Gender differences in ANG II levels and action on multiple K+ current modulation pathways in diabetic rats

Cellular and Molecular Mechanisms Underlying Arterial Baroreceptor Remodeling in Cardiovascular Diseases and Diabetes

Clinical trials and animal experimental studies have demonstrated an association of arterial baroreflex impairment with the prognosis and mortality of cardiovascular diseases and diabetes. As a primary part of the arterial baroreflex arc, the pressure sensitivity of arterial baroreceptors is blunted and involved in arterial baroreflex dysfunction in cardiovascular diseases and diabetes. Changes in the arterial vascular walls, mechanosensitive ion channels, and voltage-gated ion channels contribute to the attenuation of arterial baroreceptor sensitivity. Some endogenous substances (such as angiotensin II and superoxide anion) can modulate these morphological and functional alterations through intracellular signaling pathways in impaired arterial baroreceptors. Arterial baroreceptors can be considered as a potential therapeutic target to improve the prognosis of patients with cardiovascular diseases and diabetes.

Evaluation of electrocardiographic parameters for early diagnosis of autonomic dysfunction in children and adolescents with type-1 diabetes mellitus

BACKGROUND

The aim of this study was to identify the sensitivity of electrocardiogram (ECG) in early diagnosis of cardiac autonomic function disorder in children with type 1 diabetes mellitus.

METHODS

A total of 150 children and adolescents with type 1 diabetes mellitus were enrolled between June 2009 and June 2010, as well as 100 age- and sex-matched healthy control children. Twelve-lead ECG was done in all cases and heart rate, QT and QTc interval, dispersion of P wave (Pd), and of QT (QTd) and QTc interval (QTcd) were measured. The clinical and demographic features such as age, gender, duration of follow up and level of HbA1c and fasting glucose were obtained and the effects of these parameters on ECG measurements were investigated.

RESULTS

The mean age of the patients and controls was 11.61 ± 3.72 years and 10.92 ± 3.2 years, respectively. QT and QTc interval and QTcd interval were significantly higher in diabetic children compared to healthy controls but these ECG findings were not associated with the duration of diabetes or glycemic state. Pd was significantly higher in the diabetic patients with HbA1c >7.5% compared to control, and this was also found in patients that were followed up >1 year.

CONCLUSIONS

Cardiac autonomic function disorder, which is one of the most important causes of morbidity and mortality, may emerge in the course of type 1 diabetes mellitus. It can be diagnosed on ECG even when the patients are asymptomatic.

Angiotensin II-Superoxide Signaling and Arterial Baroreceptor Function in Type-1 Diabetes Mellitus

Diabetes is a major world health problem. Growing evidence from both clinical trials and animal experiments has clearly confirmed that arterial baroreflex dysfunction is a feature of type 1 diabetes, which links to prognosis and mortality of the type 1 diabetic patients. The arterial baroreflex normally regulates the blood pressure and heart rate through sensing changes of arterial vascular tension by the arterial baroreceptors in the aortic arch and carotid sinus. The aortic baroreceptor neuron located in the nodose ganglia is a primary afferent component of the arterial baroreflex. The functional changes of these neurons are involved in the arterial baroreflex dysfunction in the type 1 diabetes. Type 1 diabetes causes the overexpression and hyperactivation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and further reduces cell excitability of the aortic baroreceptor neurons. The alterations of the HCN channels are regulated by angiotensin II-NADPH oxidase-superoxide signaling in the aortic baroreceptor neurons. From the present review, we can understand the possible mechanisms responsible for the attenuated arterial baroreflex in the type 1 diabetes. These findings are beneficial for improving quality of life and prognosis in patients with the type 1 diabetes mellitus.

Gender, clamped hyperglycemia and arterial stiffness in patients with uncomplicated type 1 diabetes mellitus

BACKGROUND

Diabetes mellitus (DM) reduces female gender-mediated protection against the development of renal disease possibly through effects on hyperglycemia. Women with DM also exhibit increased arterial stiffness, which may promote renal disease progression. The mechanisms responsible for increased arterial stiffness in women and the possible role of acute changes in ambient glycemia remain unknown.

