Rate-dependent [K+](o) accumulation in canine right atria in vivo: electrophysiological consequences

Sudden increases in heart rate cause accumulation of K+ in the extracellular space. However, the exact relationship between rate and extracellular K+ concentration ([K+](o)) in vivo is unknown. We measured [K+](o) in right atria of anesthetized dogs by using K(+)-sensitive electrodes. Peak increase in [K+](o) ranged from 0.18 +/- 0.04 mM [means +/- SE; cycle length (CL) = 350 ms] to 0.80 +/- 0.09 mM (CL = 250 ms) above baseline (3.50 +/- 0.08 mM at CL = 380 ms; n = 5). During rapid pacing-induced atrial fibrillation, peak increase in [K+](o) averaged 0.80 +/- 0.07 mM (n = 5). Whole cell current-clamp measurements in single right atrial myocytes (n = 5) showed that raising [K+](o) from 3 to 5 mM in 1-mM steps progressively depolarized resting membrane potential and reduced both phase 0 action potential amplitude and maximal upstroke velocity. Multisite epicardial mapping (n = 4) demonstrated that sudden rate increases changed longitudinal conduction velocity (CV(L)) by -3.6 +/- 1.8% to -5.9 +/- 1.2% over a CL range of 330 to 250 ms. Our observations suggest that rate-related [K+](o) accumulation in vivo is of sufficient magnitude to modulate those cellular electrophysiological properties that determine atrial CV(L).     

Diurnal variations of the dominant cycle length of chronic atrial fibrillation

High-resolution digital Holter recording was carried out in 21 patients (15 men, 64 +/- 12 yr) with chronic atrial fibrillation. Dominating atrial cycle length (DACL) was derived by frequency domain analysis of QRST-reduced electrocardiograms. Daytime mean DACL was 150 +/- 17 ms, and nighttime mean was 157 +/- 22 ms (P = 0. 0002). Diurnal fluctuation in DACL differed among patients: it tended to be virtually absent in those with a short mean DACL, but in those with longer DACL the night-day difference was as much as 23 ms (R = 0.72, P < 0.001, correlation of mean DACL to night-day difference). Mean DACL also correlated with ventricular cycle length (R = 0.40, P < 0.001), particularly at night (r = 0.49). The shorter cycle lengths found in this study during the day are consistent with sympathetic and/or other physiological modulation, but since increased vagal tone shortens atrial refractoriness in most models, parasympathetic influences are not likely to play a major role. Alternatively, atrial effective refractory period may not be the sole determinant of atrial cycle length during atrial fibrillation.     

Characterization of fibrillatory rhythms by ensemble vector directional analysis

Recent studies have demonstrated that fibrillatory rhythms are not random phenomena but have definable patterns. However, standard mapping techniques may have limitations in their ability to identify the organization of fibrillation. The purpose of this study was to develop and apply a method, "ensemble vector mapping," for characterizing the spatiotemporal organization of fibrillation. Ventricular fibrillation was induced by burst pacing in normal mongrel dogs. In a separate protocol, atrial fibrillation was induced by epicardial aconitine application. Epicardial electrograms were recorded from a 112-electrode plaque array using a computerized mapping system. Vectors were created by summing orthogonal bipolar electrograms. The magnitude of the vectors was transformed using a logarithmic function, integrated over time, and normalized for local electrogram amplitude to produce an "ensemble vector" index whose magnitude is high when beat-to-beat activation direction is consistent and low when activation direction is variable. The mean index was 137 +/- 36 mV/s during ventricular pacing at a cycle length of 300 ms but only 39 +/- 23 mV/s during ventricular fibrillation (P < 0.001). The ensemble vector index was also lower during atrial fibrillation (60 +/- 54 mV/s) than during atrial pacing (115 +/- 27 mV/s, P < 0.01 vs. atrial fibrillation) but not as low as during ventricular fibrillation (P < 0.05, atrial vs. ventricular fibrillation). The index was also capable of distinguishing atrial tachycardia from atrial fibrillation. Ensemble vector mapping produces an objective assessment of the consistency of myocardial activation during fibrillation. The consistency of activation direction differs in different models of fibrillation and is higher during atrial than ventricular fibrillation. This technique has the potential to rapidly characterize repetitive activation patterns in fibrillatory rhythms and may help distinguish among different characteristics of fibrillatory rhythms.     

