In-vivo quantification of myocardial Na-K pump rate during β-adrenergic stimulation of intact pig hearts

Maintenance of adequate electrical activity of the heart depends critically on the ability of the Na-K pump to compensate for normal passive sodium and potassium fluxes. Using sudden injections of [³H]ouabain into the left coronary artery in anaesthetized open-chest pigs, we monitored transient changes in myocardial potassium balance by PVC-valinomycin mini-electrodes. When related to the number of pumps blocked and fractional inhibition, these data provided estimates of total Na-K pump capacity as well as actual pump rate and perturbations of the Na-K balance. Experiments were performed in hearts with and without intracoronary isoprenaline infusion (2.5 nmol min^-1). After injection of 120 nmol [³H]ouabain into the left coronary artery, myocardial [³H]ouabain concentrations were 118 (74–178) and 103 (76–145) pmol g^-1 and total concentrations of [³H]oubain binding sites were 893 (752–1076) and 785 (691–877) pmol g^-1 (median, 95% confidence interval) in isoprenaline-treated and control hearts respectively (differences not significant). The [³H]ouabain injection caused a net potassium release of 81 (56–132) and 43 (23–75) μcool 100 g^-1 (median, 95% confidence interval) in isoprenaline-treated and control hearts respectively (n= 6–8; significance of difference, P= 0.03). Na–K pump rate estimated from mono-exponential release curves was 6363 (3942–10,858) K⁺ ions min^-1 site^-1 during β-adrenoceptor stimulation and 2514 (1380–4322) in control (significance of difference, P= 0.03). This corresponds to 40 and 16%, respectively, of the maximum possible pump rate determined from ATP hydrolysis. Comparison of accumulated potassium release and relative Na-K pump rate indicates that catecholamines enhance the sensitivity of the Na-K pump for intracellular sodium.     

Comparison between isoprenaline infusions and bolus injections to assess β-adrenoceptor function in man,with special reference to cardiac contractility and the influence of autonomic reflexes

The present study was performed to characterize cardiovascular responses to isoprenaline and the influence of autonomic reflexes on these reponses. Nine healthy volunteers received infusions and bolus injections of isoprenaline before and after ‘autonomic blockade’ produced by intravenous atropine 0.04 mg kg^-1 and clonidine 300 μg. Heart rate, blood pressures, systolic time intervals and various echocardiographic measures of cardiac contractility were registered. No significant differences in responsiveness to isoprenaline were seen when infusions were repeated on the same day without ‘autonomic blockade’. After ‘blockade’, Δ responses at 1 nmol 1^-1 isoprenaline (infusions) were increased for diastolic blood pressure and decreased for systolic blood pressure and stroke volume. Bolus injections of 2 μg isoprenaline caused enhanced Δ responses after ‘autonomic blockade’ of diastolic blood pressure, left ventricular diameter in systole, ventricular circumferential fibre shortening, mean posterior wall velocity (V_{mean pw}), stroke volume, systemic vascular resistance, electromechanical systole (QS₂) and pre-ejection period. Systolic blood pressure decreased, in contrast to a small increase without ‘blockade’. These findings are explained by differences in haemodynamic effects of isoprenaline and by the dependence of responses on reflexes when isoprenaline is administered in different ways. When heart rate was increased by bolus doses of atropine, in the presence of β-blockade (propranolol), pre-ejection period and left ventricular diameter in systole were unaffected, and V_{mean pw} and ventricular circumferential fibre shortening showed only small increases (compared with alterations induced by isoprenaline). However, left ventricular ejection time, QS₂ and ejection time (by echocardiography), were markedly dependent on heart rate alterations. Thus, pre-ejection period, left ventricular diameter in systole V_{mean pw} and ventricular circumferential fibre shortening are parameters which can be useful in order to evaluate cardiac β-adrenoceptor sensitivity in vivo in man.     

