Biomedical subjects

Mary B Wagner

Publications and source records attributed to Mary B Wagner.

11 recordsLinked to original sources

PubMed2006

Propagation of pacemaker activity.

Spontaneous activity of specific regions (e.g., the Sinoatrial node, SAN) is essential for the normal activation sequence of the heart and also serve as a primary means of modulating cardiac rate by sympathetic tone and circulating catecholamines. The mechanisms of how a small SAN region can electrically drive a much larger atrium, or how a small ectopic focus can drive surrounding ventricular or atrial tissue are complex, and involve the membrane properties and electrical coupling within the SAN or focus region as well as the membrane properties, coupling conductance magnitudes and also regional distribution within the surrounding tissue. We review here studies over the past few decades in which mathematical models and experimental studies have been used to determine some of the design principles of successful propagation from a pacemaking focus. These principles can be briefly summarized as (1) central relative uncoupling to protect the spontaneously firing cells from too much electrotonic inhibition, (2) a transitional region in which the cell type and electrical coupling change from the central SAN region to the peripheral atrial region, and (3) a distributed anisotropy to facilitate focal activity.

Action Potentials↗

PubMed2006

Developmental changes in time course of recovery from inactivation in L-type calcium currents of rabbit ventricular myocytes.

The mechanisms of recovery from inactivation of the L-type calcium current (I(Ca)) are not well established, and recovery is affected by many experimental conditions. Little is known about developmental changes of recovery from inactivation of I(Ca). We studied developmental changes of recovery from inactivation in I(Ca) using isolated adult and newborn (1-4 days) rabbit ventricular myocytes. We used broken-patch and perforated-patch techniques with physiological extracellular ionic concentrations of calcium and sodium and interpulse conditioning potentials of -80 or -50 mV. We also maximized I(Ca) with forskolin. We found that recovery from inactivation did not differ between adult and newborn cells when either EGTA or BAPTA was used to buffer intracellular calcium. Maximizing I(Ca) with forskolin slowed recovery from inactivation in newborn but not in adult cells. In contrast, when the intracellular buffering of the cell was left nearly intact (perforated patch), recovery from inactivation (half-time of recovery) in the newborn cells was significantly slower than for the adult cells when either a conditioning potential of -80 mV (140 +/- 9 vs. 58 +/- 4 ms, newborn vs. adult; P < 0.05) or -50 mV (641 +/- 106 vs. 168 +/- 15 ms, newborn vs. adult; P < 0.05) was used. Forskolin significantly increased half-time of recovery for both adult and newborn cells. Dialysis with no calcium buffer showed a slower recovery from inactivation in newborn cells. Intracellular dialysis with a calcium buffer masked differences in recovery from inactivation of I(Ca) between newborn and adult rabbit ventricular cells.

Aging↗

PubMed2006

Remodeling of early-phase repolarization: a mechanism of abnormal impulse conduction in heart failure.

BACKGROUND: The early phase of action potential (AP) repolarization is critical to impulse conduction in the heart because it provides current for charging electrically coupled cells. In the present study we tested the impact of heart failure-associated electrical remodeling on AP propagation. METHODS AND RESULTS: Subepicardial, midmyocardial, and subendocardial myocytes were enzymatically dissociated from control and pressure-overload failing left ventricle (LV), and APs were recorded. A unique coupling-clamp technique was used to electrically couple 2 isolated myocytes with a controlled value of coupling conductance (Gc). In sham-operated mice, AP duration manifested a clear transmural gradient, with faster repolarization in subepicardial myocytes than in subendocardial myocytes. AP propagation from subendocardial to subepicardial myocytes required less Gc compared with conduction in the opposite direction. In failing heart, AP morphology was dramatically altered, with a significantly elevated plateau potential and prolonged AP duration. Spatially nonuniform alteration of AP duration in failing heart blunted the transmural gradient of repolarization. Furthermore, increased pacing rate prolonged AP duration exclusively in myocytes from failing heart, and the critical conductance required for successful AP propagation decreased significantly at high frequencies. Finally, in failing heart, asymmetry of transmural electrical propagation was abolished. CONCLUSIONS: In failing heart, preferential conduction from subendocardial to subepicardial myocytes is lost, and failing myocytes manifest facilitated AP propagation at fast rates. Together, these electrical remodeling responses may promote conduction of premature impulses and heighten the risk of malignant arrhythmia, a prominent feature of heart failure.

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PubMed2006

Effects of single-site versus biventricular epicardial pacing on myocardial performance in an immature animal model of atrioventricular block.