Measurements of calcium transients in ventricular cells during discontinuous action potential conduction

The L-type calcium current (I(Ca)) is important in sustaining propagation during discontinuous conduction. In addition, I(Ca) is altered during discontinuous conduction, which may result in changes in the intracellular calcium transient. To study this, we have combined the ability to monitor intracellular calcium concentration ([Ca(2+)](i)) in an isolated cardiac cell using confocal scanning laser fluorescence microscopy with our "coupling clamp" technique, which allows action potential propagation from the real cell to a real-time simulation of a model cell. Coupling a real cell to a model cell with a value of coupling conductance (G(C) = 8 nS) just above the critical value for action potential propagation results in both an increased amplitude and an increased rate of rise of the calcium transient. Similar but smaller changes in the calcium transient are caused by increasing G(C) to 20 nS. The increase of [Ca(2+)](i) by discontinuous conduction is less than the increase of I(Ca), which may indicate that much of [Ca(2+)](i) is the result of calcium released from the sarcoplasmic reticulum rather than the integration of I(Ca).     

Functional gap junctions in the early sea urchin embryo are localized to the vegetal pole.

Using the whole-cell voltage-clamp technique we have studied electrical coupling and dye coupling between pairs of blastomeres in 16- to 128-cell-stage sea urchin embryos. Electrical coupling was established between macromeres and micromeres at the 16-cell stage with a junctional conductance (G(j)) of 26 nS that decreased to 12 nS before the next cleavage division. G(j) between descendants of macromeres and micromeres was 12 nS falling to 8 nS in the latter half of the cell cycle. Intercellular current intensity was independent of transjunctional voltage, nondirectional, and sensitive to 1-octanol and therefore appears to be gated through gap junction channels. There was no significant coupling between other pairs of blastomeres. Lucifer yellow did not spread between these electrically coupled cell pairs and in fact significant dye coupling between nonsister cells was observed only at the 128-cell stage. Since 1-octanol inhibited electrical communication between blastomeres at the 16- to 64-cell stage and also induced defects in formation of the archenteron, it is possible that gap junctions play a role in embryonic induction.     

Effects of amiodarone on wave front dynamics during ventricular fibrillation in isolated swine right ventricle

The effects of acute amiodarone infusion on dynamics of ventricular fibrillation (VF) are unclear. Six isolated swine right ventricles (RVs) were studied in vitro. Activation patterns during VF were mapped optically, whereas action potentials were recorded with a glass microelectrode. At baseline, VF was associated with frequent spontaneous wave breaks. Amiodarone (2.5 microg/ml) reduced spontaneous wave breaks and increased the cycle length (CL) of VF from 83.3 +/- 17.8 ms at baseline to 118.4 +/- 25.8 ms during infusion (P < 0.05). Amiodarone increased the reentrant wave front CL (114.4 +/- 15.5 vs. 78.2 +/- 19.0 ms, P < 0.05) and central core area (4.1 +/- 3.8 vs. 0.9 +/- 0.3 mm2, P < 0.05). Within 30 min of infusion, VF terminated (n = 1), converted to ventricular tachycardia (VT) (n = 1) or continued at a slower rate (n = 4). Amiodarone flattened the APD restitution curves. We conclude that amiodarone reduced spontaneous wave breaks. It might terminate VF or convert VF to VT. These effects were associated with the flattening of APD restitution slope and increased core size of reentrant wave fronts.     

Diversity of K+ channels in circular smooth muscle of opossum lower esophageal sphincter

We previously demonstrated that a balance of K+ and Ca2+-activated Cl- channel activity maintained the basal tone of circular smooth muscle of opossum lower esophageal sphincter (LES). In the current studies, the contribution of major K+ channels to the LES basal tone was investigated in circular smooth muscle of opossum LES in vitro. K+ channel activity was recorded in dispersed single cells at room temperature using patch-clamp recordings. Whole-cell patch-clamp recordings displayed an outward current beginning to activate at -60 mV by step test pulses lasting 400 ms (-120 mV to +100 mV) with increments of 20 mV from holding potential of -80 mV ([K+]I = 150 mM, [K+]o = 2.5 mM). However, no inward rectification was observed. The outward current peaked within 50 ms and showed little or no inactivation. It was significantly decreased by bath application of nifedipine, tetraethylammonium (TEA), 4-aminopyridine (4-AP), and iberiotoxin (IBTN). Further combination of TEA with 4-AP, nifedipine with 4-AP, and IBTN with TEA, or vice versa, blocked more than 90% of the outward current. Ca2+-sensitive single channels were recorded at asymetrical K+ gradients in cell-attached patch-clamp configurations (100.8+/-3.2 pS, n = 8). Open probability of the single channels recorded in inside-out patch-clamp configurations were greatly decreased by bath application of IBTN (100 nM) (Vh = -14.4+/-4.8 mV in control vs. 27.3+/-0.1 mV, n = 3, P < 0.05). These data suggest that large conductance Ca2+-activated K+ and delayed rectifier K+ channels contribute to the membrane potential, and thereby regulate the basal tone of opossum LES circular smooth muscle.     

