ENaC plays a role in regulated antibody secretion by hybridomas

Mechanisms of renal NaCl retention in proteinuric disease

In diseases with proteinuria, for example nephrotic syndrome and pre-eclampsia, there often are suppression of plasma renin-angiotensin-aldosterone system components, expansion of extracellular volume and avid renal sodium retention. Mechanisms of sodium retention in proteinuria are reviewed. In animal models of nephrotic syndrome, the amiloride-sensitive epithelial sodium channel ENaC is activated while more proximal renal Na(+) transporters are down-regulated. With suppressed plasma aldosterone concentration and little change in ENaC abundance in nephrotic syndrome, the alternative modality of proteolytic activation of ENaC has been explored. Proteolysis leads to putative release of an inhibitory peptide from the extracellular domain of the γ ENaC subunit. This leads to full activation of the channel. Plasminogen has been demonstrated in urine from patients with nephrotic syndrome and pre-eclampsia. Urine plasminogen correlates with urine albumin and is activated to plasmin within the urinary space by urokinase-type plasminogen activator. This agrees with aberrant filtration across an injured glomerular barrier independent of the primary disease. Pure plasmin and urine samples containing plasmin activate inward current in single murine collecting duct cells. In this study, it is shown that human lymphocytes may be used to uncover the effect of urine plasmin on amiloride- and aprotinin-sensitive inward currents. Data from hypertensive rat models show that protease inhibitors may attenuate blood pressure. Aberrant filtration of plasminogen and conversion within the urinary space to plasmin may activate γ ENaC proteolytically and contribute to inappropriate NaCl retention and oedema in acute proteinuric conditions and to hypertension in diseases with chronic microalbuminuria/proteinuria.

ENaCs and ASICs as therapeutic targets

The epithelial Na(+) channel (ENaC) and acid-sensitive ion channel (ASIC) branches of the ENaC/degenerin superfamily of cation channels have drawn increasing attention as potential therapeutic targets in a variety of diseases and conditions. Originally thought to be solely expressed in fluid absorptive epithelia and in neurons, it has become apparent that members of this family exhibit nearly ubiquitous expression. Therapeutic opportunities range from hypertension, due to the role of ENaC in maintaining whole body salt and water homeostasis, to anxiety disorders and pain associated with ASIC activity. As a physiologist intrigued by the fundamental mechanics of salt and water transport, it was natural that Dale Benos, to whom this series of reviews is dedicated, should have been at the forefront of research into the amiloride-sensitive sodium channel. The cloning of ENaC and subsequently the ASIC channels has revealed a far wider role for this channel family than was previously imagined. In this review, we will discuss the known and potential roles of ENaC and ASIC subunits in the wide variety of pathologies in which these channels have been implicated. Some of these, such as the role of ENaC in Liddle's syndrome are well established, others less so; however, all are related in that the fundamental defect is due to inappropriate channel activity.

Spontaneous pulmonary vein firing in man: relationship to tachycardia-pause early afterdepolarizations and triggered arrhythmia in canine pulmonary veins in vitro

INTRODUCTION

Rapid firing originating within pulmonary veins (PVs) initiates atrial fibrillation (AF). The following studies were performed to evaluate spontaneous PV firing in patients with AF to distinguish focal versus reentrant mechanisms.

METHODS

Intracardiac recordings were obtained in 18 patients demonstrating paroxysmal AF. Microelectrode (ME) recordings were obtained from superfused canine PV sleeves (N = 48).

RESULTS

Spontaneous PV firing (566 +/- 16 bpm; 127 +/- 6 ms cycle length) giving rise to AF (52 episodes) was observed. Tachycardia-pause initiation was present in 132 of 200 episodes of rapid PV firing and 34 of 52 AF episodes. The pause cycle length preceding PV firing was 1,039 +/- 86 ms following tachycardia (420 +/- 40 ms cycle length). The remaining episodes were initiated following a 702 +/- 32 ms pause during sinus rhythm (588 +/- 63 ms). Spontaneous firing recorded with a multipolar mapping catheter did not detect electrical activity bridging the diastolic interval between the initial ectopic and preceding post-pause sinus beat. Tachycardia-pause initiated PV firing (138 +/- 7 ms coupling interval) in patients correlated with tachycardia-pause enhanced isometric force, early afterdepolarization (EAD) amplitude, and triggered firing within canine PVs. Rapid firing (1,172 +/- 134 bpm; 51 +/- 8 ms cycle length) following an abbreviated coupling interval (69 +/- 12 ms) was initiated in 13 of 18 canine PVs following tachycardia-pause pacing during norepinephrine + acetylcholine superfusion. Stimulation selectively activating local autonomic nerve terminals facilitated tachycardia-pause triggered firing in canine PVs (5 of 15 vs 0 of 15; P < 0.05).

