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The mouse is widely used as a genetic platform to investigate the molecular mechanisms of sinoatrial node (SAN) pacemaking. Recently, it has been shown that isolated SAN cells from the ankyrin-B (AnkB)-deficient mice display severe pacemaking dysfunction similar to individuals harboring ankyrin 2 allele variants. However, these results have been limited to isolated SAN cells only and thus did not evaluate the functional anatomy of the widely distributed atrial pacemaker complex (e.g., the dynamic interaction of primary and subsidiary pacemakers). We studied pacemaker function in an intact mouse atrial preparation, which included the SAN, atrioventricular junction (AVJ), and both atria, excluding most of the septum. Optical mapping with a voltage-sensitive dye and CMOS camera ULTIMA-L was used to map spontaneous pacemaker activity with or without autonomic modulation in wild-type (WT) mice (n = 7) and in the AnkB heterozygous ([AnkB.sup.+/-]; n = 9) mouse model of human SAN disease. In WT mice, isoproterenol accelerated the SAN rate (for 10 [micro]M: from 325 [+ or -] 19 to 510 [+ or -] 33 beat/min, P < 0.01) and shifted the leading pacemaker site superiorly by 0.77 [+ or -] 0.11 mm within the SAN. ACh decreased the SAN rate (from 333 [+ or -] 26 to 96 [+ or -] 22 beats/min, P < 0.01) and shifted the leading pacemaker either inferiorly within the SAN or abruptly toward the AVJ. After isoproterenol, [AnkB.sup.+/-] mice exhibited a larger beat-to-beat variability (SD of a cycle length: 13.4 [+ or -] 3.6 vs. 2.5 [+ or -] 0.8 ms, P < 0.01 vs. WT mice), disorganized shift of the leading pacemaker (2.04 [+ or -] 0.37 ram, P < 0.05 vs. WT mice), and competing multiple pacemakers, resulting in beat-to-beat changes of the leading pacemaker location site between the SAN and AVJ regions. Notably, [AnkB.sup.+/-] mice also displayed a reduced sensitivity to ACh (rate slowing by 32 [+ or -] 12% vs. 67 [+ or -] 4%, P < 0.05, [AnkB.sup.+/-] vs. WT mice, respectively). In conclusion, AnkB dysfunction results in SAN abnormalities in an isolated mouse atria preparation. While AnkB dysfunction dramatically alters single SAN cell function, the mechanisms underlying cardiac automaticity are clearly complex, and phenotypes may be partially compensated by the dynamic interaction of cells within the pacemaker complex. These new findings highlight the importance of the functional anatomy of the entire atrial distributed pacemaker complex, including the SAN and AVJ, and clearly demonstrate the role of AnkB in cardiac automaticity. pacemaker; autonomic control; action potential; sinoatrial node doi: 10.1152/ajpheart.00756.2009.

Classical numerical models have attributed the regulation of normal cardiac automaticity in sinoatrial node cells (SANCs) largely to G protein-coupled receptor (GPCR) modulation of sarcolemmal ion currents. More recent experimental evidence, however, has indicated that GPCR modulation of SANCs automaticity involves spontaneous, rhythmic, local [Ca.sup.2+] releases (LCRs) from the sarcoplasmic reticulure (SR). We explored the GPCR rate modulation of SANCs using a unique and novel numerical model of SANCs in which [Ca.sup.2+]-release characteristics are graded by variations in the SR [Ca.sup.2+] pumping capability, mimicking the modulation by phospholamban regulated by cAMP-mediated, PKA-activated signaling. The model faithfully predicted the entire range of physiological chronotropic modulation of SANCs by the activation of [beta]-adrenergic receptors or cholinergic receptors only when experimentally documented changes of sarcolemmal ion channels are combined with a simultaneous increase/ decrease in SR [Ca.sup.2+] pumping capability. The novel numerical mechanism of GPCR rate modulation is based on numerous complex synergistic interactions between sarcolemmal and intracellular processes via membrane voltage and [Ca.sup.2+]. Major interactions include changes of diastolic [Na.sup.+]/[Ca.sup.2+] exchanger current that couple earlier/ later diastolic [Ca.sup.2+] releases (predicting the experimentally defined LCR period shill) of increased/decreased amplitude (predicting changes in LCR signal mass, i.e, the product of LCR spatial size. amplitude, and number per cycle) to the diastolic depolarization and ultimately to the spontaneous action potential firing rate. Concomitantly, larger/smaller and more/less frequent activation of L-type [Ca.sup.2+] current shifts the cellular [Ca.sup.2+] balance to support the respective [Ca.sup.2+] cycling changes. In conclusion, our model simulations corroborate recent experimental results in rabbit SANCs pointing to a new paradigm for GPCR heart rate modulation by a complex system of dynamically coupled sarcolemmal and intracellular proteins. calcium; sarcoplasmic reticulum; ion channels; numerical modeling; sinoatrial node cell doi: 10.1152/ajpheart.00783.2009.