Existence of a transient outward K(+) current in guinea pig cardiac myocytes

A novel transient outward K(+) current that exhibits inward-going rectification (I(to.ir)) was identified in guinea pig atrial and ventricular myocytes. I(to.ir) was insensitive to 4-aminopyridine (4-AP) but was blocked by 200 micromol/l Ba(2+) or removal of external K(+). The zero current potential shifted 51-53 mV/decade change in external K(+). I(to.ir) density was twofold greater in ventricular than in atrial myocytes, and biexponential inactivation occurs in both types of myocytes. At -20 mV, the fast inactivation time constants were 7.7 +/- 1.8 and 6.1 +/- 1.2 ms and the slow inactivation time constants were 85.1 +/- 14.8 and 77.3 +/- 10.4 ms in ventricular and atrial cells, respectively. The midpoints for steady-state inactivation were -36.4 +/- 0.3 and -51.6 +/- 0.4 mV, and recovery from inactivation was rapid near the resting potential (time constants = 7.9 +/- 1.9 and 8.8 +/- 2.1 ms, respectively). I(to.ir) was detected in Na(+)-containing and Na(+)-free solutions and was not blocked by 20 nmol/l saxitoxin. Action potential clamp revealed that I(to.ir) contributed an outward current that activated rapidly on depolarization and inactivated by early phase 2 in both tissues. Although it is well known that 4-AP-sensitive transient outward current is absent in guinea pig, this Ba(2+)-sensitive and 4-AP-insensitive K(+) current has been overlooked.     

Effects of pituitary adenylate cyclase-activating polypeptide on canine atrial electrophysiology

We hypothesized that pituitary adenylate cyclase-activating polypeptide (PACAP) activates intracardiac postganglionic parasympathetic nerves and has a different effect than cervical vagal stimulation. We measured effective refractory period (ERP) and conduction velocity at four atrial sites [high right atrium (HRA), low right atrium (LRA), high left atrium (HLA), and low left atrium (LLA)] and minimum atrial fibrillation (AF) cycle length at 12 atrial sites during cervical vagal stimulation and after PACAP in 26 autonomically decentralized, open-chest, anesthetized dogs. PACAP shortened ERP to a similar extent at all four sites (HRA, 58 +/- 2.0 ms; LRA, 60 +/- 6.3 ms; HLA, 68 +/- 11.5 ms; and LLA, 60 +/- 8.3 ms). Low- and high-intensity vagal stimulation shortened ERP at the HRA, but not in the other atrial sites (low-intensity stimulation: HRA, 64 +/- 4.0 ms; LRA, 126 +/- 5.1 ms; HLA, 110 +/- 9.5 ms; and LLA, 102 +/- 11.5 ms; high-intensity stimulation: HRA, 58 +/- 4.2 ms; and HLA, 101 +/- 4.0 ms). Conduction velocity was not altered by any intervention. Minimum AF cycle length after PACAP was similar in both atria but was shorter in the right atrium than in the left atrium during vagal stimulation. After atropine administration, no interventions changed ERP. These results suggest that PACAP shortens atrial refractoriness uniformly in both atria through activation of intrinsic cardiac nerves, not all of which are activated by cervical vagal stimulation.     