METHODS

Blood pressure, augmentation index (AIx), pulse wave velocity (PWV) and circulating mediators of the renin angiotensin system and nitric oxide (cGMP) were measured in men (n = 22) and women (n = 19) with uncomplicated type 1 DM under clamped euglycemic and hyperglycemic conditions.

RESULTS

At baseline, men exhibited higher levels of angiotensin II (p = 0.030) and lower cGMP levels (p = 0.004), higher systolic blood pressure (124 ± 2 versus 109 ± 2 mmHg, p < 0.0001) and pulse pressure (42 ± 2 versus 58 ± 2 beats per minute, p < 0.0001). For arterial stiffness, radial (-8.0 ± 2.6% versus +5.4 ± 3.7%, p < 0.0001) and carotid AIx (-4.7 ± 2.9 versus +12.5 ± 3.0, p < 0.0001) were lower in men versus women. In contrast, carotid-femoral PWV was similar in men and women (5.20 ± 0.30 versus 5.13 ± 0.17, respectively, p = 0.853). In response to clamped hyperglycemia, systolic blood pressure increased in women (109 ± 2 to 112 ± 2 mmHg, p = 0.005) but not men. Serum aldosterone increased and cGMP declined in women but not in men. Clamped hyperglycemia did not influence arterial stiffness in either group and radial and carotid AIx remained higher in women.

CONCLUSIONS

Arterial stiffness is higher in women with type 1 DM. This effect is not dependent on the effects of clamped hyperglycemia or neurohormonal activation.

Ryanodine receptor: a new therapeutic target to control diabetic cardiomyopathy

Diabetes mellitus is a major risk factor for cardiovascular complications. Intracellular Ca(2+) release plays an important role in the regulation of muscle contraction. Sarcoplasmic reticulum Ca(2+) release is controlled by dedicated molecular machinery, composed of a complex of cardiac ryanodine receptors (RyR2s). Acquired and genetic defects in this complex result in a spectrum of abnormal Ca(2+) release phenotypes in heart. Cardiovascular dysfunction is a leading cause for mortality of diabetic individuals due, in part, to a specific cardiomyopathy, and to altered vascular reactivity. Cardiovascular complications result from multiple parameters, including glucotoxicity, lipotoxicity, fibrosis, and mitochondrial uncoupling. In diabetic subjects, oxidative stress arises from an imbalance between production of reactive oxygen and nitrogen species and capability of the system to readily detoxify reactive intermediates. To date, the etiology underlying diabetes-induced reductions in myocyte and cardiac contractility remains incompletely understood. However, numerous studies, including work from our laboratory, suggest that these defects stem in part from perturbation in intracellular Ca(2+) cycling. Since the RyR2s are one of the well-characterized redox-sensitive ion channels in heart, this article summarizes recent findings on redox regulation of cardiac Ca(2+) transport systems and discusses contributions of redox regulation to pathological cardiac function in diabetes.

Angiotensin II-NADPH oxidase-derived superoxide mediates diabetes-attenuated cell excitability of aortic baroreceptor neurons

Overactivation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels is involved in diabetes-depressed excitability of aortic baroreceptor neurons in nodose ganglia. This involvement links to the autonomic dysfunction associated with high morbidity and mortality in diabetic patients. The present study examined the effects of an angiotensin II type I receptor (AT(1)R) antagonist (losartan), a NADPH oxidase inhibitor (apocynin), and a superoxide dismutase mimetic (tempol) on the enhanced HCN currents and attenuated cell excitability in diabetic nodose neurons. In sham and streptozotocin-induced type 1 diabetic rats, HCN currents and cell excitability of aortic baroreceptor neurons were recorded by the whole cell patch-clamp technique. The angiotensin II level in nodose ganglia from diabetic rats was higher than that from sham rats (101.6 ± 4.8 vs. 38.9 ± 4.2 pg/mg protein, P < 0.05). Single-cell RT-PCR, Western blot, immunofluorescence staining, and chemiluminescence data showed that mRNA and protein expression of AT(1)R, protein expression of NADPH oxidase components, and superoxide production in nodose neurons were increased in diabetic rats compared with those from sham rats. HCN current density was higher and cell excitability was lower in aortic baroreceptor neurons from diabetic rats than that from sham rats. Losartan (1 μM), apocynin (100 μM), and tempol (1 mM) normalized the enhanced HCN current density and increased the cell excitability in the aortic baroreceptor neurons of diabetic rats. These findings suggest that endogenous angiotensin II-NADPH oxidase-superoxide signaling contributes to the enhanced HCN currents and the depressed cell excitation in the aortic baroreceptor neurons of diabetic rats.