Hydrogen peroxide-induced impairment of reactivity in rat isolated aorta: potentiation by 3-amino-1,2,4-triazole

1. In this study the impairment induced by hydrogen peroxide of vascular reactivity and the role of endogenous catalase in protection against this impairment was assessed in isolated rings of rat aorta.
2. Incubation with hydrogen peroxide at 1 mM, but not at 0.1 mM, for 15, 30 or 60 min followed by washout depressed, in a time-dependent manner, the subsequent ability of endothelium-containing and endothelium-denuded rings to contract to phenylephrine.
3. Incubation with 3-amino-1,2,4-triazole (50 mM, 90 min, followed by washout) to inhibit endogenous catalase had no effect by itself on subsequent phenylephrine-induced contraction. However, pretreatment with 3-amino-1,2,4-triazole did lead to a profound enhancement of the ability of hydrogen peroxide (1 mM, present for the final 30 min of the 90 min incubation, followed by washout) to depress phenylephrine-induced contraction in both endothelium-containing and endothelium-denuded rings.
4. Incubation with hydrogen peroxide at 1 mM, but not at 0.1 mM, for 15, 30 or 60 min followed by washout inhibited, in a time-dependent manner, the subsequent ability of acetylcholine (10 nM–3 μM) to induce endothelium-dependent relaxation. Furthermore, incubation with hydrogen peroxide 1 mM (30 min, followed by washout) also inhibited relaxation induced by glyceryl trinitrate (1–100 nM) or isoprenaline (10 nM–3 μM) in endothelium-denuded rings.
5. Incubation with 3-amino-1,2,4-triazole (50 mM, 90 min, followed by washout) had no effect by itself on relaxation induced by acetylcholine, glyceryl trinitrate or isoprenaline. In contrast, pretreatment with 3-amino-1,2,4-triazole led to profound enhancement of the ability of hydrogen peroxide (1 mM, present for final 30 min of the 90 min incubation) to block relaxation to acetylcholine, glyceryl trinitrate or isoprenaline.
6. On the basis of the actions of 3-amino-1,2,4-triazole, it is likely that endogenous catalase plays an important role in the protection of vascular reactivity of rat aorta against oxidant damage by high (1 mM) but not lower (0.1 mM) concentrations of hydrogen peroxide. The data are consistent with the promotion of non-selective damage to the vascular smooth muscle cells by hydrogen peroxide, but endothelial damage may also be sustained.

     

Thiopentone inhibits beta-adrenergic responses in myocardial tissue

The purpose of this study was to determine the importance of inhibition of beta-adrenergic function in thiopentone-induced myocardial depression. Using an isolated, electrically stimulated rat left atria model, contractile dose-response curves to thiopentone (200 μM, 400 μM, 600 μM, 800 μM) were shifted to the right in preparations treated with 10^{− 3} M dibutyryl cyclic adenosine monophosphate (cAMP) compared with atria stimulated with 10^{− 6} M isoprenaline, demonstrating that inhibition of beta-adrenergic mechanisms by thiopentone is physiologically important. Depression by thiopentone was similar in atria treated with 10^{− 5} M forskolin compared with preparations stimulated with 10^{− 6} M isoprenaline, indicating that thiopentone does not block beta-adrenergic receptors. It is concluded that thiopentone depresses myocardial function by several mechanisms, one of which involves inhibition of the adenyl cyclase cascade. The adenyl cyclase enzyme is a likely site where thiopentone inhibits the system; however, other components of the cascade may also be involved. L’objectif de cette étude consiste à déterminer l’influence de l’inhibition de l’activité β-adrenergique sur la dépression myocardique induite par le thiopentone. A l’aide d’un modèle constitué d’une oreillette gauche de rat stimulée électriquement, la relation dose-effet du thiopentone sur la contractilité (200 μM, 400 μM, 600 μM, 800 μM) se déplace vers la droite dans des préparations traitées avec de l’adénosine monophosphorique cyclique (cAMP) 10^{− 3} M comparativement à des oreillettes stimulées avec de l’isoprénaline 10^{− 6} M, ce qui démontre que l’inhibition β-adrénergique provoquée par le thiopentone est physiologiquement importante. La dépression de l’oreillette provoquée par le thiopentone est identique à celle que produit la forskoline 10^{− 5} M comparativement à celle de l’isoprénaline 10^{− 6} M, ce qui indique que le thiopentone n’inhibe pas les récepteurs β-adrénergiques. Les auteurs concluent que le thiopentone déprime la fonction myocardique par plusieurs mécanismes qui impliquent l’inhibition de la cascade de l’adényl cyclase. L’inhibition du système se produit vraisemblablement au niveau de l’enzyme adényl cyclase; cependant, il est possible que d’autres éléments de la cascade de l’adényl cyclase soient impliqués.     

