Adrenergic Modulation of Potassium Currents in Isolated Human Atrial Myocytes

Evolution Under Thermal Stress Affects Escherichia coli's Resistance to Antibiotics

Exposure to both antibiotics and temperature changes can induce similar physiological responses in bacteria. Thus, changes in growth temperature may affect antibiotic resistance. Previous studies have found that evolution under antibiotic stress causes shifts in the optimal growth temperature of bacteria. However, little is known about how evolution under thermal stress affects antibiotic resistance. We examined 100+ heat-evolved strains of Escherichia coli that evolved under thermal stress. We asked whether evolution under thermal stress affects optimal growth temperature, if there are any correlations between evolving in high temperatures and antibiotic resistance, and if these strains' antibiotic efficacy changes depending on the local environment's temperature. We found that: (1) surprisingly, most of the heat-evolved strains displayed a decrease in optimal growth temperature and overall growth relative to the ancestor strain, (2) there were complex patterns of changes in antibiotic resistance when comparing the heat-evolved strains to the ancestor strain, and (3) there were few significant correlations among changes in antibiotic resistance, optimal growth temperature, and overall growth.

Adrenoceptor sub-type involvement in Ca2+ current stimulation by noradrenaline in human and rabbit atrial myocytes

Atrial fibrillation (AF) from elevated adrenergic activity may involve increased atrial L-type Ca²⁺ current (I_CaL) by noradrenaline (NA). However, the contribution of the adrenoceptor (AR) sub-types to such I_CaL-increase is poorly understood, particularly in human. We therefore investigated effects of various broad-action and sub-type-specific α- and β-AR antagonists on NA-stimulated atrial I_CaL. I_CaL was recorded by whole-cell-patch clamp at 37 °C in myocytes isolated enzymatically from atrial tissues from consenting patients undergoing elective cardiac surgery and from rabbits. NA markedly increased human atrial I_CaL, maximally by ~ 2.5-fold, with EC₇₅ 310 nM. Propranolol (β₁ + β₂-AR antagonist, 0.2 microM) substantially decreased NA (310 nM)-stimulated I_CaL, in human and rabbit. Phentolamine (α₁ + α₂-AR antagonist, 1 microM) also decreased NA-stimulated I_CaL. CGP20712A (β₁-AR antagonist, 0.3 microM) and prazosin (α₁-AR antagonist, 0.5 microM) each decreased NA-stimulated I_CaL in both species. ICI118551 (β₂-AR antagonist, 0.1 microM), in the presence of NA + CGP20712A, had no significant effect on I_CaL in human atrial myocytes, but increased it in rabbit. Yohimbine (α₂-AR antagonist, 10 microM), with NA + prazosin, had no significant effect on human or rabbit I_CaL. Stimulation of atrial I_CaL by NA is mediated, based on AR sub-type antagonist responses, mainly by activating β₁- and α₁-ARs in both human and rabbit, with a β₂-inhibitory contribution evident in rabbit, and negligible α₂ involvement in either species. This improved understanding of AR sub-type contributions to noradrenergic activation of atrial I_CaL could help inform future potential optimisation of pharmacological AR-antagonism strategies for inhibiting adrenergic AF.

Leptin modulates electrophysiological characteristics and isoproterenol-induced arrhythmogenesis in atrial myocytes

Background

Obesity is an important risk factor for atrial fibrillation (AF). Leptin is an important adipokine. However, it is not clear whether leptin directly modulates the electrophysiological characteristics of atrial myocytes.