Age-related regulation of excitation-contraction coupling in rat heart

Hearts from subjects with different ages have different Ca(2+) signaling. Release of Ca(2+) from intracellular stores in response to an action potential initiates cardiac contraction. Both depolarization-stimulated and spontaneous Ca(2+) releases, Ca(2+) transients and Ca(2+) sparks, demonstrate the main events of excitation-contraction coupling (ECC). Global increase in free Ca(2+) concentration ([Ca(2+)]( i )) consists of summation of Ca(2+) release events in cardiomyocytes. Since the Ca(2+) flux induced by Ca(2+) sparks reports a summation of ryanodine-sensitive Ca(2+) release channels (RyR2s)'s behavior in a spark cluster, evaluation of the properties of Ca(2+) sparks and Ca(2+) transients may provide insight into the role of RyR2s on altered heart function between 3-month-old (young adult) and 6-month-old (mature adult) rats. Basal [Ca(2+)]( i ) and Ca(2+) sparks frequency were significantly higher in mature adult rats compared to those of young adults. Moreover, amplitudes of Ca(2+) sparks and Ca(2+) transients were significantly smaller in mature adults than those of young adults with longer time courses. A smaller L-type Ca(2+) current density and decreased SR Ca(2+) load was observed in mature adult rats. In addition, RyR2s were markedly hyperphosphorylated, and phosphorylation levels of PKA and CaMKII were higher in heart from mature adults compared to those of young adults, whereas their SERCA protein levels were similar. Our data demonstrate that hearts from rats with different ages have different Ca(2+) signaling including hyperphosphorylation of RyR2s and higher basal [Ca(2+)]( i ) together with increased oxidized protein-thiols in mature adult rats compared to those of young adults, which play important roles in ECC. Finally, we report that ECC efficiency changes with age during maturation, partially related with an increased cellular oxidation level leading to reduced free protein-thiols in cardiomyocytes.

Elevated angiotensin II in rat nodose ganglia primes diabetes-blunted arterial baroreflex sensitivity: involvement of NADPH oxidase-derived superoxide

Clinical trials and experimental animal studies have confirmed the contribution of arterial baroreflex impairment in causing excess morbidity and mortality in type-1 diabetes. Our previous study has shown that angiotensin II (Ang II)-NADPH oxidase-superoxide signaling is associated with the reduced cell excitability in the aortic baroreceptor neurons (a primary afferent limb of the arterial baroreflex) from diabetic rats. In this study, we examined whether above-mentioned signaling might contribute to the blunted baroreflex sensitivity in streptozotocin-induced diabetic rats. Using Ang II (125)I radioimmunoassay and lucigenin chemiluminescence method, we found Ang II concentration, NADPH oxidase activity, and superoxide production in the nodose ganglia were enhanced in diabetic rats, compared to sham rats. As an index of the arterial baroreflex sensitivity, the reflex decreases in blood pressure and heart rate evoked by unilateral steady-frequency aortic depressor nerve stimulation were attenuated in diabetic rats. Local microinjection (50 nl) of losartan (an AT(1) receptor antagonist, 1 nmol), apocynin (a NADPH oxidase inhibitor, 1 nmol), and tempol (a superoxide dismutase mimetic, 10 nmol) into the nodose ganglia significantly improved the arterial baroreflex sensitivity in diabetic rats. In addition, these three chemicals also normalized exogenous Ang II-attenuated arterial baroreflex sensitivity in sham rats. These results indicate that overactivation of the Ang II-NADPH oxidase-superoxide signal pathway in the nodose ganglia contributes to the blunted baroreflex sensitivity in diabetes.

