree C than at 22 degree C. Those beta-antagonists which additionally possess pharmacological "quinidine-like' activity (e.g. propranolol, alprenolol) were potent competitors at 4 degree C, but competition was non-stereoselective. In contrast atenolol, a beta-antagonist devoid of "quinidine-like' activity was ineffective at 4 degree C. Furthermore, procaine, and quinidine itself were potent competitors of [3H]dihydroalprenolol binding at 4 degree C. Thus the specificity of [3H]dihydroalprenolol binding to rat heart membranes at 4 degree C appears to be directed non-stereoselectively at that portion of the competing molecule which recognized "quinidine-like' as opposed to adrenergic activity.