INTRODUCTION: Single-site ventricular pacing results in dyssynchronous ventricular activation and may contribute to ventricular dysfunction. We evaluated epicardial biventricular (BiV) pacing as a means of maintaining synchronous ventricular activation in an acute piglet model of AV block with normal ventricular anatomy and function. METHODS AND RESULTS: We used left ventricular (LV) impedance catheters and tissue Doppler imaging to assess the cardiodynamics of immature piglets (n = 6, 33-78 days, 9.35 +/- 0.85 kg). Following catheter ablation of the AV node, a pacemaker was programmed 20 beats per minute above the intrinsic atrial rate. The animals were paced at 5-minute intervals via the following AV sequential configurations: Right atrial appendage (RA)-RV apex (RVA), RA-LV apex (LVA), and RA-biventricular (RVA/LVA). RA-RVA was the experimental control. LV systolic mechanics, measured by the slope of the end-systolic pressure-volume relationship (E(es)), increased with BiV pacing (12.8 +/- 3.4 mmHg/mL, P < 0.02) or single-site LVA pacing (10.6 +/- 3.4 mmHg/mL, P < 0.05) compared with single-site RVA pacing (8.3 +/- 1.4 mmHg/mL). QRS duration lengthened compared with sinus rhythm (42 +/- 8 msec) with either RVA (56 +/- 9 msec, P < 0.02) or LVA (54 +/- 7 msec, P < 0.03), but not with BiV (48 +/- 7 msec, P = 0.08) pacing. Tissue Doppler imaging showed LV dyssynchrony with RVA (septal-to-lateral delay 46.0 +/- 51.7 msec), with return toward normal with LVA (-9.6 +/- 33.6 msec, P < 0.04) or BiV (-4.1 +/- 33.8 msec, P < 0.04) pacing. CONCLUSIONS: In this acute immature piglet model of AV block, LV performance improved with single-site pacing from the LVA and BiV pacing (RVA/LVA), as compared with single-site pacing from the RVA. These changes correlated with tissue Doppler indices of mechanical synchrony, though not necessarily with QRS duration.

Animals↗

PubMed2005

Upregulation of hypoxia inducible factor is associated with attenuation of neuronal injury in neonatal piglets undergoing deep hypothermic circulatory arrest.

BACKGROUND: Prolonged deep hypothermic circulatory arrest is known to cause neurological injury. Hypoxia inducible factor, a transcription factor that mediates adaptive changes during hypoxia, is neuroprotective in models of ischemic brain injury, in part by upregulating erythropoietin. This study tested the hypothesis that upregulation of hypoxia inducible factor and erythropoietin by preconditioning with hypoxia or the hypoxia-mimetic agents deferoxamine and cobalt chloride would be neuroprotective in a piglet model of deep hypothermic circulatory arrest. METHODS: Anesthetized neonatal piglets were randomized to 4 preconditioning groups (15 per group): hypoxia, deferoxamine, cobalt chloride, or control (NaCl vehicle). Brain hypoxia inducible factor and erythropoietin contents were assessed by means of Western blotting at 3, 8, and 24 hours after treatment (n = 3 per time point). Twenty-four hours after treatment, 6 to 7 animals per group underwent cardiopulmonary bypass and 110 minutes of deep hypothermic circulatory arrest. After recovery, serial neurobehavioral examinations were conducted for 6 days, after which histopathologic brain injury and neuronal apoptosis (cleaved caspase 3) were assessed. RESULTS: Erythropoietin expression was not significantly increased by any of the pretreatment strategies. In contrast, there was a significant upregulation of hypoxia inducible factor by pretreatment with deferoxamine and cobalt chloride (P = .002). Neurobehavioral measures revealed no significant differences in time to recovery or extent of injury. Examination of histopathologic brain injury in the hippocampus revealed that pretreatment with deferoxamine (0.4 +/- 0.3) and cobalt chloride (0.5 +/- 0.3) were associated with significantly less neuronal loss than pretreatment with hypoxia or control (2.8 +/- 0.5, P = .004). Finally, cleaved caspase 3 (a marker of apoptotic cell death) was also shown to be diminished in the cobalt and deferoxamine groups, but the difference was not significantly different from the value in the control group. CONCLUSIONS: In contrast to hypoxia, deferoxamine and cobalt chloride preconditioning upregulated hypoxia inducible factor and were associated with histopathologic neuroprotection after exposure to cardiopulmonary bypass and prolonged deep hypothermic circulatory arrest.

Animals↗

PubMed2004

Calcium transients in infant human atrial myocytes.