Streptozotocin-induced diabetes modulates action potentials and ion channel currents from the rat atrioventricular node

The present study was conducted to evaluate whether experimentally induced type 1 diabetes results in alterations to atrioventricular nodal (AVN) electrophysiology at the cellular level. Spontaneous action potentials (APs) and ionic currents were recorded from AVN myocytes isolated from the hearts of control rats and from those with streptozotocin-induced diabetes. Perforated patch-clamp recordings were used to assess changes in cellular AP parameters and in ionic currents. Type 1 diabetes significantly increased AP duration, whilst reducing AP firing rate, upstroke velocity and rate of diastolic depolarization. The diabetes-induced changes in AP parameters were accompanied by a significant leftward shift in the zero current potential under voltage clamp, a reduction in peak L-type Ca(2+) current density and reduced amplitude of delayed rectifier and hyperpolarization-activated currents. These findings demonstrate that experimentally induced type 1 diabetes can lead to remodelling of AVN cellular electrophysiology.

Angiotensin II enhances hyperpolarization-activated currents in rat aortic baroreceptor neurons: involvement of superoxide

As an endogenous physiologically active peptide, angiotensin (ANG) II plays an important role in the maintenance of blood pressure. In the arterial baroreceptor reflex (a pivotal regulator of blood pressure), aortic baroreceptor (AB) neurons located in the nodose ganglia (NG) are a primary afferent limb of the baroreflex. Hyperpolarization-activated currents ( Ih) in the AB neurons contribute to the excitability of the AB neurons. Therefore, the present study was to measure the modulating effect of ANG II on the Ih in the primary AB neurons isolated from rats. Data from immunofluorescent and Western blot analyses showed that protein of AT1 and AT2 receptors was expressed in the nodose neurons. In the whole cell patch-clamp recording, ANG II concentration dependently enhanced the Ih density in the AB neurons (100 nM ANG II-induced 53.8 ± 3.8% increase for A-type AB neurons and 30.4 ± 7.7% increase for C-type AB neurons at test pulse −140 mV, P < 0.05). ANG II also decreased membrane excitability in the AB neurons. AT1 receptor antagonist (1 μM losartan) but not AT2 receptor antagonist (1 μM PD-123,319) totally abolished the effect of ANG II on the Ih and neuronal excitability. In addition, NADPH oxidase inhibitor (100 μM apocynin) and superoxide scavenger (1 mM tempol) also significantly blunted the ANG II-induced increase of the Ih and decrease of the membrane excitability in the AB neurons. Furthermore, losartan, apocynin, or tempol significantly attenuated the superoxide overproduction in the NG tissues induced by ANG II. These results suggest that ANG II-NADPH oxidase-superoxide signaling can activate the Ih and subsequently decrease the membrane excitability of rat AB neurons.

Molecular determinants of cardiac transient outward potassium current (I(to)) expression and regulation

Rapidly activating and inactivating cardiac transient outward K(+) currents, I(to), are expressed in most mammalian cardiomyocytes, and contribute importantly to the early phase of action potential repolarization and to plateau potentials. The rapidly recovering (I(t)(o,f)) and slowly recovering (I(t)(o,s)) components are differentially expressed in the myocardium, contributing to regional heterogeneities in action potential waveforms. Consistent with the marked differences in biophysical properties, distinct pore-forming (alpha) subunits underlie the two I(t)(o) components: Kv4.3/Kv4.2 subunits encode I(t)(o,f), whereas Kv1.4 encodes I(t)(o,s), channels. It has also become increasingly clear that cardiac I(t)(o) channels function as components of macromolecular protein complexes, comprising (four) Kvalpha subunits and a variety of accessory subunits and regulatory proteins that influence channel expression, biophysical properties and interactions with the actin cytoskeleton, and contribute to the generation of normal cardiac rhythms. Derangements in the expression or the regulation of I(t)(o) channels in inherited or acquired cardiac diseases would be expected to increase the risk of potentially life-threatening cardiac arrhythmias. Indeed, a recently identified Brugada syndrome mutation in KCNE3 (MiRP2) has been suggested to result in increased I(t)(o,f) densities. Continued focus in this area seems certain to provide new and fundamentally important insights into the molecular determinants of functional I(t)(o) channels and into the molecular mechanisms involved in the dynamic regulation of I(t)(o) channel functioning in the normal and diseased myocardium.