Adrenergic receptors in the forehead microcirculation

The presence of - and -adrenoceptors in the forehead microcirculation was investigated in 49 healthy subjects. Local vascular responses to noradrenaline, isoprenaline and adrenergic antagonists, administered transcutaneously by iontophoresis, were monitored via laser Doppler flowmetry. Iontophoresis of the -adrenergic antagonist phentolamine induced a persistent increase in skin blood flow, whereas iontophoresis of saline induced a minor increase in skin blood flow which subsided rapidly. Skin blood flow increased moderately after the iontophoresis of the -adrenergic antagonist propranolol. Pretreatment of the experimental site with phentolamine blocked the normal vasoconstrictor response to noradrenaline, and unmasked a minor vasodilator component of response in some subjects. Iontophoresis of the -adrenergic agonist isoprenaline induced dose-dependent vasodilatation which was antagonised by propranolol. These findings indicate that -adrenoceptors in the forehead microcirculation normally mediate a vasoconstrictor response to iontophoretically-applied noradrenaline. In addition, -adrenoceptors appear to mediate a minor vasodilator component of response.     

Influence of propranolol and pindolol on the haemodynamic effects of papaverine,isoprenaline and noradrenaline in hypertensive patients

Summary The influence of two -adrenoceptor antagonists, propranolol and pindolol, on the haemodynamic effects of papaverine, isoprenaline and noradrenaline was investigated in 9 male patients with first degree essential hypertension. Propranolol and pindolol were given according to a doubleblind, crossover scheme. Heart rate and blood pressure were measured before and after each treatment. Propranolol 670 µg/kg i. v. reduced the supine and standing systolic blood pressures by 2.3% and 1.6%, respectively. Similarly, the intravenous administration of pindolol 35 µg/kg reduced supine and standing systolic blood pressure by 5.5% and 8.3% respectively (clinically insignificant). Neither drug affected diastolic blood pressure. Following propranolol, there were moderate reductions in supine and standing heart rates, respectively by 24% and 20% (p<0.001). Similarly, but to a lesser extent, pindolol reduced supine and standing heart rate by 12% and 17% (p<0.001). The effects of papaverine, which, at 1.5 mg/kg i. v. reduced systolic blood pressure by 5–10% and increased heart rate by 8–15%, were not significantly influenced by the -blockers. The blood pressure and heart rate responses to isoprenaline, on the other hand, were attenuated or inhibited by both -blockers. While the -blockers inhibited the -adrenoceptor component of noradrenaline, the pressor component of noradrenaline, which is mediated through the -adrenoceptors, was not influenced by propranolol, but was inhibited after pindolol. It is concluded that pindolol differs qualitatively from propranolol in that it inhibited both the -and -adrenoceptor effects of noradrenaline.Abbreviations BP blood pressure - ECG electrocardiogram - HR heart rate - ISA intrinsic sympathomimetic activity     

The effect of urapidil on responses to phenylephrine,angiotensin and isoprenaline in man

Summary Intravenous urapidil, 40 mg bolus followed by an infusion of 18 mg·h^–1 for 2 h was administered to 6 female non-patient volunteers. Randomised cumulative dose response curves to angiotensin, phenylephrine and isoprenaline were performed before and commencing 30 min after the start of the infusion of urapidil. Urapidil significantly reduced supine systolic blood pressure, 118.5 mm Hg to 105.3. The diastolic blood pressure was not significantly reduced, heart rate was not affected. Urapidil did not affect the responses to angiotensin or isoprenaline. Urapidil inhibited the pressor response to phenylephrine. The dose required to increase systolic blood pressure by 20 mm Hg increased from 156.9 g·min^–1 before to 685 g·min^–1 during urapidil; Dose ratio from individual values of 4.58. Urapidil concentrations were not significantly different before and after each agonist infusion. It is concluded that urapidil has ₁-adrenoceptor blocking activity in man without any non specific vasodilator action and that it is devoid of beta adrenoceptor blocking action.     