Peristaltic motor activity of the gut is an essential activity to sustain life. In each gut organ, a multitude of overlapping mechanisms has developed to acquire the ability of coordinated contractile activity under a variety of circumstances and in response to a variety of stimuli. The presence of several simultaneously operating control systems is a challenge for investigators who focus on the role of one particular control activity since it is often not possible to decipher which control systems are operating or dominant in a particular situation. A crucial advantage of multiple control systems is that gut motility control can withstand injury to one or more of its components. Our efforts to increase understanding of control mechanism are not helped by recent attempts to eliminate proven control systems such as interstitial cells of Cajal (ICC) as pacemaker cells, or intrinsic sensory neurons, nor does it help to view peristalsis as a simple reflex. This review focuses on the role of ICC as slow-wave pacemaker cells and places ICC into the context of other control mechanisms, including control systems intrinsic to smooth muscle cells. It also addresses some areas of controversy related to the origin and propagation of pacemaker activity. The urge to simplify may have its roots in the wish to see the gut as a consequence of a single perfect design experiment whereas in reality the control mechanisms of the gut are the messy result of adaptive changes over millions of years that have created complementary and overlapping control systems. All these systems together reliably perform the task of moving and mixing gut content to provide us with essential nutrients. interstitial cells of Cajal; motility; pacemaker; transit; slow waves

For disease states characterized by oscillatory ventilation, an ideal dynamic therapy would apply a counteracting oscillation in ventilation. Modulating respiratory gas transport through the circulation might allow this. We explore the ability of repetitive alternations in heart rate, using a cardiac pacemaker, to elicit oscillations in respiratory variables and discuss the potential for therapeutic exploitation. By incorporating acute cardiac output manipulations into an integrated mathematical model, we observed that a rise in cardiac output should yield a gradual rise in end-tidal C[O.sub.2] and, subsequently, ventilation. An alternating pattern of cardiac output might, therefore, create oscillations in C[O.sub.2] and ventilation. We studied the effect of repeated alternations in heart rate of 30 beats/rain with periodicity of 60 s, on cardiac output, respiratory gases, and ventilation in 22 subjects with implanted cardiac pacemakers and stable breathing patterns. End-tidal C[O.sub.2] and ventilation developed consistent oscillations with a period of 60 s during the heart rate alternations, with mean peak-to-trough relative excursions of 8.4 [+ or -] 5.0% (P < 0.0001) and 24.4 [+ or -] 18.8% (P < 0.0001), respectively. Furthermore, we verified the mathematical prediction that the amplitude of these oscillations would depend on those in cardiac output (r = 0.59, P = 0.001). Repetitive alternations in heart rate can elicit reproducible oscillations in end-tidal C[O.sub.2] and ventilation. The size of this effect depends on the magnitude of the cardiac output response. Harnessed and timed appropriately, this cardiorespiratory mechanism might be exploited to create an active dynamic responsive pacing algorithm to counteract spontaneous respiratory oscillations, such as those causing apneic breathing disorders. periodic breathing; physiology; pacing; reflex