Sex-dependent impairment of cardiac action potential conduction in type 1 diabetic rats

The incidence of diabetes mellitus is increasing. Cardiac dysfunction often develops, resulting in diverse arrhythmias. These arise from ion channel remodeling or from altered speed and pattern of impulse propagation. Few studies have investigated impulse propagation in the diabetic heart. We previously showed a reduced conduction reserve in the diabetic heart, with associated changes in intercellular gap junctions. The present study investigated whether these effects are sex specific. Hearts from control and streptozotocin-diabetic male and female rats were used. Optical mapping was performed with the voltage-sensitive dye di-4-ANEPPS, using Langendorff-perfused hearts. Isolated ventricular cells and tissue sections were used for immunofluorescent labeling of the gap junction protein connexin43 (Cx43). The gap junction uncoupler heptanol (0.75 mM) or elevated K+ (9 mM, to reduce cell excitability) produced significantly greater slowing of propagation in diabetic males than females. In ovariectomized diabetic females, 9 mM K+ slowed conduction significantly more than in nonovariectomized females. The subcellular redistribution (lateralization) of the gap junction protein Cx43 was smaller in diabetic females. Pretreatment of diabetic males with the angiotensin-converting enzyme inhibitor quinapril reduced Cx43 lateralization and the effects of 9 mM K+ on propagation. In conclusion, the slowing of cardiac impulse propagation in type 1 diabetes is smaller in female rats, partly due to the presence of female sex hormones. This difference is (partly) mediated by sex differences in activation of the cardiac renin-angiotensin system.

Aldosterone and the autocrine modulation of potassium currents and oxidative stress in the diabetic rat heart

BACKGROUND AND PURPOSE

Aldosterone plays a major role in cardiac pathology. This study was designed to investigate the role of cardiac aldosterone in modulating K(+) currents and oxidative stress in the streptozotocin-induced diabetic rat heart.

EXPERIMENTAL APPROACH

Transient and sustained K(+) currents were measured in ventricular myocytes by voltage clamp. Plasma and cellular aldosterone were measured by ELISA. Fluorescent dihydroethidium (DHE) was used to assess superoxide ions as markers of oxidative stress.

KEY RESULTS

The mineralocorticoid antagonist spironolactone (1 microM, 5-9 h) significantly augmented both K(+) currents in diabetic males, with a concomitant shortening of the action potential but had no effect in myocytes from control males or from diabetic females. Effects of spironolactone were restored in ovariectomized diabetic females and abolished in orchidectomized diabetic males. The aldosterone synthase inhibitor FAD286 (1 microM, 5-9 h) significantly augmented K(+) currents in cells from diabetic males, but not females. Spironolactone and FAD286 significantly reduced oxidative stress in cells from diabetic males. Plasma aldosterone content was elevated in diabetic males (relative to control), but not in females. Cellular aldosterone was also elevated, but not significantly. The elevation in aldosterone was only partly dependent on a concomitant increase in cellular angiotensin II.

CONCLUSIONS AND IMPLICATIONS

A gender-related, sex-hormone-dependent elevation in plasma and cardiac cell aldosterone contributed to oxidative stress and to attenuation of K(+) currents in diabetic male rats. Aldosterone may thus contribute to diabetes-associated cardiac arrhythmias. Aldosterone elevation was partly related to levels of angiotensin II, but residual, angiotensin II-independent, aldosterone maintains functional relevance.

Role of slow delayed rectifier K+-current in QT prolongation in the alloxan-induced diabetic rabbit heart

AIM

In diabetes mellitus, several cardiac electrophysiological parameters are known to be affected. In rodent experimental diabetes models, changes in these parameters were reported, but only limited relevant information is available in other species, having cardiac electrophysiological properties more resembling the human, including the rabbit. The present study was designed to analyse the effects of experimental type 1 diabetes on ventricular repolarization and the underlying transmembrane potassium currents in rabbit hearts.

METHODS

Diabetes was induced by a single injection of alloxan (145 mg kg(-1) i.v.). After the development of diabetes (3 weeks), electrophysiological studies were performed using whole cell voltage clamp and ECG measurements.