Differential modulation of cardiac Ca2+ channel gating by beta-subunits

To investigate the mechanisms that increase ionic currents when Ca(2+) channels' alpha(1) subunits are co-expressed with the beta-subunits, we compared channel activity of Ca(V)1.2 (alpha(1C)) co-expressed with beta(1a) and beta(2a) in Xenopus oocytes. Normalized by charge movement, ionic currents were near threefold larger with beta(2a) than with beta(1a). At the single-channel level, the open probability (P(o)) was over threefold larger with beta(2a), and traces with high P(o) were more frequent. Among traces with P(o) > 0.1, the mean duration of burst of openings (MBD) were nearly twice as long for alpha(1C)beta(2a) (15.1 +/- 0.7 ms) than for alpha(1C)beta(1a) (8.4 +/- 0.5 ms). Contribution of endogenous beta(3xo) was ruled out by comparing MBDs with alpha(1C)-cRNA alone (4.7 +/- 0.1 ms) with beta(3xo) (14.3 +/- 1.1 ms), and with beta(1b) (8.2 +/- 0.5 ms). Open-channel current amplitude distributions were indistinguishable for alpha(1C)beta(1a) and alpha(1C)beta(2a), indicating that opening and closing kinetics are similar with both subunits. Simulations with constant opening and closing rates reproduced the microscopic kinetics accurately, and therefore we conclude that the conformational change-limiting MBD is differentially regulated by the beta-subunits and contributes to the larger ionic currents associated with beta(2a), whereas closing and opening rates do not change, which should reflect the activity of a separate gate.     

A partial agonist of the A(1)-adenosine receptor selectively slows AV conduction in guinea pig hearts

The use of full agonists of the A(1)-adenosine receptor (A(1)-ADOR) as antiarrhythmic agents is limited by their actions to cause high-grade atrioventricular (AV) block, profound bradycardia, atrial fibrillation, and vasodilation. It may be possible to avoid these undesired actions by use of partial agonists. We determined the effects of CVT-2759, a potential partial agonist of A(1)-ADORs, on guinea pig hearts. CVT-2759 (0.1-100 microM) increased the S-H interval of the isolated heart from 45 +/- 1 to 60 +/- 3 ms (P < 0. 01) with a half-maximal effect at 3.1 microM. CVT-2759 did not cause second-degree AV block. CVT-2759 significantly attenuated the actions of the full agonists N(6)-cyclopentyladenosine and adenosine. CVT-2759 caused a moderate slowing of atrial rate by 10 microM. In contrast, CVT-2759 was a full agonist to decrease cAMP content of rat adipocytes and Fischer rat thyroid line 5 cells. Results of radioligand binding assays indicated that CVT-2759 stabilized a high-affinity, G protein-coupled state of the A(1)-ADOR in membranes prepared from rat adipocytes but not in membranes prepared from the guinea pig brain. The results suggest that a weak A(1)-ADOR agonist, such as CVT-2759, may be useful to slow AV nodal conduction and thereby ventricular rate without causing AV block, bradycardia, atrial arrhythmias, or vasodilation.     

Effects of ion transport inhibitors on MCh-mediated secretion from porcine airway submucosal glands.

Submucosal glands secrete macromolecules and liquid that are essential for normal airway function. To determine the mechanisms responsible for airway gland secretion and the interaction between gland secretion and epithelial ion transport, studies were performed in porcine tracheal epithelia by using the hillocks and Ussing techniques. No significant baseline gland fluid flux (J(G)) was measured by the hillocks technique after 3 min, and the epithelia had an average potential difference of 7.5 +/- 0.5 mV (lumen negative) with a short-circuit current of 73 +/- 4 microA/cm(2), as measured by the Ussing technique. The secretagogue methacholine induced concentration-dependent increases in J(G) after 3 min from 0.003 microl. min(-1). cm(-2) at 0.1 microM to 0.41 +/- 0.04 microl. min(-1). cm(-2) at 1,000 microM, with a 0.9 +/- 0.1 mV hyperpolarization of the epithelium at 1,000 microM. When the epithelium was pretreated for 3 min with the sodium channel blocker amiloride, the methacholine (1,000 microM)-induced J(G) increased to 0.67 +/- 0.09 microl. min(-1). cm(-2), and the hyperpolarization increased to 2.2 +/- 0.5 mV over the amiloride-pretreated level. When pretreated for 3 min with the chloride channel blocker diphenylamine-2-carboxylic acid, the methacholine (1,000 microM)-induced J(G) was inhibited to 0.20 +/- 0.06 microl. min(-1). cm(-2), and the methacholine-induced hyperpolarization was abolished. These data indicate that, in porcine airways, methacholine-induced J(G) may be increased by inhibition of sodium absorption and decreased by inhibition of chloride secretion.     

