The mechanisms of the vulnerable window: the role of virtual electrodes and shock polarity

Vulnerability and defibrillation are mechanistically dependent upon shock strength, polarity, and timing. We have recently demonstrated that shock-induced virtual electrode polarization (VEP) may induce reentry. However, it remains unclear how the VEP mechanism may explain the vulnerable window and polarity dependence of vulnerability. We used a potentiometric dye and optical mapping to assess the anterior epicardial electrical activity of Langendorff-perfused rabbit hearts (n = 7) during monophasic shocks (+/-100 V and +/-200 V, duration of 8 ms) applied from a transvenous defibrillation lead at various coupling intervals. Arrhythmias were induced in a coupling interval and shock polarity dependent manner: (i) anodal and cathodal shocks induced arrhythmias in 33.2 +/- 30.1% and 53.1 +/- 39.3% cases (P < 0.01), respectively, and (ii) the vulnerable window was located near the T-wave. Optical maps revealed that VEP was also modulated by the coupling interval and shock polarity. Recovery of excitability produced by negative polarization, known as de-excitation, and the resulting reentry was more readily achieved during the relative refractory period than the absolute refractory period. Furthermore, anodal shocks produced wavefronts propagating in an inward direction with respect to the electrode, whereas cathodal shocks propagated in an outward direction. Wavefronts produced by anodal shocks were more likely to collide and annihilate each other than those caused by cathodal shocks. The probability of degeneration of the VEP-induced phase singularity into a sustained arrhythmia depends upon the gradient of VEP and the direction of the VEP-induced wavefront. The VEP gradient depends upon the coupling interval, while the direction depends upon shock polarity; these factors explain the vulnerable window and polarity-dependence of vulnerability, respectively.     

Comparative study of the space constant of electrotonic decay in the endocardium and epicardium of the rabbit right atrium

The distribution of the electronic potential in the endocardium and epicardium of the rabbit right atrium was studied. The distribution of the electronic potential was studied by the method of intracellular polarization of cardiac fibers via a suction electrode perfused from the inside with a KCl isotonic solution. The space constant of electronic decay along and across cardiac fibers in the endocardium and epicardium of rabbit right atrium was measured. It was shown that the space constant of electronic potential decay in rabbit the right atrium endocardium in the areas of ordered arrangement of trabecules both along (lambda x = 2117 +/- 653 microns) and across (lambda y = 394 +/- 212 microns) the fiber was higher (p < 0.001) than that in epicardium (lambda x and lambda y equal to 1361 +/- 486 microns and 212 +/- 63 microns, respectively). The values lambda x and lambda y do not significantly differ between separate areas of the epicardium. The degree of electrotonic anisotropy in all the structures investigated was almost the same, its value ranging from 2 to 17.     

Electric inhomogeneity in membranes of Characean internode influenced by light/dark transition, O2, N2, CO2-free air and extracellular pH

The electric membrane potential as functions of position and time of Characean internode has been studied using a modified water-film electrode technique. Between the low-conductance hyperpolarized region (called the H-region or acidic region) and the high-conductance depolarized region (D-region), there is a difference in the direction of responses to light-off and -on stimulations. In darkness the membrane potential becomes hyperpolarized in the D-region, whereas it is depolarized in the H-region at the steady state. The potential difference between D- and H-regions, delta Vm, is increased by exposure to pure O2, N2, or CO2-free air. When the amount of water surrounding the internode is limited, the formation of an electric pattern occurs rapidly. In contrast, the recovery is delayed. The membrane potential of the D-region is sometimes hyperpolarized significantly with lowering of the extracellular pH to 7.5, while the potential of the H-region is slightly depolarized. This seems to be an all-or-none type response. However, the electric profile is always homogenized with the pH of 6.8. Thus, the pH around 7.5 may be a threshold level to open/close putative OH- (or H+) channels of the D-region.     