RESULTS

The QT(c) interval in diabetic rabbits was moderately but statistically significantly longer than measured in the control animals (155 +/- 1.8 ms vs. 145 +/- 2.8 ms, respectively, n = 9-10, P < 0.05). This QT(c)-lengthening effect of diabetes was accompanied by a significant reduction in the density of the slow delayed rectifier K(+) current, I(Ks) (from 1.48 +/- 0.35 to 0.86 +/- 0.17 pA pF(-1) at +50 mV, n = 19-21, P < 0.05) without changes in current kinetics. No differences were observed either in the density or in the kinetics of the inward rectifier K(+) current (I(K1)), the rapid delayed rectifier K(+) current (I(Kr)), the transient outward current (I(to)) and the L-type calcium current (I(CaL)) between the control and alloxan-treated rabbits.

CONCLUSION

It is concluded that type 1 diabetes mellitus, although only moderately, lengthens ventricular repolarization. Diabetes attenuates the repolarization reserve by decreasing the density of I(Ks) current, and thereby may enhance the risk of sudden cardiac death.

Sex-related effects on diabetes-induced alterations in calcium release in the rat heart

The present study was designed to determine whether the properties of local Ca(2+) release and its related regulatory mechanisms might provide insight into the role of sex differences in heart functions of control and streptozotocin-induced diabetic adult rats. Left ventricular developed pressure, the rates of pressure development and decay (+/-dP/dt), basal intracellular Ca(2+) level ([Ca(2+)](i)), and spatiotemporal parameters of [Ca(2+)](i) transients were found to be similar in male and female control rats. However, spatiotemporal parameters of Ca(2+) sparks in cardiomyocytes isolated from control females were significantly larger and slower than those in control males. Diabetes reduced left ventricular developed pressure to a lower extent in females than in males, and the diabetes-induced depressions in both +dP/dt and -dP/dt were less in females than in males. Diabetes elicited a smaller reduction in the amplitude of [Ca(2+)](i) transients in females than in males, a smaller reduction in sarcoplasmic reticulum-Ca(2+) load, and less increase in basal [Ca(2+)](i). Similarly, the elementary Ca(2+) events and their control proteins were clearly different in both sexes, and these differences were more marked in diabetes. Diabetes-induced depression of the Ca(2+) spark amplitude was significantly less in females than in matched males. Levels of cardiac ryanodine receptors (RyR2) and FK506-binding protein 12.6 in control females were significantly higher than those shown in control males. Diabetes induced less RyR2 phosphorylation and FK506-binding protein 12.6 unbinding in females. Moreover, total and free sulfhydryl groups were significantly less reduced, and PKC levels were less increased, in diabetic females than in diabetic males. The present data related to local Ca(2+) release and its related proteins describe some of the mechanisms that may underlie sex-related differences accounting for females to have less frequent development of cardiac diseases.

Gender-dependent effects of selenite on the perfused rat heart: a toxicological study

Gender differences are related to the manner in which the heart responds to chronic and acute stress conditions of physiological and pathological nature. Depending on dose, sodium selenite acts as an antioxidant proven to have beneficial effects in several pathological conditions G. Drasch, J. Schopfer, and G. N. Schrauzer, Selenium/cadmium ratios in human prostates: indicators of prostate cancer risk of smokers and nonsmokers, and relevance to the cancer protective effects of selenium, Biol. Trace Element Res. 103(2), 103-107 (2005); R. G. Kasseroller and G. N. Schrauzer, Treatment of secondary lymphedema of the arm with physical decongestive therapy and sodium selenite: a review, Am. J. Ther. 7(4), 273-279 (2000); G. N. Schrauzer, Anticarcinogenic effects of selenium, Cell. Mol. Life Sci. 57(13-14), 1864-1873 (2000); I. S. Palmer and O. E. Olson, Relative toxicities of selenite and selenate in the drinking water of rats, J. Nutr. 104(3), 306-314 (1974). To date, little is known about the gender-dependent direct effects of toxic doses of selenite on electrophysiology of the cardiovascular system H. A. Schroeder and M. Mitchener, Selenium and tellurium in rats: effect on growth, survival and tumors, J. Nutr. 101(11), 1531-1540 (1971); G. N. Schrauzer, The nutritional significance, metabolism and toxicology of selenomethionine, Adv. Food Nutr. Res. 47, 73-112 (2003). In the present study, the effects of in vitro toxic concentrations of sodium selenite ranging from 10-6 M to 10-3 M were tested on both male and female rat heart preparations. The toxic effects seen in an electrocardiogram and left ventricular pressure were dose and sex dependent at most of the tested concentrations. The present study clearly shows that at toxic doses, stress conditions are induced by selenite, resulting in genderdependent modifications of the heart function. This modification is more pronounced in the contraction cascade of female rats. Males, on the other hand, had been much more affected in excitation-related parameters.