Isolated infant human atrial cells have a slower early repolarization than adult human atrial cells. In addition, from room temperature voltage-clamp studies, infant cells have lower basal L-type calcium currents than adult cells. We hypothesized that the slower repolarization increases the calcium transient of infant human atrial cells. Atrial myocytes were enzymatically dissociated from biopsies of human right atrial appendages of infant (3-8 mo) patients who were undergoing open-heart surgery. Intracellular calcium transients were measured with fluorescence microscopy with application of either square waves or action potential waveforms at physiologic temperature. After repetitive application (1 Hz) of 100-ms duration conditioning depolarizations to 10 mV (from -80 mV), a test pulse of varying duration (DeltaT; 2-100 ms) produced smaller transients (expressed as percentage of the last conditioning pulse) at shorter durations (33 +/- 7% for DeltaT = 2 ms, 80 +/- 4% for DeltaT = 25 ms). With repetitive application of either adult or infant prerecorded action potentials to infant cells, the cells had a decreased calcium transient with the adult action potential (F/F(0) 2.2 +/- 0.4 for infant action potential versus 1.6 +/- 0.2 for adult action potential; n = 7; p < 0.05). The delayed early repolarization of infant cells alters the Ca(2+) transient, which may compensate for the lower availability of basal calcium current in infant cells. The steep relationship that we have demonstrated between test-pulse duration and the calcium transient suggests that modulation of the early repolarization phase of the action potential may be of great significance in modulating excitation-contraction coupling.

Action Potentials↗

PubMed2004

Induced automaticity in isolated rat atrial cells by incorporation of a stretch-activated conductance.

Stretch of the atrium and sympathetic activity have been implicated as substrates for atrial fibrillation. We investigate how a model of stretch in combination with sympathetic stimulation can induce automaticity in atrial cells. We adapted our coupling clamp circuit so that a model ionic current that represents stretch-activated channels (SACs) was injected into an isolated rat atrial cell in real time. This current was calculated as ISAC= GSAC (Vm-ESAC), where GSAC and ESAC are the conductance and reversal potential of SACs and Vm is the cell's membrane potential. Repetitive automaticity was induced by a sufficiently large GSAC and this critical value of GSAC was decreased by exposure to isoproterenol. The critical value of GSAC decreased from 0.63+/-0.05 nS (mean+/-SE) in control to 0.40+/-0.07 nS in isoproterenol (P<0.05). Additionally, after exposure to isoproterenol, automaticity continued after GSAC was no longer applied and was accompanied by delayed after-depolarizations. In three cells, repetitive automaticity could not be induced at any value of GSAC. Exposure to 10 nM isoproterenol converted these cells to cells with repetitive automaticity in response to GSAC. We conclude that automaticity can be induced in isolated rat atrial cells by application of a model of SACs. Exposure to isoproterenol enhances this effect.

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PubMed2004

Developmental differences in L-type calcium current of human atrial myocytes.

We investigated differences in L-type Ca2+ current (ICa) between infant (INF, 1-12 mo old), young adult (YAD, 14-18 yr old), and older adult (AD) myocytes from biopsies of right atrial appendages. Basal ICa was smaller in INF myocytes (1.2 +/- 0.1 pA/pF, n = 29, 6 +/- 1 mo old, 11 patients) than in YAD (2.5 +/- 0.2 pA/pF, n = 20, 16 +/- 1 yr old, 5 patients) or AD (2.6 +/- 0.3 pA/pF, n = 19, 66 +/- 3 yr old, 9 patients) myocytes (P < 0.05). Maximal ICa produced by isoproterenol (Iso) was similar in INF, YAD, and AD cells: 8.4 +/- 1.1, 9.6 +/- 1.0, and 9.2 +/- 1.3 pA/pF, respectively. Efficacy (Emax) was larger in INF (607 +/- 50%) than for YAD (371 +/- 29%) or AD (455 +/- 12%) myocytes. Potency (EC50) was 8- to 10-fold higher in AD (0.82 +/- 0.09 nM) or YAD (0.41 +/- 0.14 nM) than in INF (7.6 +/- 3.5 nM) myocytes. Protein levels were similar for Gialpha2 but much greater for Gialpha3 in INF than in AD or YAD atrial tissue. When Gialpha3 activity was inhibited by inclusion of a Gialpha3 COOH-terminal decapeptide in the pipette, basal ICa and the response to 10 nM Iso were increased in INF, but not in YAD, cells. We propose that basal ICa and the response to low-dose beta-adrenergic stimulation are inhibited in INF (but not YAD or AD) cells as a result of constitutive inhibitory effects of Gialpha3.

Adolescent↗

PubMed2003

Inhibition of fast sodium current in rabbit ventricular myocytes by protein tyrosine kinase inhibitors.