Examination of stimulation mechanism and strength-interval curve in cardiac tissue

Understanding the basic mechanisms of excitability through the cardiac cycle is critical to both the development of new implantable cardiac stimulators and improvement of the pacing protocol. Although numerous works have examined excitability in different phases of the cardiac cycle, no systematic experimental research has been conducted to elucidate the correlation among the virtual electrode polarization pattern, stimulation mechanism, and excitability under unipolar cathodal and anodal stimulation. We used a high-resolution imaging system to study the spatial and temporal stimulation patterns in 20 Langendorff-perfused rabbit hearts. The potential-sensitive dye di-4-ANEPPS was utilized to record the electrical activity using epifluorescence. We delivered S1-S2 unipolar point stimuli with durations of 2-20 ms. The anodal S-I curves displayed a more complex shape in comparison with the cathodal curves. The descent from refractoriness for anodal stimulation was extremely steep, and a local minimum was clearly observed. The subsequent ascending limb had either a dome-shaped maximum or was flattened, appearing as a plateau. The cathodal S-I curves were smoother, closer to a hyperbolic shape. The transition of the stimulation mechanism from break to make always coincided with the final descending phase of both anodal and cathodal S-I curves. The transition is attributed to the bidomain properties of cardiac tissue. The effective refractory period was longer when negative stimuli were delivered than for positive stimulation. Our spatial and temporal analyses of the stimulation patterns near refractoriness show always an excitation mechanism mediated by damped wave propagation after S2 termination.     

Analysis of nerve conduction block induced by direct current

The mechanisms of nerve conduction block induced by direct current (DC) were investigated using a lumped circuit model of the myelinated axon based on Frankenhaeuser–Huxley (FH) model. Four types of nerve conduction block were observed including anodal DC block, cathodal DC block, virtual anodal DC block, and virtual cathodal DC block. The concept of activating function was used to explain the blocking locations and relation between these different types of nerve block. Anodal/cathodal DC blocks occurred at the axonal nodes under the block electrode, while virtual anodal/cathodal DC blocks occurred at the nodes several millimeters away from the block electrode. Anodal or virtual anodal DC block was caused by hyperpolarization of the axon membrane resulting in the failure of activating sodium channels by the arriving action potential. Cathodal or virtual cathodal DC block was caused by depolarization of the axon membrane resulting in inactivation of the sodium channel. The threshold of cathodal DC block was lower than anodal DC block in most conditions. The threshold of virtual anodal/cathodal blocks was about three to five times higher than the threshold of anodal/cathodal blocks. The blocking threshold was decreased with an increase of axonal diameter, a decrease of electrode distance to axon, or an increase of temperature. This simulation study, which revealed four possible mechanisms of nerve conduction block in myelinated axons induced by DC current, can guide future animal experiments as well as optimize the design of electrodes to block nerve conduction in neuroprosthetic applications.     

Cardiac excitation mechanisms, wavefront dynamics and strength-interval curves predicted by 3D orthotropic bidomain simulations

The assessment and understanding of cardiac excitation mechanisms is very important for the development and improvement of implantable cardiac devices, pacing protocols, and arrhythmia treatments. Previous bidomain simulation studies have investigated cathodal and anodal make/break mechanisms of cardiac excitation and strength-interval (S-I) curves in two-dimensional sheets or cylindrical domains, that by symmetry reduce to the two-dimensional case. In this work, cathodal and anodal S-I curves are studied by means of detailed bidomain simulations which include: (i) three-dimensional cardiac slabs; (ii) transmural fiber rotation; (iii) unequal orthotropic anisotropy of the conducting media; (iv) incorporation of funny and electroporation currents in the ventricular membrane model. The predicted shape of cathodal and anodal S-I curves exhibit the same features of the S-I curves observed experimentally and the break/make transition coincides with the final descending phase of the S-I curves. Away from the break/make transition, only the break or make excitation mechanism is observed independently of the stimulus strength, whereas within an interval at the break/make transition, new paradoxical excitation behaviors are observed that depend on the stimulus strength.     

Paced monophasic and biphasic waveforms alter transmembrane potentials and metabolism of human fibroblasts

Resting transmembrane potential (TMP) of primary human fibroblast cells was altered in predictable directions by subjecting cell cultures to specific monophasic and biphasic waveforms. Cells electrically stimulated with an anodal pulse resulted in hyperpolarization while a cathodal waveform depolarized the TMP to below that of non-paced control cells. The biphasic waveform, consisting of an anodal pulse followed immediately by an inverse symmetric cathodal pulse, also lessened the TMP similar to that of the cathodal pulse. The effect of short-term pacing on the TMP can last up to 4 h before the potentials equilibrate back to baseline. While subjecting the cells to this electrical field stimulation did not appear to damage the integrity of the cells, the three paced electrical stimulation waves inhibited expansion of the cultures when compared to non-paced control cells. With longer pacing treatments, elongation of the cells and electrotaxis towards the anodal polarity were observed. Pacing the fibroblasts also resulted in modest, yet very statistically significant (and likely underestimated) changes to cellular adenosine-5'-triphosphate (ATP) levels, and cells undergoing anodal and biphasic (anodal/cathodal) stimulation also exhibited altered mitochondrial morphology. These observations indicate an active role of electrical currents, especially with anodal content, in affecting cellular metabolism and function, and help explain accumulating evidence of cellular alterations and clinical outcomes in pacing of the heart and other tissues in general.     