Angiotensin II-induced changes of calcium sparks and ionic currents in human atrial myocytes: potential role for early remodeling in atrial fibrillation

AIMS

Atrial angiotensin II (ANG II) levels have been shown to be increased in atrial fibrillation (AF). The purpose of the study was to evaluate a potential role of ANG II in the early remodeling and susceptibility to chronicization of AF.

METHODS AND RESULTS

Isolated human atrial myocytes were incubated in ANG II and/or angiotensin type 1 receptor blocker candesartan. ANG II markedly increased the frequency of spontaneous Ca(2+) sparks, spark full duration, time to peak Ca(2+) fluorescence and decay time measured by confocal imaging. Sarcoplasmic reticulum calcium content estimated by caffeine-evoked calcium release did not differ between ANG II-treated cells and controls. Patch-clamp recordings revealed that ANG II significantly decreased I(to) and increased I(Ca,L) current densities. Candesartan blocked these ANG II-mediated alterations. ANG II exhibited no effect on I(K1), I(Kur) and I(f) current size. Expression of connexin 40 and connexin 43 was not significantly changed by ANG II as assessed by immunohistochemistry and Western blot analysis.

CONCLUSION

ANG II-induced alterations of calcium handling and electrophysiological changes in human atrial cells similar to those previously observed in the onset of AF. Prevention of these alterations by candesartan might constitute a useful pharmacological strategy for the treatment of AF.

Differential autocrine modulation of atrial and ventricular potassium currents and of oxidative stress in diabetic rats

The autocrine modulation of cardiac K(+) currents was compared in ventricular and atrial cells (V and A cells, respectively) from Type 1 diabetic rats. K(+) currents were measured by using whole cell voltage clamp. ANG II was measured by ELISA and immunofluorescent labeling. Oxidative stress was assessed by immunofluorescent labeling with dihydroethidium, a measure of superoxide ions. In V cells, K(+) currents are attenuated after activation of the renin-angiotensin system (RAS) and the resulting ANG II-mediated oxidative stress. In striking contrast, these currents are not attenuated in A cells. Inhibition of the angiotensin-converting enzyme (ACE) also has no effect, in contrast to current augmentation in V cells. ANG II levels are enhanced in V, but not in A, cells. However, the high basal ANG II levels in A cells suggest that in these cells, ANG II-mediated pathways are suppressed, rather than ANG II formation. Concordantly, superoxide ion levels are lower in diabetic A than in V cells. Several findings indicate that high atrial natriuretic peptide (ANP) levels in A cells inhibit RAS activation. In male diabetic V cells, in vitro ANP (300 nM-1 muM, >5 h) decreases oxidative stress and augments K(+) currents, but not when excess ANG II is present. ANP has no effect on ventricular K(+) currents when the RAS is not activated, as in control males, in diabetic males treated with ACE inhibitor and in diabetic females. In conclusion, the modulation of K(+) currents and oxidative stress is significantly different in A and V cells in diabetic rat hearts. The evidence suggests that this is largely due to inhibition of RAS activation and/or action by ANP in A cells. These results may underlie chamber-specific arrhythmogenic mechanisms.