The present study investigated the effects of protein tyrosine kinase inhibitors on the fast sodium current ( I(Na)) in rabbit ventricular myocytes. Single rabbit ventricular myocytes were isolated enzymatically using Langendorff perfusion. I(Na) was recorded using the whole-cell patch-clamp technique at room temperature. The protein tyrosine kinase inhibitors genistein, AG957, ST638, and PP2 reversibly inhibited I(Na) in a concentration-dependent manner. At a test pulse potential of -30 mV, genistein (n=7) inhibited I(Na) by 37.7+/-3.2%, 53.4+/-2.5%, and 71.8+/-2.7% at concentrations of 15, 50, and 100 microM, respectively, without changing the voltage dependence of activation, while 100 microM AG957, 100 microM ST638, and 30 microM PP2 inhibited I(Na) by 38.7+/-2.4, 35.8+/-3.4, and 21.1+/-3.9%, respectively. Genistein (100 microM) and AG957 (100 microM) shifted the voltage for half-maximal inactivation of I(Na) from -76.7+/-2.0 mV (n=10) in control to -88.37+/-2.6 mV (n=6, P<0.05), and -82.9+/-1.7 (n=4, P<0.05), respectively, without changing the slope factor. Genistein and AG957 also significantly prolonged the time course of I(Na) recovery from inactivation. Daidzein and PP3, inactive analogs of genistein and PP2, respectively, did not inhibit I(Na) significantly. We conclude that protein tyrosine kinase signaling pathways may play an important role in regulation of I(Na) in cardiac myocytes.

Animals↗

PubMed2003

Differences in transient outward current properties between neonatal and adult human atrial myocytes.

UNLABELLED: Knowledge of postnatal modulation of I(to) in human atrial myocytes is quite limited. The present study investigated the differences in I(to) properties between neonatal and adult human atrial myocytes. METHODS: Atrial myocytes were dissociated enzymatically from biopsies of human right atrial appendage. I(to) and action potentials were recorded by whole-cell patch-clamp technique. The expressed protein levels of Kv4.3 and KChIP2 in atrial tissue were detected by western blot technique. RESULTS: I(to) was present in all atrial cells (n = 37) from 10 neonatal patients (2.5-7 months). The mean value of I(to) density in neonatal atrial cells was significantly larger than in adult atrial cells. The time constants for I(to) current decay were significantly faster for neonatal cells, compared to adult cells. I(to) recovery from inactivation at holding potential of - 80 mV was significantly slower for neonatal atrial cells than for adult atrial cells. There was no difference in the voltage dependence of I(to) activation between neonatal and adult cells. The voltage-dependent inactivation slope factor was smaller for neonatal compared to adult atrial cells. A more significant frequency-dependent suppression of I(to) peak current and a more significant lengthening of APD(30) were observed in neonatal atrial cells compared to adult atrial cells. Western blots showed both Kv4.3 and KChIP2 are expressed in neonatal atria, but with significantly higher level of Kv4.3 and lower level of KChIP2 protein compared to adult. CONCLUSION: There are significant differences in the properties of I(to) between neonatal and adult human atrial cells, including a larger current density, faster inactivation and slower recovery from inactivation in the neonatal atrial cells. The physiological differences of I(to) are consistent with the different expression protein levels of Kv4.3 and KChIP2.

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PubMed2002

Regulation of transient outward current in human atrial myocytes by protein tyrosine kinase pathway.

INTRODUCTION: Regulation of transient outward current (I(to)) in human myocytes is unclear. The present study investigated the effect of protein tyrosine kinase (PTK) inhibitors on I(to) in human atrial myocytes. METHODS AND RESULTS: Atrial myocytes were isolated enzymatically from biopsies of human right atrial appendage obtained from patients undergoing coronary artery bypass surgery. I(to) was recorded by the whole-cell patch-clamp technique in voltage-clamp configuration. Two groups of PTK inhibitors, the ATP binding site PTK inhibitors genistein and AG957 and the protein substrate PTK inhibitors ST638 and PP2, significantly inhibited I(to) in a concentration-dependent manner, with a potency order of genistein>AG957>ST638>PP2. At test pulse potential of +60 mV, I(to) was inhibited by 28% +/- 3%, 59% +/- 3%, and 89% +/- 3% by 15, 50, and 100 microM genistein, respectively. Daidzein and PP3, inactive analogues of genistein and PP2, respectively, did not produce any inhibitory effects on I(to). In addition to the inhibition of I(to) amplitude, the protein substrate PTK inhibitors ST638 and PP2 significantly accelerated I(to) inactivation (current decay) and delayed recovery from inactivation. Inhibition of protein tyrosine phosphatase partially reversed the effect of genistein. Stimulation or inhibition of serine/ threonine kinases (PKA, PKC, and PKG) did not change I(to) or alter the inhibitory response of PTK inhibitors on I(to). CONCLUSION: In human atrial cells, the PTK pathway plays an important role in the regulation of basal I(to), independent of serine/threonine kinases.

Aged↗