Effects of anodal vs. cathodal pacing on the mechanical performance of the isolated rabbit heart.

Previous studies have suggested that anodal pacing enhances electrical conduction in the heart near the pacing site. It was hypothesized that enhanced conduction by anodal pacing would also enhance ventricular pressure in the heart. Left ventricular pressure measurements were made in isolated, Langendorff-perfused rabbit hearts by means of a Millar pressure transducer with the use of a balloon catheter fixed in the left ventricle. The pressure wave was analyzed for maximum pressure (Pmax) generated in the left ventricle and the work done by the left ventricle (Parea). Eight hearts were paced with monophasic square-wave pulses of varying amplitudes (2, 4, 6, and 8 V) with 100 pulses of each waveform delivered to the epicardium. Anodal stimulation pulses showed statistically significant improvement in mechanical response at 2, 4, and 8 V. Relative to unipolar cathodal pacing, unipolar anodal pacing improved Pmax by 4.4 +/- 2.3 (SD), 5.3 +/- 3.1, 3.5 +/- 4.9, and 4.8 +/- 1.9% at 2, 4, 6, and 8 V, respectively. Unipolar anodal stimulation also improved Parea by 9.0 +/- 3.0, 12.0 +/- 6.0, 10.1 +/- 7.7, and 11.9 +/- 6.0% at 2, 4, 6, and 8 V, respectively. Improvements in Pmax and Parea indicate that an anodally paced heart has a stronger mechanical response than does a cathodally paced heart. Anodal pacing might be useful as a novel therapeutic technology to treat mechanically impaired or failed hearts.     

Effects of electroporation on optically recorded transmembrane potential responses to high-intensity electrical shocks

The outcome of defibrillation shocks is determined by the nonlinear transmembrane potential (DeltaVm) response induced by a strong external electrical field in cardiac cells. We investigated the contribution of electroporation to DeltaVm transients during high-intensity shocks using optical mapping. Rectangular and ramp stimuli (10-20 ms) of different polarities and intensities were applied to the rabbit heart epicardium during the plateau phase of the action potential (AP). DeltaVm were optically recorded under a custom 6-mm-diameter electrode using a voltage-sensitive dye. A gradual increase of cathodal and well as anodal stimulus strength was associated with 1) saturation and subsequent reduction of DeltaVm; 2) postshock diastolic resting potential (RP) elevation; and 3) postshock AP amplitude (APA) reduction. Weak stimuli induced a monotonic DeltaVm response and did not affect the RP level. Strong shocks produced a nonmonotonic DeltaVm response and caused RP elevation and a reduction of postshock APA. The maximum positive and maximum negative DeltaVm were recorded at 170 +/- 20 mA/cm2 for cathodal stimuli and at 240 +/- 30 mA/cm2 for anodal stimuli, respectively (means +/- SE, n = 8, P = 0.003). RP elevation reached 10% of APA at a stimulus strength of 320 +/- 40 mA/cm2 for both polarities. Strong ramp stimuli (20 ms, 600 mA/cm2) induced a nonmonotonic DeltaVm response, reaching the same largest positive and negative values as for rectangular shocks. The transition from monotonic to nonmonotonic morphology correlates with RP elevation and APA reduction, which is consistent with cell membrane electroporation. Strong shocks resulted in propidium iodide uptake, suggesting sarcolemma electroporation. In conclusion, electroporation is a likely explanation of the saturation and nonmonotonic nature of cellular responses reported for strong electric stimuli.     

Mechanism of anode break stimulation in the heart.