CONCLUSIONS

The studies demonstrate (1) tachycardia-pause initiation of rapid, short-coupled PV firing in AF patients and (2) tachycardia-pause facilitation of isometric force, EAD formation, and autonomic-dependent triggered firing within canine PVs, suggestive of a common arrhythmia mechanism.

Sodium channels and multiple sclerosis: roles in symptom production, damage and therapy

Our understanding of the potential role of sodium channels in multiple sclerosis (MS) has grown substantially in recent years. The channels have long had a recognized role in the symptomatology of the disease, but now also have suspected roles in causing permanent axonal destruction, and a potential role in modulating the intensity of immune activity. Sodium channels might also provide an avenue to achieve axonal and neuronal protection in MS, thereby impeding the otherwise relentless advance of permanent neurological deficit. The symptoms of MS are largely determined by the conduction properties of axons and these, in turn, are largely determined by sodium channels. The number, subtype and distribution of the sodium channels are all important, together with the way that channel function is modified by local factors, such as those resulting from inflammation (eg, nitric oxide). Suspicion is growing that sodium channels may also contribute to the axonal degeneration primarily responsible for permanent neurological deficits. The proposed mechanism involves intra-axonal sodium accumulation which promotes reverse action of the sodium/calcium exchanger and thereby a lethal rise in intra-axonal calcium. Partial blockade of sodium channels protects axons from degeneration in experimental models of MS, and therapy based on this approach is currently under investigation in clinical trials. Some recent findings suggest that such systemic inhibition of sodium channels may also promote axonal protection by suppressing inflammation within the brain.

Late-phase 3 EAD. A unique mechanism contributing to initiation of atrial fibrillation

Early (EAD) and delayed (DAD) afterdepolarizations-induced triggered activity is capable of initiating and maintaining cardiac arrhythmias. EAD-induced triggered responses are traditionally thought to be involved in the generation of ventricular arrhythmias under long QT conditions and are precipitated by bradycardia or long pauses. In contrast, DAD-induced triggered activity commonly underlies arrhythmias precipitated by tachycardia. Spontaneous release of calcium from the sarcoplasmic reticulum (SR) secondary to cellular calcium overload induces DADs and some forms of EADs. Recent studies from our laboratory have uncovered a novel mechanism giving rise to triggered activity, termed "late-phase 3 EAD," which combines properties of both EAD and DAD, but has its own unique character. Late-phase 3 EAD-induced triggered extrasystoles represent a new concept of arrhythmogenesis in which abbreviated repolarization permits "normal SR calcium release" to induce an EAD-mediated closely coupled triggered response, particularly under conditions permitting intracellular calcium loading. This review briefly describes the mechanisms and properties of late-phase 3 EADs, how they differ from conventional EADs and DADs, as well as their role in the initiation of cardiac arrhythmias, such as atrial fibrillation.

Protein kinase Cepsilon activates protein kinase B/Akt via DNA-PK to protect against tumor necrosis factor-alpha-induced cell death

We have previously shown that protein kinase Cepsilon (PKCepsilon) protects breast cancer cells from tumor necrosis factor-alpha (TNF)-induced cell death. In the present study, we have investigated if the antiapoptotic function of PKCepsilon is mediated via Akt and the mechanism by which PKCepsilon regulates Akt activity. TNF caused a transient increase in Akt phosphorylation at Ser473 in MCF-7 cells. Overexpression of PKCepsilon in MCF-7 cells increased TNF-induced Akt phosphorylation at Ser473 resulting in its activation. Knockdown of PKCepsilon by small interfering RNA (siRNA) decreased TNF-induced Akt phosphorylation/activation and increased cell death. Introduction of constitutively active Akt protected breast cancer MCF-7 cells from TNF-mediated cell death and partially restored cell survival in PKCepsilon-depleted cells. Depletion of Akt in MCF-7 cells abolished the antiapoptotic effect of PKCepsilon on TNF-mediated cell death. Akt was constitutively associated with PKCepsilon and DNA-dependent protein kinase (DNA-PK), and this association was increased by TNF treatment. Overexpression of PKCepsilon enhanced the interaction between Akt and DNA-PK. Knockdown of DNA-PK by siRNA inhibited TNF-induced Akt phosphorylation and the antiapoptotic effect of Akt and PKCepsilon. These results suggest that PKCepsilon activates Akt via DNA-PK to mediate its antiapoptotic function. Furthermore, we report for the first time that DNA-PK can regulate receptor-initiated apoptosis via Akt.

