Role of NADPH oxidase and xanthine oxidase in mediating inducible VT/VF and triggered activity in a canine model of myocardial ischemia.

BACKGROUND: Ventricular tachycardia or fibrillation (VT/VF) of focal origin due to triggered activity (TA) from delayed afterdepolarizations (DADs) is reproducibly inducible after anterior coronary artery occlusion. Both VT/VF and TA can be blocked by reducing reactive oxygen species (ROS). We tested the hypothesis that inhibition of NADPH oxidase and xanthine oxidase would block VT/VF. METHODS: 69 dogs received apocynin (APO), 4 mg/kg intraveneously (IV), oxypurinol (OXY), 4 mg/kg IV, or both APO and OXY (BOTH) agents, or saline 3 h after coronary occlusion. Endocardium from ischemic sites (3-D mapping) was sampled for Rac1 (GTP-binding protein in membrane NADPH oxidase) activation or standard microelectrode techniques. Results (mean±SE, * p<0.05): VT/VF originating from ischemic zones was blocked by APO in 6/10 *, OXY in 4/9 *, BOTH in 5/8 * or saline in 1/27; 11/16 VT/VFs blocked were focal. In isolated myocardium, TA was blocked by APO (10(-6) M) or OXY (10(-8) M). Rac1 levels in ischemic endocardium were decreased by APO or OXY. CONCLUSION: APO and OXY suppressed focal VT/VF due to DADs, but the combination of the drugs was not more effective than either alone. Both drugs inhibited ischemic Rac1 with inhibition by OXY suggesting ROS-induced ROS. The inability to totally prevent VT/VF suggests that other mechanisms also contribute to ischemic VT.

Association among intracardiac T-wave alternans, ischemia, and spontaneous ventricular arrhythmias after coronary artery occlusion in a canine model.

T-wave alternans (TWA) has been investigated as a marker for susceptibility to lethal ventricular arrhythmia. In this article, we studied intracardiac TWA and ischemia as predictors of spontaneous ventricular tachycardia (VT) or ventricular fibrillation (VF) in a canine model of coronary artery occlusion (CAO). Anesthetized, open-chest dogs were studied. Electrograms from intracardiac bipolar electrodes (IBEs) were assessed for TWA and spontaneous VT or VF. TWA was defined on IBE as T wave voltage change on every other complex. In each heart, we examined 62 electrograms measured in the risk zone and surrounding normal sites, filtered from 3 to 1300 Hz. Ischemia was measured as percent of all IBE recorded that had QRS voltage drop >45%. Mapping localized the three-dimensional origin of spontaneous VT or VF. The data from dogs with VF (n = 5), VT (n = 8), or controls (no VT or VF, n = 8) were analyzed before left CAO, at the 20th min after CAO and times immediately preceding VT and VF. We found a correlation between intracardiac TWA and ischemia. More importantly, increases in intracardiac TWA peaked immediately preceding spontaneous VF and VT and were significantly higher compared to controls at comparable times. At VT/VF origins and adjacent sites, the mean TWA magnitude and discordance of TWA distinguished between VT/VF and controls at comparable times but not between VT and VF or between reentry and focal mechanisms. TWA was more common than ischemia at VT/VF origins. In summary, changes in intracardiac TWA and ischemia correlate with impending spontaneous VT/VF in a clinically applicable canine model of CAO.

Catecholamine-independent heart rate increases require Ca2+/calmodulin-dependent protein kinase II.

BACKGROUND: Catecholamines increase heart rate by augmenting the cAMP-responsive hyperpolarization-activated cyclic nucleotide-gated channel 4 pacemaker current (I(f)) and by promoting inward Na(+)/Ca(2+) exchanger current (I(NCX)) by a "Ca(2+) clock" mechanism in sinoatrial nodal cells (SANCs). The importance, identity, and function of signals that connect I(f) and Ca(2+) clock mechanisms are uncertain and controversial, but the multifunctional Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is required for physiological heart rate responses to ß-adrenergic receptor (ß-AR) stimulation. The aim of this study was to measure the contribution of the Ca(2+) clock and CaMKII to cardiac pacing independent of ß-AR agonist stimulation. METHODS AND RESULTS: We used the L-type Ca(2+) channel agonist Bay K8644 (BayK) to activate the SANC Ca(2+) clock. BayK and isoproterenol were similarly effective in increasing rates in SANCs and Langendorff-perfused hearts from wild-type control mice. In contrast, SANCs and isolated hearts from mice with CaMKII inhibition by transgenic expression of an inhibitory peptide (AC3-I) were resistant to rate increases by BayK. BayK only activated CaMKII in control SANCs but increased L-type Ca(2+) current (I(Ca)) equally in all SANCs, indicating that increasing I(Ca) was insufficient and suggesting that CaMKII activation was required for heart rate increases by BayK. BayK did not increase I(f) or protein kinase A-dependent phosphorylation of phospholamban (at Ser16), indicating that increased SANC Ca(2+) by BayK did not augment cAMP/protein kinase A signaling at these targets. Late-diastolic intracellular Ca(2+) release and I(NCX) were significantly reduced in AC3-I SANCs, and the response to BayK was eliminated by ryanodine in all groups. CONCLUSIONS: The Ca(2+) clock is capable of supporting physiological fight-or-flight responses, independent of ß-AR stimulation or I(f) increases. Complete Ca(2+) clock and ß-AR stimulation responses require CaMKII.

I(f) and SR Ca(2+) release both contribute to pacemaker activity in canine sinoatrial node cells.