The hypothesis of pacemaker level origin of thermal compensation in heart rate was tested by recording action potentials (AP) in intact sinoatrial tissue and enzymatically isolated pacemaker cells of rainbow trout acclimated at 4[degrees]C (cold) and 18[degrees]C (warm). With electrophysiological recordings, the primary pacemaker was located at the base of the sinoatrial valve, where a morphologically distinct ring of tissue comprising myocytes and neural elements was found by histological examination. Intrinsic beating rate of this pacemaker was higher in cold-acclimated (46 [+ or -] 6 APs/min) than warm-acclimated trout (38 [+ or -] 3 APs/min; P < 0.05), and a similar difference was seen in beating rate of isolated pacemaker cells (44 [+ or -] 6 vs. 38 [+ or -] 6 APs/min; P < 0.05), supporting the hypothesis that thermal acclimation modifies the intrinsic pacemaker mechanism of fish heart. Inhibition of sarcoplasmic reticulum (SR) with 10 [micro]M ryanodine and 1 [micro]M thapsigargin did not affect heart rate in either warm- or cold-acclimated trout at 11[degrees]C but reduced heart rate in warm-acclimated trout from 74 [+ or -] 2 to 42 [+ or -] 6 APs/min (P < 0.05) at 18[degrees]C. At 11[degrees]C, a half-maximal blockade of the delayed rectifier [K.sup.+] current ([I.sub.Kr]) with 0.1 [micro]M E-4031 reduced heart rate more in warm-acclimated (from 45 [+ or -] 1 to 24 [+ or -] 5 APs/min) than cold-acclimated trout (56 [+ or -] 3 vs. 48 [+ or -] 2 APs/min), whereas [I.sub.Kr] density was higher and AP duration less in cold-acclimated trout (P > 0.05). Collectively, these findings suggest that a cold-induced increase in AP discharge frequency is at least partly due to higher density of the [I.sub.Kr] in the cold-acclimated trout, whereas contribution of SR [Ca.sup.2+] release to thermal compensation of heart rate is negligible. cardiac pacemaker; heart rate; sarcoplasmic reticulum; delayed rectifier potassium current

Voltage-dependent sodium ([Na.sup.+]) channels are heterogeneously distributed through the pacemaker of the heart, the sinoatrial node (SA node). The measured sodium channel current ([i.sub.Na]) density is higher in the periphery but low or zero in the center of the SA node. The functional roles of [i.sub.Na] in initiation and conduction of cardiac pacemaker activity remain uncertain. We evaluated the functional roles of [i.sub.Na] by computer modeling. A gradient model of the intact SA node and atrium of the rabbit heart was developed that incorporates both heterogeneities of the SA node electrophysiology and histological structure. Our computations show that a large [i.sub.Na] in the periphery helps the SA node to drive the atrial muscle. Removal [i.sub.Na] from the SA node slows down the pacemaking rate and increases the sinoatrial node-atrium conduction time. In some cases, reduction of the SA node [i.sub.Na] results in impairment of impulse initiation and conduction that leads to the SA node-atrium conduction exit block. Decrease in active SA node cell population has similar effects. Combined actions of reduced cell population and removal of [i.sub.Na] from the SA node have greater impacts on weakening the ability of the SA node to pace and drive the atrium. sodium current; sinoatrial node; aging; dysfunction; conduction block; computer simulation

Intraventricular synchrony of cardiac activation is important for efficient pump function. Ventricular pacing restores the beating frequency but induces more asynchronous depolarization and more inhomogeneous contraction than in the normal heart. We investigated whether the increased inhomogeneity in the left ventricle can be described by a relatively simple mathematical model of cardiac electromechanics, containing normal mechanical and impulse conduction properties. Simulations of a normal heartbeat and of pacing at the right ventricular apex (RVA) were performed. All properties in the two simulations were equal, except for the depolarization sequence. Simulation results of RVA pacing on local depolarization time and systolic midwall circumferential strain were compared with those measured in dogs, using an epicardial sock electrode and MRI tagging, respectively. We used the same methods for data processing for simulation and experiment. Model and experiment agreed in the following aspects. 1) Ventricular pacing decreased systolic pressure and ejection fraction relative to natural sinus rhythm. 2) Shortening during ejection and stroke work declined in early depolarized regions and increased in late depolarized regions. 3) The relation between epicardial depolarization time and systolic midwall circumferential strain was linear and similar for the simulation (slope = -3.80 [+ or -] 0.28 [s.sup.-1], [R.sup.2] = 0.87) and the experiments [slopes for 3 animals -2.62 [+ or -] 0.43 [s.sup.-1] ([R.sup.2] = 0.59), -2.97 [+ or -] 0.38 [s.sup.-1] ([R.sup.2] = 0.69), and -4.44 [+ or -] 0.51 [s.sup.-1] ([R.sup.2] = 0.76)]. We conclude that our model of electromechanics is suitable to simulate ventricular pacing and that the apparently complex events observed during pacing are caused by well-known basic physiological processes. eikonal-diffusion equation; electromechanics; ventricular pacing; finite elements