Baroreflex modulation of peripheral vasoconstriction during progressive hypothermia in anesthetized humans

Mild hypothermia is a major concomitant of surgery under general anesthesia. We examined the hypothesis that baroreceptor loading/unloading modifies thermoregulatory peripheral vasoconstriction and, consequently, body core temperature in subjects undergoing lower abdominal surgery with general anesthesia. Thirty-six patients were divided into four groups: control group (C), applied positive end-expiratory pressure (PEEP; 10 cmH(2)O) group (P), applied leg-up position group (L), and a group of leg-up position patients with PEEP starting 90 min after induction of anesthesia (L + P). The esophageal temperature (T(es)) and the forearm-fingertip temperature gradient, as an index of peripheral vasoconstriction, were monitored for 3 h after induction of anesthesia. Mean arterial pressure and pulse pressure did not change during the study in any group. The change in right atrial transmural pressure from the baseline value was 0.3 +/- 0.1 mmHg in C, -3.0 +/- 0.5 mmHg in P, and 2.3 +/- 0.4 mmHg in L (P < 0.01). The change in T(es) at the end of the study was -1.7 +/- 0.1 (35.1 +/- 0.1) degrees C in C, -1.1 +/- 0.1 (35.7 +/- 0.1) degrees C in P, and -2.7 +/- 0.1 (34.1 +/- 0.1) degrees C in L, showing significant differences (P < 0.01). The T(es) threshold for thermal peripheral vasoconstriction was 35.6 +/- 0.1 degrees C in C, 36.2 +/- 0.2 degrees C in P, and 34.8 +/- 0.2 degrees C in L (P < 0.01). Excessive T(es) decrease in the leg-up-position operation was attenuated by applying PEEP (L + P group; P < 0.05). Our data indicate that baroreceptor loading augments and unloading prevents perioperative hypothermia in anesthetized and paralyzed subjects by reducing and increasing the body temperature threshold for peripheral vasoconstriction, respectively.     

Voltage-dependent K+ channels regulate the duration of reactive hyperemia in the canine coronary circulation

We previously demonstrated a role for voltage-dependent K(+) (K(V)) channels in coronary vasodilation elicited by myocardial metabolism and exogenous H(2)O(2), as responses were attenuated by the K(V) channel blocker 4-aminopyridine (4-AP). Here we tested the hypothesis that K(V) channels participate in coronary reactive hyperemia and examined the role of K(V) channels in responses to nitric oxide (NO) and adenosine, two putative mediators. Reactive hyperemia (30-s occlusion) was measured in open-chest dogs before and during 4-AP treatment [intracoronary (ic), plasma concentration 0.3 mM]. 4-AP reduced baseline flow 34 +/- 5% and inhibited hyperemic volume 32 +/- 5%. Administration of 8-phenyltheophylline (8-PT; 0.3 mM ic or 5 mg/kg iv) or N(G)-nitro-L-arginine methyl ester (L-NAME; 1 mg/min ic) inhibited early and late portions of hyperemic flow, supporting roles for adenosine and NO. 4-AP further inhibited hyperemia in the presence of 8-PT or L-NAME. Adenosine-induced blood flow responses were attenuated by 4-AP (52 +/- 6% block at 9 microg/min). Dilation of arterioles to adenosine was attenuated by 0.3 mM 4-AP and 1 microM correolide, a selective K(V)1 antagonist (76 +/- 7% and 47 +/- 2% block, respectively, at 1 microM). Dilation in response to sodium nitroprusside, an NO donor, was attenuated by 4-AP in vivo (41 +/- 6% block at 10 microg/min) and by correolide in vitro (29 +/- 4% block at 1 microM). K(V) current in smooth muscle cells was inhibited by 4-AP (IC(50) 1.1 +/- 0.1 mM) and virtually eliminated by correolide. Expression of mRNA for K(V)1 family members was detected in coronary arteries. Our data indicate that K(V) channels play an important role in regulating resting coronary blood flow, determining duration of reactive hyperemia, and mediating adenosine- and NO-induced vasodilation.     