Increasing evidence suggests that cardiac pacemaking is the result of two sinoatrial node (SAN) cell mechanisms: a 'voltage clock' and a Ca(2+) dependent process, or 'Ca(2+) clock.' The voltage clock initiates action potentials (APs) by SAN cell membrane potential depolarization from inward currents, of which the pacemaker current (I(f)) is thought to be particularly important. A Ca(2+) dependent process triggers APs when sarcoplasmic reticulum (SR) Ca(2+) release activates inward current carried by the forward mode of the electrogenic Na(+)/Ca(2+) exchanger (NCX). However, these mechanisms have mostly been defined in rodents or rabbits, but are unexplored in single SAN cells from larger animals. Here, we used patch-clamp and confocal microscope techniques to explore the roles of the voltage and Ca(2+) clock mechanisms in canine SAN pacemaker cells. We found that ZD7288, a selective I(f) antagonist, significantly reduced basal automaticity and induced irregular, arrhythmia-like activity in canine SAN cells. In addition, ZD7288 impaired but did not eliminate the SAN cell rate acceleration by isoproterenol. In contrast, ryanodine significantly reduced the SAN cell acceleration by isoproterenol, while ryanodine reduction of basal automaticity was modest ( approximately 14%) and did not reach statistical significance. Importantly, pretreatment with ryanodine eliminated SR Ca(2+) release, but did not affect basal or isoproterenol-enhanced I(f). Taken together, these results indicate that voltage and Ca(2+) dependent automaticity mechanisms coexist in canine SAN cells, and suggest that I(f) and SR Ca(2+) release cooperate to determine baseline and catecholamine-dependent automaticity in isolated dog SAN cells.

Angiotensin II effects on ischemic focal ventricular tachycardia are predominantly mediated through myocardial AT(2) receptor.

Ischemic focal ventricular tachycardia (VT) occurs in animals and humans. Angiotensin-converting enzyme inhibitors and receptor blockers reduce sudden death in patients with ischemic heart disease. In our dog model of coronary artery occlusion (CAO), we tested the hypothesis that angiotensin II (AGII) will selectively promote focal VT and that the specific AT(2) blocker PD-123319 (PD), or AT(1) blocker losartan, will affect this VT. Anesthetized dogs (n = 90) underwent CAO, followed by three-dimensional activation mapping of inducible VT. Dogs without VT in 1-3 h after CAO received AGII, and those with VT received either PD or losartan. Focal endocardium excised from ischemic sites was studied in vitro with standard microelectrode. Of 33 dogs with no inducible VT, AGII infusion resulted in sustained VT of only focal Purkinje origin in 13 (39%) compared with 0 of 20 dogs with saline. Of 26 dogs with inducible VT at baseline, given PD, reinduction was blocked in 8 of 10 (P < 0.05) focal VT, but only 1 of 15 with reentry. In contrast, of 11 dogs given losartan, reinduction of either mechanism was not blocked. In vitro triggered activity in Purkinje was blocked by PD in 13 of 19 (P < 0.05), but not by losartan in 8. Also, triggered activity was promoted by AGII, losartan, or the combination in 9 of 12 tissues. AGII promotes only focal, mainly Purkinje ischemic VT. PD, but not losartan, preferentially blocked focal VT, which is likely due to triggered activity due to delayed afterdepolarizations in Purkinje.

Free radical scavenger specifically prevents ischemic focal ventricular tachycardia.

BACKGROUND: Focal ventricular tachycardia (VT) in acute myocardial ischemia is closely related to triggered activity (TA), which may be blocked by scavenging reactive oxygen species (ROS). OBJECTIVE: This study analyzed effects of acutely administered ROS scavenger-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) on VT in vivo and TA in vitro. METHODS: Forty-three alpha chloralose anesthetized dogs with coronary artery occlusion were studied. Three-dimensional activation mapping helped to locate the origin of focal or reentrant VT. TEMPO (30 mg/kg intravenously) or vehicle was given. Endocardium excised from the site of origin of VT was studied using standard microelectrode techniques and measures of ROS. RESULTS: Reentry and focal VT induction were both highly reproducible. TEMPO blocked focal VT in 6 of 11 dogs (P <.05), but 9 of 9 dogs with reentrant VT continued to have VT re-induced after TEMPO. TEMPO did not alter effective refractory period (168 +/- 3 to 171 +/- 3 ms), mean blood pressure (88 +/- 3 to 81 +/- 3 mm Hg), and size of ischemia (42% +/- 3% vs 40% +/- 4%). In vitro, TEMPO (10(-3) M, n = 14) produced no change in action potentials. Nevertheless, TA was reversibly attenuated from 5.3 +/- 1.1 to 0.4 +/- 0.4 complexes with TEMPO (n = 15, P <.05). Lucigenin-enhanced chemiluminescence and dihydroethidium staining showed increased ROS in ischemic endocardium; TEMPO dramatically reduced ROS in ischemic sites. CONCLUSION: TEMPO, a scavenger of ROS, prevented triggered activity associated with focal VT during myocardial ischemia in areas of increased ROS. Antioxidant therapy may play an important role in blockade of focal VT under the conditions of myocardial ischemia.

Rapunzel syndrome reviewed and redefined.

Rapunzel syndrome is a rare presentation of a trichobezoar, with 24 cases having been reported in literature so far. This syndrome has been reported in 23 female and 1 male patient with a mean age of 10.8 years. The patients commonly present with abdominal pain, nausea, vomiting and signs of obstruction. The distal end of the bezoar may be in the jejunum, ileum or the colon. We evaluate the various cases reported and discuss the various modes of presentation of this uncommon syndrome, and also present a series of 3 cases, all females, aged 16, 18 and 21 years, and had a trichobezoar in the stomach with a tail extending into the ileum.