Anodal stimulation is routinely observed in cardiac tissue, but only recently has a mechanism been proposed. The bidomain cardiac tissue model proposes that virtual cathodes induced at sites distant from the electrode initiate the depolarization. In contrast, none of the existing cardiac action potential models (Luo-Rudy phase I and II, or Oxsoft) predict anodal stimulation at the single-cell level. To determine whether anodal stimulation has a cellular basis, we measured membrane potential and membrane current in mammalian ventricular myocytes by using whole-cell patch clamp. Anode break responses can be readily elicited in single ventricular cells. The basis of this anodal stimulation in single cells is recruitment of the hyperpolarization-activated inward current I(f). The threshold of activation for I(f) is -80 mV in rat cells and -120 mV in guinea pig or canine cells. Persistent I(f) "tail" current upon release of the hyperpolarization drives the transmembrane potential toward the threshold of sodium channels, initiating an action potential. Time-dependent block of the inward rectifier, I(K1), at hyperpolarized potentials decreases membrane conductance and thereby potentiates the ability of I(f) to depolarize the cell on the break of an anodal pulse. Inclusion of I(f), as well as the block and unblock kinetics of I(K1), in the existing Luo-Rudy action potential model faithfully reproduces anode break stimulation. Thus active cellular properties suffice to explain anode break stimulation in cardiac tissue.     

Virtual electrodes and the induction of fibrillation in Langendorff-perfused rabbit ventricles: the role of intracellular calcium

A strong premature electrical stimulus (S(2)) induces both virtual anodes and virtual cathodes. The effects of virtual electrodes on intracellular Ca(2+) concentration ([Ca(2+)](i)) transients and ventricular fibrillation thresholds (VFTs) are unclear. We studied 16 isolated, Langendorff-perfused rabbit hearts with simultaneous voltage and [Ca(2+)](i) optical mapping and for vulnerable window determination. After baseline pacing (S(1)), a monophasic (10 ms anodal or cathodal) or biphasic (5 ms-5 ms) S(2) was applied to the left ventricular epicardium. Virtual electrode polarizations and [Ca(2+)](i) varied depending on the S(2) polarity. Relative to the level of [Ca(2+)](i) during the S(1) beat, the [Ca(2+)](i) level 40 ms after the onset of monophasic S(2) increased by 36+/-8% at virtual anodes and 20+/-5% at virtual cathodes (P<0.01), compared with 25+/-5% at both virtual cathode-anode and anode-cathode sites for biphasic S(2). The VFT was significantly higher and the vulnerable window significantly narrower for biphasic S(2) than for either anodal or cathodal S(2) (n=7, P<0.01). Treatment with thapsigargin and ryanodine (n=6) significantly prolonged the action potential duration compared with control (255+/-22 vs. 189+/-6 ms, P<0.05) and eliminated the difference in VFT between monophasic and biphasic S(2), although VFT was lower for both cases. We conclude that virtual anodes caused a greater increase in [Ca(2+)](i) than virtual cathodes. Monophasic S(2) is associated with lower VFT than biphasic S(2), but this difference was eliminated by the inhibition of the sarcoplasmic reticulum function and the prolongation of the action potential duration. However, the inhibition of the sarcoplasmic reticulum function also reduced VFT, indicating that the [Ca(2+)](i) dynamics modulate, but are not essential, to ventricular vulnerability.     

Effects of bicarbonate on resting potentials in mammalian skeletal muscle

In the absence of external HCO3, resting membrane potentials (Vm) in extensor digitorum longus muscle were depolarized as compared to the normal Vm in the presence of HCO3. Removal of Na or Cl form the HCO3-free media induced repolarization. In muscle in HCO3 buffer at 20 degrees C, internal K, Na, and Cl activities were analyzed with liquid ion selective microelectrodes. The averages were respectively, 119.7 +/- 2.1, 6.69 +/- 0.3, and 3.41 +/- 0.06 mM. In a high proportion of cells analyzed, the equilibrium potential for Cl was negative to Vm. Removing external HCO3, decreased internal K while internal Na and Cl increased. An increase in temperature and the application of HCO3 significantly lowered internal activities of both Na and Cl. Removal of HCO3 with temperature held constant caused a rapid depolarization, an increase in internal Na and Cl, and a decrease in internal K. Furosemide (10 microM) induced a repolarization of cells that were previously depolarized in the HCO3-free state, but the drug does not decrease internal Na.     