Results

Whole cell patch clamp and indo-1 fluorescence were used to record the action potentials (APs) and ionic currents in isolated rabbit left atrial (LA) myocytes incubated with and without (control) leptin (100 nM) for 1 h to investigate the role of leptin on atrial electrophysiology. Leptin-treated LA myocytes (n = 19) had longer 20% of AP duration (28 ± 3 vs. 21 ± 2 ms, p < 0.05), but similar 50% of AP duration (51 ± 4 vs. 50 ± 3 ms, p > 0.05), and 90% of AP duration (89 ± 5 vs. 94 ± 4 ms, p > 0.05), as compared to the control (n = 22). In the presence of isoproterenol (10 nM), leptin-treated LA myocytes (n = 21) showed a lower incidence (19% vs. 54.2%, p < 0.05) of delayed afterdepolarization (DAD) than the control (n = 24). Leptin-treated LA myocytes showed a larger sodium current, but a smaller ultra-rapid delayed rectifier potassium current, and sodium-calcium exchanger current than the control. Leptin-treated and control LA myocytes exhibited a similar late sodium current, inward rectifier potassium current, transient outward current and L-type calcium current. In addition, the leptin-treated LA myocytes (n = 38) exhibited a smaller intracellular Ca²⁺ transient (0.21 ± 0.01 vs. 0.26 ± 0.01 R410/485, p < 0.05) and sarcoplasmic reticulum Ca²⁺ content (0.35 ± 0.02 vs. 0.43 ± 0.03 R410/485, p < 0.05) than the control LA myocytes (n = 42).

Conclusions

Leptin regulates the LA electrophysiological characteristics and attenuates isoproterenol-induced arrhythmogenesis.

Altered Excitation-Contraction Coupling in Human Chronic Atrial Fibrillation

This review focuses on the (mal)adaptive processes in atrial excitation-contraction coupling occurring in patients with chronic atrial fibrillation. Cellular remodeling includes shortening of the atrial action potential duration and effective refractory period, depressed intracellular Ca²⁺ transient, and reduced myocyte contractility. Here we summarize the current knowledge of the ionic bases underlying these changes. Understanding the molecular mechanisms of excitation-contraction-coupling remodeling in the fibrillating human atria is important to identify new potential targets for AF therapy.

Ionic mechanisms of electrophysiological properties and repolarization abnormalities in rabbit Purkinje fibers

Purkinje cells play an important role in drug-induced arrhythmogenesis and are widely used in preclinical drug safety assessments. Repolarization abnormalities such as action potential (AP) prolongation and early afterdeploarizations (EAD) are often observed in vitro upon pharmacological interventions. However, because drugs do not act on only one defined target, it is often difficult to fully explain the mechanisms of action and their potential arrhythmogenicity. Computational models, when appropriately detailed and validated, can be used to gain mechanistic insights into the mechanisms of action of certain drugs. Nevertheless, no model of Purkinje electrophysiology that is able to reproduce characteristic Purkinje responses to drug-induced changes in ionic current conductances such as AP prolongation and EAD generation currently exists. In this study, a novel biophysically detailed model of rabbit Purkinje electrophysiology was developed by integration of data from voltage-clamp and AP experimental recordings. Upon validation, we demonstrate that the model reproduces many key electrophysiological properties of rabbit Purkinje cells. These include: AP morphology and duration, both input resistance and rate dependence properties as well as response to hyperkalemia. Pharmacological interventions such as inward rectifier K(+) current and rapid delayed rectifier K(+) current block as well as late Na(+) current increase result in significant AP changes. However, enhanced L-type Ca(2+) current (i(CaL)) dominates in EAD genesis in Purkinje fibers. In addition, i(CaL) inactivation dynamics and intercellular coupling in tissue strongly modulate EAD formation. We conclude that EAD generation in Purkinje cells is mediated by an increase in i(CaL) and modulated by its inactivation kinetics.

Cardiac adrenergic control and atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia, and it causes substantial mortality. The autonomic nervous system, and particularly the adrenergic/cholinergic balance, has a profound influence on the occurrence of AF. Adrenergic stimulation from catecholamines can cause AF in patients. In human atrium, catecholamines can affect each of the electrophysiological mechanisms of AF initiation and/or maintenance. Catecholamines may produce membrane potential oscillations characteristic of afterdepolarisations, by increasing Ca(2+) current, [Ca(2+)](i) and consequent Na(+)-Ca(2+) exchange, and may also enhance automaticity. Catecholamines might affect reentry, by altering excitability or conduction, rather than action potential terminal repolarisation or refractory period. However, which arrhythmia mechanisms predominate is unclear, and likely depends on cardiac pathology and adrenergic tone. Heart failure (HF), a major cause of AF, causes adrenergic activation and adaptational changes, remodelling, of atrial electrophysiology, Ca(2+) homeostasis, and adrenergic responses. Chronic AF also remodels these, but differently to HF. Myocardial infarction and AF cause neural remodelling that also may promote AF. beta-Adrenoceptor antagonists (beta-blockers) are used in the treatment of AF, mainly to control the ventricular rate, by slowing atrioventricular conduction. beta-Blockers also reduce the incidence of AF, particularly in HF or after cardiac surgery, when adrenergic tone is high. Furthermore, the chronic treatment of patients with beta-blockers remodels the atria, with a potentially antiarrhythmic increase in the refractory period. Therefore, the suppression of AF by beta-blocker treatment may involve an attenuation of arrhythmic activity that is caused by increased [Ca(2+)](i), coupled with effects of adaptation to the treatment. An improved understanding of the involvement of the adrenergic system and its control in basic mechanisms of AF under differing cardiac pathologies might lead to better treatments.