Dexamethasone and cardiac potassium currents in the diabetic rat

Experiments were designed to compare effects of dexamethasone on transient (Ipeak) and sustained (Isus) K+ currents in control and diabetic rat myocytes. Ventricular myocytes were isolated from control or type 1 streptozotocin (STZ)-induced diabetic male and female rats. Currents were measured using whole-cell voltage-clamp methods. Incubation of cells from control males or females with 100 nM dexamethasone (5-9 h) significantly (P<0.005) augmented Isus (by 28-31%). Ipeak was unchanged. Isus augmentation was abolished by cycloheximide or cytochalasin D, but not by inhibition of protein kinases A or C. Inhibition of tyrosine kinases by genistein (but not its inactive analog genistin) prevented the increase of Isus by dexamethasone. In marked contrast, dexamethasone had a significantly (P<0.015) smaller effect on Isus (11% increase) in cells from male STZ-diabetic rats, as compared to control cells. However, Isus augmentation in cells from female STZ-diabetic rats was normal (31% increase). In ovariectomized-diabetic rats, Isus was unchanged by dexamethasone. The reduced effect in diabetic males might be due to preactivation of tyrosine kinases linking dexamethasone to current modulation. In conclusion, type I diabetes is associated with gender-specific changes in sensitivity of K+ currents to glucocorticoids, linked to alterations in tyrosine-phosphorylated proteins.

Effects of diabetes on ryanodine receptor Ca release channel (RyR2) and Ca2+ homeostasis in rat heart

The defects identified in the mechanical activity of the hearts from type 1 diabetic animals include alteration of Ca2+ signaling via changes in critical processes that regulate intracellular Ca2+ concentration. These defects result partially from a dysfunction of cardiac ryanodine receptor calcium release channel (RyR2). The present study was designed to determine whether the properties of the Ca2+ sparks might provide insight into the role of RyR2 in the altered Ca2+ signaling in cardiomyocytes from diabetic animals when they were analyzed together with Ca2+ transients. Basal Ca2+ level as well as Ca2+-spark frequency of cardiomyoctes isolated from 5-week streptozotocin (STZ)-induced diabetic rats significantly increased with respect to aged-matched control rats. Ca2+ transients exhibited significantly reduced amplitude and prolonged time courses as well as depressed Ca2+ loading of sarcoplasmic reticulum in diabetic rats. Spatio-temporal properties of the Ca2+ sparks in cardiomyocytes isolated from diabetic rats were also significantly altered to being almost parallel to the changes of Ca2+ transients. In addition, RyR2 from diabetic rat hearts were hyperphosphorylated and protein levels of both RyR2 and FKBP12.6 depleted. These data show that STZ-induced diabetic rat hearts exhibit altered local Ca2+ signaling with increased basal Ca2+ level.

Modulation of potassium currents by angiotensin and oxidative stress in cardiac cells from the diabetic rat

Diabetes induces oxidative stress and leads to attenuation of cardiac K+ currents. We investigated the role of superoxide ions and angiotensin II (ANG II) in generating and linking oxidative stress to the modulation of K+ currents under diabetic conditions. K+ currents were measured using patch-clamp methods in ventricular myocytes from streptozotocin (STZ)-induced diabetic rats. Superoxide ion levels, indicating oxidative stress, were measured by fluorescent labelling with dihydroethidium (DHE). ANG II content was measured using enzyme-linked immunosorbent asssay (ELISA). The results showed DHE fluorescence to be significantly higher in cells from diabetic males, compared to controls. Relief of stress by the NADPH oxidase inhibitor apocynin or by superoxide dismutase (SOD) but not by catalase reversed the attenuation of K+ currents and reduced DHE fluorescence. In cells from diabetic females, neither apocynin nor SOD augmented K+ currents, ANG II was not elevated and DHE fluorescence was significantly weaker than in cells from males. Reduced glutathione (GSH) also augmented K+ currents in cells from diabetic males but not females. In ovariectomized diabetic females K+ currents were augmented by GSH and apocynin. Current augmentation and the attenuation of DHE fluorescence by apocynin were significantly blunted by excess ANG II (300 nm). Diabetic male rats pretreated with the angiotensin-converting enzyme (ACE) inhibitor quinapril were hyperglycaemic, but their cellular ANG II levels and DHE fluorescence were significantly decreased. In cells from these rats, K+ currents were insensitive to apocynin. In conclusion, diabetes-related oxidative stress attenuates K+ currents through ANG II-generated increased superoxide ion levels. When ANG II levels are lower, as in diabetic females or following ACE inhibition in males, oxidative stress is reduced, with blunted alterations in K+ currents.