BRL-37344对心力衰竭大鼠β肾上腺素受体表达水平的影响(英文)

目的　探讨 β3 肾上腺素受体 (β3 AR)及其激动剂BRL 37344 (BRL)在心力衰竭中的作用。方法大鼠分为对照组、异丙肾上腺素 (Iso)组和Iso +BRL组。Iso组和Iso +BRL组大鼠间隔 2 4h ,scIso 340mg·kg- 12次制作心力衰竭模型。 8周后 ,Iso +BRL组大鼠从尾静脉给予BRL 0 .4nmol·kg- 1·min- 1,10min ,每周 2次 ,给药 2或 6周。分别于给Iso后 10或 14周测定死亡率、血流动力学、左室重 /体重、心肌组织 β1 ,β2 和 β3 ARmRNA水平。结果　 14周时对照组、Iso组和Iso +BRL组分别死亡 0 / 2 0 ,3/ 18和 5 / 2 2 (P >0 .0 5 )。Iso使左室收缩末压、±dp/dtmax明显降低 ,左室等容舒张时间常数、左室舒张末压明显增高 ,左室重 /体重明显增加 ;与Iso组比 ,Iso +BRL组的心脏舒缩功能进一步下降 ,左室重 /体重进一步增加。Iso组 β1 ARmRNA水平降低 ,β3 ARmRNA水平升高 ,与Iso组比较 ,Iso +BRL组 β1 ARmRNA水平更低 ,β3 ARmRNA水平更高。β2 ARmRNA 3组均有下降趋势 ,但无统计学差异。结论衰竭心脏β1 AR水平下降及 β3 AR水平增高可导致心功能降低。β3 ARmRNA水平在衰竭心脏比非衰竭心脏明显增高 ,应用 β3 AR激动剂可明显加重心力衰竭。     

1,6-二磷酸果糖镁盐对大鼠实验性心肌线粒体损伤的治疗作用

 目的观察1,6-二磷酸果糖镁盐(FDP-Mg)对由异丙肾上腺素(Iso)诱导的心肌线粒体缺血性损伤的治疗作用。方法采用皮下多点注射Iso致大鼠心肌缺血损伤模型,静脉分别予FDP-Mg100mg·kg^-1及FDP-Na120mg·kg^-1和MgSO₄30mg·kg^-1进行治疗,观察心肌细胞及线粒体超微结构的变化,测定药物对缺血损伤的心肌细胞内的线粒体膜电位的影响及对线粒体肿胀的抑制率。同时利用黄嘌呤/黄嘌呤氧化酶氧自由基发生系统和CuSO₄/细胞色素C羟自由基发生系统分别检测药物对超氧阴离子(O₂^-)和羟自由基(·OH)的清除率。结果FDP-Mg可以显著改善由Iso所诱导的线粒体膜电位异常改变,抑制线粒体肿胀。且对·OH和O₂^-有较高的清除率,与对照组相比有显著差异。电镜显示FDP-Mg对维持线粒体的正常形态有明显作用。结论FDP-Mg可有效保护由Iso所致的心肌细胞的线粒体实验性缺血损伤。     

Beta-receptor activation increases sodium current in guinea pig heart

Aim： To study the influence of β-receptor activation on sodium channel current and the physiological significance of increased sodium current with regard to the increased cardiac output caused by sympathetic excitation. Methods： Multiple experimental approaches, including ECG, action potential recording with conventional microelectrodes, whole-cell current measurements, single-channel recordings, and pumping-force measurements, were applied to guinea pig hearts and isolated ventricular myocytes. Results： lsoprenaline was found to dose-dependently shorten QRS waves, increase the amplitude and the Vmax of action potentials, augment the fast sodium current, and increase the occurrence frequencies and open time constants of the long-open and burst modes of the sodium channel. Increased levels of membrane-permeable cAMP have similar effects. In the presence of a calcium channel blocker, TTX reversed the increased pumping force produced by isoprenaline. Conclusion： Beta-adrenergic modulation increases the inward sodium current and accelerates the conduction velocity within the ventricles by changing the sodium channel modes, which might both be conducive to the synchronous contraction of the heart and enhance its pumping function.