Interstitial cells of Cajal (ICC) undergo marked morphological changes on contraction of the musculature, making it essential to understand properties of mechanosensitive ion channels. The whole cell patch-clamp technique was used to identify and to characterize volume-activated [Cl.sup.-] currents in ICC cultured through the explant technique. Hypotonic solutions ([approximately equal to] 210 mosM) activated an outwardly rectifying current, which reversed near the equilibrium potential for [Cl.sup.-]. Time-dependent inactivation occurred only at pulse potentials of +80 mV, with a time constant of 478 [+ or -] 182 ms. The degree of outward rectification was calculated using a rectification index, the ratio between the slope conductances of +65 and -55 mV, which was 13.9 [+ or -] 1.5 at 76 mM initial extracellular [Cl.sup.-] concentration. The sequence of relative anion permeability of the outwardly rectifying [Cl.sup.-] channel was [I.sup.-] > [Cl.sup.-] > [aspartate.sup.-]. The chloride channel blockers, DIDS and 5-nitro-2-(3-phenlypropl-amino)benzoic acid, caused a voltage-dependent block of the outwardly rectifying [Cl.sup.-] current, inhibition occurring primarily at depolarized potentials. On exposure to hypotonic solution, the slope conductance significantly increased at the resting membrane potential (-70 reV) from 1.2 [+ or -] 0.2 to 2.0 [+ or -] 0.4 nS and at the slow-wave plateau potential (-35 reV) from 2.1 [+ or -] 0.3 to 5.0 [+ or -] 1.0 nS. The current was constitutively active in ICC and contributed to the resting membrane potential and excitability at the slow-wave plateau. In conclusion, swelling or volume change will depolarize ICC through activation of outwardly rectifying chloride channels, thereby increasing cell excitability. gut motility; intestinal peristalsis; distension; pacemaker cells

The existence of a pacemaker system in the urinary tract capable of orchestrating the movement of filtrated urine from the ureteral pelvis to the distal ureter and lower urinary tract seems intuitive. The coordinated activity necessary for such movement or 'peristalsis' would likely require an intricate network of cells with pacemakerlike activity, as is the case with the interstitial cells of Cajal (ICC) of the gut. We investigated whether these putative pacemaker cells of the urinary tract are antigenically similar to ICC of the gut by using immunofluorescence staining for c-kit, a cell-surface marker specific for ICC. Ureteral, urinary bladder, and urethral tissues were harvested from female mice of the WBB6F1 strain, and fixed sections were prepared and stained for c-kit. Cell networks composed of stellate-appearing, c-kit-positive, ICC-like cells were found in the lamina propria and at the interface of the inner longitudinal and outer circular muscle layers of the ureteral pelvis but not in the urinary bladder or urethra. Thus, like in the gut, c-kit-positive, ICC-like cells are present in the urinary tract but appear to be restricted to the proximal ureter of this murine species. urine; urethral tissue; pacemaker; motility

The impact of right atrial pressure on the cardiac pacemaker response to vagal stimulation is examined in rats and rabbits. Cardiac pacemaker response is analyzed in situ at different values of right atrial pressure in isolated rat right atrium and in the rabbit heart. Results demonstrate that the level of right atrial pressure modulates the vagal bradycardia and indicate that normally right atrial pressure may interact with cardiac vagal function in heart rate regulation.

A study was conducted to determine whether the phase-response curves had a 'dead zone' of insensitivity to bright light during the subjective day. The core body temperature measured one to two days after the application of the stimulus and the modulation of the phase-shifting effects of the bright-light stimulus were also investigated in the human samples studied. Human circadian pacemaker was found to be sensitive to light throughout subjective day even with the absence of transients.

A transfer function analysis framework was developed to predict instantaneous heart rate (HR) based on sympathetic nerve activity (SNA). The goal was to provide the principle for the development of cardiac pacemakers directly regulated by cardiac sympathetic nerve activity. Tests on rabbits prove that the framework can accurately predict instantaneous HR response based on the decoding and transduction of SNA activity.

Research was conducted to examine the parasympathetic modulation of sinoatrial node pacemaker activity in rabbit heart. A compartmental model of the single rabbit sinoatrial node cell was extended to simulate cellular responses to bath applications of ACh and isoprenaline and the effects of neuronally released ACh. Results suggest that the model can be used over the full range of intensity and pattern of vagal input and can provided biophysically based explanations for various phenomena related to the autonomic control of cardiac pacemaking.