Effects of thyroid hormone on action potential and repolarizing currents in rat ventricular myocytes

Thyroid hormones play an important role in cardiac electrophysiology through both genomic and nongenomic mechanisms of action. The effects of triiodothyronine (T(3)) on the electrophysiological properties of ventricular myocytes isolated from euthyroid and hypothyroid rats were studied using whole cell patch clamp techniques. Hypothyroid ventricular myocytes showed significantly prolonged action potential duration (APD(90)) compared with euthyroid myocytes, APD(90) of 151 +/- 5 vs. 51 +/- 8 ms, respectively. Treatment of hypothyroid ventricular myocytes with T(3) (0.1 microM) for 5 min significantly shortened APD by 24% to 115 +/- 10 ms. T(3) similarly shortened APD in euthyroid ventricular myocytes, but only in the presence of 4-aminopyridine (4-AP), an inhibitor of the transient outward current (I(to)), which prolonged the APD by threefold. Transient outward current (I(to)) was not affected by the acute application of T(3) to either euthyroid or hypothyroid myocytes; however, I(to) density was significantly reduced in hypothyroid compared with euthyroid ventricular myocytes.     

Transitions among atrial fibrillation, atrial flutter, and sinus rhythm during procainamide infusion and vagal stimulation in dogs with sterile pericarditis

The mechanism of atrial flutter and fibrillation induced by rapid pacing in 22 dogs with 3-day-old sterile pericarditis was investigated by computerized epicardial mapping of atrial activation before and after administration of agents known to modify atrial electrophysiologic properties: procainamide, isoproterenol, and electrical stimulation of the vagosympathetic trunks. Before the administration of any of these agents, a total of 30 episodes of sustained atrial flutter (greater than 1 min duration, monomorphic; regular cycle length, 127 +/- 12 ms, mean +/- SD) was induced in 15 out of 22 dogs and 9 episodes of unstable atrial flutter (duration, less than 1 min; cycle length, 129 +/- 34 ms; monomorphic, alternating with fibrillation) were induced in the remaining 7 preparations. In the latter, administration of procainamide transformed unstable atrial flutter and atrial fibrillation to sustained atrial flutter (cycle length, 142 +/- 33 ms; n = 9 episodes). During control atrial flutter, atrial maps displayed circus movement of excitation in the right atrial free wall with faster conduction parallel to the orientation of intra-atrial myocardial bundles. Vagal stimulation changed atrial flutter to atrial fibrillation in 32 of 73 trials; this was associated with acceleration of conduction in the lower right atrium, leading to fragmentation of the major wave front. Isoproterenol produced a 6-25% increase of the atrial rate in 6 out of 14 trials of atrial flutter and induced atrial fibrillation in 4. After procainamide, the reentrant pathway was lengthened and conduction was slowed further in the right atrium. Maps obtained during unstable atrial flutter showed incomplete circuits involving the right atrium. Following procainamide infusion, the area of functional dissociation or block was enlarged and a stable circus movement pattern, which was similar to the pattern seen in control atrial flutter, was established in the right atrium. We conclude that (1) the transitions among atrial fibrillation, atrial flutter, and sinus rhythm occur between different functional states of the same circus movement substratum primarily located in the lower right atrial free wall, and (2) the anisotropic conduction properties of the right atrium may contribute to these reentrant arrhythmias and may be potentiated by acute pericarditis.     

High sensitivity of the sheep pulmonary vein antrum to acetylcholine stimulation.

Isolation of the pulmonary vein antrum can terminate atrial fibrillation, but the rationale has not been elucidated. In the present study, we show that sheep atrial effective refractory period (ERP) was heterogeneously shortened by acetylcholine administration. After perfusion with 15 muM acetylcholine, the shortest ERP occurred in the pulmonary vein antrum, which was recorded with the standard intracellular microelectrode technique (the ERP results in the pulmonary vein antrum, left atrial posterior wall, roof, free wall and appendage, and right atrial free wall were 52.0 +/- 1.6, 75.1 +/- 2.0, 77.2 +/- 1.7, 85.6 +/- 1.7, 64.3 +/- 2.1, and 90.5 +/- 1.3 ms, respectively; P < 0.05). Immunofluorescent staining revealed that muscarinic type 2 receptors (M(2)R) were also distributed heterogeneously in the atrial myocardium, with the highest density in the antrum (the relative fluorescent intensity results of the M(2)R in the pulmonary vein antrum, left atrial posterior wall, roof, free wall and appendage, and right atrial free wall were 62.64 +/- 2.56, 53.12 +/- 2.76, 51.83 +/- 2.45, 47.90 +/- 2.33, 55.27 +/- 2.08, and 45.53 +/- 2.02, respectively; P < 0.05), which was in accordance with the heterogeneity of ERP distribution. Thus the pulmonary vein antrum is a unique electrophysiological region with high sensitivity to acetylcholine, and its intensive response to acetylcholine is most likely associated with the dense M(2)R distribution of this region. Such an acetylcholine-induced ERP heterogeneity is possibly a substrate for atrial fibrillation and hence one of the potential electrophysiological bases for the isolation therapy.     