FUNCTIONAL IMPAIRMENT OF CARDIAC TRANSIENT OUTWARD K+CURRENT AS A RESULT OF ABNORMALLY ALTERED CELLULAR ENVIRONMENT

1. Physiological functions of cardiac cells require a normal cellular environment. Under pathological conditions, there is a loss of normal cellular environment due to metabolic perturbations and other abnormalities. To test the hypothesis that cellular environmental stresses can create an electrophysiological substrate for electrical disorders in the heart, we investigated the effects of hypoxia, acidosis and ischaemia on transient outward K+ current (I(to)) in single canine ventricular myocytes. 2. The I(to) was studied because it plays a critical role in initiating cardiac repolarization and, thereby, arrhythmias. It was found that I(to) was significantly depressed by some 30% under hypoxic conditions relative to that in a normal cellular environment with normal Tyrode's solution. 3. Acidosis created by lowering the pH of the external solution from 7.4 to 7.2 produced a substantial (approximately 35%) reduction of the I(to) amplitude. 4. A marked impairment of I(to) function was consistently observed in ischaemic hearts in the canine coronary artery ligation model, with an approximate 30% decrease in the size of I(to). 5. Importantly, the impairment of I(to) under these environmental stresses was largely reversible following restoration to normal conditions. 6. The results of the present study suggest that I(to) is susceptible to changes in the cellular environment and the functional impairment of I(to) under environmental stresses contributes to arrhythmias under relevant pathological conditions of the heart.

Pharmacological evaluation of ocoteine, isolated from Cassytha filiformis, as an alpha 1-adrenoceptor antagonist in rat thoracic aorta

Ocoteine, isolated from Cassytha filiformis, was found to be an alpha 1-adrenoceptor blocking agent in rat thoracic aorta as revealed by its competitive antagonism of phenylephrine-induced vasoconstriction (pA2 = 7.67 +/- 0.09). Removal of endothelium from the aorta did not affect its antagonistic potency (pA2 = 7.97 +/- 0.07). [3H]-Inositol monophosphate formation caused by noradrenaline (3 microM) was suppressed by ocoteine (10 microM) and prazosin (3 microM). Ocoteine did not affect the contraction induced by U-46619, prostaglandin F2 alpha or angiotensin II, but inhibited slightly those by high K+ and endothelin I. Neither the cyclic AMP nor cyclic GMP content of rat thoracic aorta was changed by ocoteine (10 microM). Comparing the EC50 values, the potency of ocoteine against 5-hydroxytryptamine (5-HT) was about 60 times less than that against phenylephrine. Ocoteine (10 microM) also slightly antagonized the clonidine-induced inhibition of the twitch response evoked by field stimulation in rat vas deferens. In guinea pig trachea, the contraction caused by carbachol, histamine, neurokinin A or leukotriene C4 and beta 2-adrenoceptor-mediated relaxing responses induced by isoprenaline were not affected by ocoteine (10 microM). The voltage clamp study in rat ventricular single myocytes revealed that ocoteine (3, 10 microM) inhibited steady state outward currents, but not transient outward currents or slow inward Ca2+ currents. It is concluded that ocoteine is a selective alpha 1-adrenoceptor antagonist in isolated rat thoracic aorta. At high concentrations, it also blocks 5-HT receptors and Na+ and steady state outward currents in rat ventricular myocytes.