Electrophysiology of Pulmonary Veins

Pulmonary veins were found to be important foci for the genesis and maintenance of atrial fibrillation. Morphological studies have demonstrated the presence of complex anatomic structures and different types of cardiomyocytes in pulmonary veins. Numerous studies have suggested that the combination of reentrant and nonreentrant mechanisms (automaticity and triggered activity) are the underlying arrhythmogenic mechanisms of atrial fibrillation initiation from the pulmonary veins. Electropharmacological studies further indicated that pulmonary veins contained distinct arrhythmogenic activity. Several experimental models have been used to study the pulmonary vein electrical activity and demonstrate the precipitating factors for enhancing the pulmonary vein arrhythmogenic activity. The aim of this review article is to provide a critical overview of the current understanding of the basic and clinical electrophysiology of pulmonary veins and to underscore the importance of future research in this field.

Mechanism for Atrial Tachyarrhythmia in Chronic Volume Overload-Induced Dilated Atria

Mechanism of chronic volume overload-induced AT.

INTRODUCTION

Atrial dilatation associated with chronic volume overload (CVO) plays an important role in the development of atrial fibrillation (AF). However, the underlying mechanisms are unknown.

METHODS AND RESULTS

CVO-induced atrial dilatation was created in Japanese white rabbits using arteriovenous shunt formation for 6 weeks. Epicardial action potentials were measured from both atria in Langendorff-perfused sham-operated control hearts (n=8) and in CVO hearts (n=8) using high-resolution optical mapping techniques. The left atrial diameter was greater in CVO hearts (16.0+/-0.4 mm) compared to control hearts (11.0+/-0.8 mm). During steady-state pacing, right and left atrial conduction velocities were significantly lower in CVO hearts compared to control hearts (P<0.01). Rapid atrial pacing did not induce atrial tachyarrhythmia (AT) in any control hearts. However, in seven of eight CVO hearts 16 episodes of AT were induced, of which 9 exhibited a single reentrant circuit. The remaining 7 episodes exhibited a focal pattern of excitation without evidence of reentry. Interestingly, the activation rate was higher during reentry (16.1+/-1.5 Hz) compared to focal AT (9.8+/-1.0 Hz). In addition, 15 of 16 episodes occurred in the posterior left atrium (PLA). In all seven CVO hearts, AT was self-sustained for more than 10 minutes.

CONCLUSION

CVO caused atrial dilatation, conduction slowing, and AT associated with reentrant and focal excitation originating from the PLA. These results suggest that the PLA may play an important role in AT induction associated with CVO-induced atrial dilation.

Preservation of base-line hemodynamic function and loss of inducible cardioprotection in adult mice lacking protein kinase C epsilon

Signaling pathways involving protein kinase C isozymes are modulators of cardiovascular development and response to injury. Protein kinase C epsilon activation in cardiac myocytes reduces necrosis caused by coronary artery disease. However, it is unclear whether protein kinase C epsilon function is required for normal cardiac development or inducible protection against oxidative stress. Protein kinase C delta activation is also observed during cardiac preconditioning. However, its role as a promoter or inhibitor of injury is controversial. We examined hearts from protein kinase C epsilon knock-out mice under physiological conditions and during acute ischemia reperfusion. Null-mutant and wild-type mice displayed equivalent base-line morphology and hemodynamic function. Targeted disruption of the protein kinase C epsilon gene blocked cardioprotection caused by ischemic preconditioning and alpha(1)-adrenergic receptor stimulation. Protein kinase C delta activation increased in protein kinase C epsilon knock-out myocytes without altering resistance to injury. These observations support protein kinase C epsilon activation as an essential component of cardioprotective signaling. Our results favor protein kinase C delta activation as a mediator of normal growth. This study advances the understanding of cellular mechanisms responsible for preservation of myocardial integrity as potential targets for prevention and treatment of ischemic heart disease.