Estimation of the total peripheral resistance baroreflex impulse response from spontaneous hemodynamic variability

We previously developed a mathematical analysis technique for estimating the static gain values of the arterial total peripheral resistance (TPR) baroreflex (G(A)) and the cardiopulmonary TPR baroreflex (G(C)) from small, spontaneous beat-to-beat fluctuations in arterial blood pressure, cardiac output, and stroke volume. Here, we extended the mathematical analysis so as to also estimate the entire arterial TPR baroreflex impulse response [h(A)(t)] as well as the lumped arterial compliance (AC). The extended technique may therefore provide a linear dynamic characterization of TPR baroreflex systems during normal physiological conditions from potentially noninvasive measurements. We theoretically evaluated the technique with respect to realistic spontaneous hemodynamic variability generated by a cardiovascular simulator with known system properties. Our results showed that the technique reliably estimated h(A)(t) [error = 30.2 +/- 2.6% for the square root of energy (E(A)), 19.7 +/- 1.6% for absolute peak amplitude (P(A)), 37.3 +/- 2.5% for G(A), and 33.1 +/- 4.9% for the overall time constant] and AC (error = 17.6 +/- 4.2%) under various simulator parameter values and reliably tracked changes in G(C). We also experimentally evaluated the technique with respect to spontaneous hemodynamic variability measured from seven conscious dogs before and after chronic arterial baroreceptor denervation. Our results showed that the technique correctly predicted the abolishment of h(A)(t) [E(A) = 1.0 +/- 0.2 to 0.3 +/- 0.1, P(A) = 0.3 +/- 0.1 to 0.1 +/- 0.0 s(-1), and G(A) = -2.1 +/- 0.6 to 0.3 +/- 0.2 (P < 0.05)] and the enhancement of G(C) [-0.7 +/- 0.44 to -1.8 +/- 0.2 (P < 0.05)] following the chronic intervention. Moreover, the technique yielded estimates whose values were consistent with those reported with more invasive and/or experimentally difficult methods.     

Refractoriness of the sheep superior vena cava myocardial sleeve

The cardiomyocytes in the superior vena cava (SVC) myocardial sleeve have distinct action potentials and ionic current profiles, but the refractoriness of these cells has not been reported. Using standard intracellular microelectrode techniques, we demonstrated in sheep that the effective refractory period (ERP) of the cardiomyocytes in the SVC (114.7 +/- 6.5 ms) is shorter than that in the inferior vena cava (IVC) (166.7 +/- 6.2 ms), right atrial free wall (RAFW) (201.0 +/- 6.0 ms) and right atrial appendage (RAA) (203.1 +/- 5.8 ms) (P < 0.05). The right atrial cardiomyocyte ERP was heterogeneously shortened by acetylcholine, a muscarinic type 2 receptor (M(2)R) agonist. After perfusion with 15 microM acetylcholine, the shortest ERP occurred in the SVC (the ERP in the SVC, IVC, RAFW and RAA was 53.6 +/- 2.7, 98.9 +/- 2.2, 121.8 +/- 6.0 and 109.7 +/- 5.1 ms, respectively; P < 0.05). Carbachol (1 microM), another M(2)R agonist, produced a similar effect as acetylcholine. Furthermore, we used methoctramine, a M(2)R blocker, 4-DAMP, a muscarinic type 3 receptor (M(3)R) blocker, and tropicamide, a muscarinic type 4 receptor (M(4)R) blocker to inhibit the acetylcholine-induced ERP shortening of SVC cardiomyocytes, and found that the 50% inhibitory concentration for methoctramine, 4-DAMP and tropicamide was 5.91, 45.72 and 80.34 nM, respectively. Therefore, we conclude that the sheep SVC myocardial sleeve is a unique electrophysiological region of the right atrium with the shortest ERP both under physiological condition and under cholinergic agonist stimulation. M(2)R might play a major role in the response of the SVC myocardial sleeve to parasympathetic nerve tone. The association between the distinct refractoriness in SVC and atrial fibrillation originating from the region deserves further investigation.