Mechanical and electrical effects of high-frequency and high-intensity stimulation of muscle

A unique tension response can be obtained by stimulating an isometrically held skeletal muscle or a single muscle fiber by a train of high-frequency pulses (2,000 pps) at higher-than-normal intensity, or by a long DC pulse. It is called the tetanoid response, and it is composed of three well-defined stages. Initially, tension develops rapidly, and mechanical output (Po) reaches about 0.35. Subsequently, this tension is maintained at a nearly steady level for the remainder of stimulation. After stimulation, a final increase of tension takes place. Intracellular electrical recordings show that the initial development of tension is elicited by two or three action potentials generated at the beginning of the stimulation, and that no additional action potentials are generated for the remainder of stimulation. During stimulation, part of the fiber membrane (regarded in cross-section) is depolarized, which generates tension, and part of the membrane is hyperpolarized. With termination of stimulation, a single action potential is elicited via anode-break excitation (ABE) on the hyperpolarized portion of the membrane, which gives rise to the final increase of tension.     

Nonselective cation channels in intact human T lymphocytes.

Nonselective cation channels were found in single channel recordings from cell-attached patches on human T lymphocytes. These channels were active under conditions that should lead to cell swelling (hypotonic bath solutions with NaCl or KCl); however, a definite dependence of activity on cell swelling has not been proven. Under these conditions similar channels were found in 20 of 23 patches from 11 different blood donors. The current-voltage relation was approximately linear for outward current (11-14 pS) and inwardly rectifying (to 23 pS) when the intact cells were depolarized with high KCl in the bath. The voltage dependence of channel activity is consistent with closing at hyperpolarized membrane potentials (Vm less than or equal to -50 mV) and block of open channels at strongly depolarized membrane potentials (Vm greater than 0 mV). Reversal potentials under all ionic gradients tested are consistent with a channel that is poorly selective between Na+ and K+ ions. Active channels in cell-attached patches were rapidly blocked by bath addition of the membrane-permeant inhibitor quinine. Channels that were active in cell-attached became quiescent after patch excision; however, two patches remained active long enough to obtain current-voltage relations. These were linear with a slope conductance for outward current of 8-11 pS. Because of the clustering of single-channel openings, detailed voltage dependence of kinetics and probability of opening were not studied.     

Electrochemical Regulation of Budding Yeast Polarity

Cells are naturally surrounded by organized electrical signals in the form of local ion fluxes, membrane potential, and electric fields (EFs) at their surface. Although the contribution of electrochemical elements to cell polarity and migration is beginning to be appreciated, underlying mechanisms are not known. Here we show that an exogenous EF can orient cell polarization in budding yeast (Saccharomyces cerevisiae) cells, directing the growth of mating projections towards sites of hyperpolarized membrane potential, while directing bud emergence in the opposite direction, towards sites of depolarized potential. Using an optogenetic approach, we demonstrate that a local change in membrane potential triggered by light is sufficient to direct cell polarization. Screens for mutants with altered EF responses identify genes involved in transducing electrochemical signals to the polarity machinery. Membrane potential, which is regulated by the potassium transporter Trk1p, is required for polarity orientation during mating and EF response. Membrane potential may regulate membrane charges through negatively charged phosphatidylserines (PSs), which act to position the Cdc42p-based polarity machinery. These studies thus define an electrochemical pathway that directs the orientation of cell polarization.     

Analysis of electric field stimulation of single cardiac muscle cells.

Electrical stimulation of cardiac cells by imposed extracellular electric fields results in a transmembrane potential which is highly nonuniform, with one end of the cell depolarized and the other end hyperpolarized along the field direction. To date, the implications of the close proximity of oppositely polarized membranes on excitability have not been explored. In this work we compare the biophysical basis for field stimulation of cells at rest with that for intracellular current injection, using three Luo-Rudy type membrane patches coupled together as a lumped model to represent the cell membrane. Our model shows that cell excitation is a function of the temporal and spatial distribution of ionic currents and transmembrane potential. The extracellular and intracellular forms of stimulation were compared in greater detail for monophasic and symmetric biphasic rectangular pulses, with duration ranging from 0.5 to 10 ms. Strength-duration curves derived for field stimulation show that over a wide range of pulse durations, biphasic waveforms can recruit and activate membrane patches about as effectively as can monophasic waveforms having the same total pulse duration. We find that excitation with biphasic stimulation results from a synergistic, temporal summation of inward currents through the sodium channel in membrane patches at opposite ends of the cell. Furthermore, with both waveform types, a net inward current through the inwardly rectifying potassium channel contributes to initial membrane depolarization. In contrast, models of stimulation by intracellular current injection do not account for the nonuniformity of transmembrane potential and produce substantially different (even contradictory) results for the case of stimulation from rest.     

大鼠颈动脉体球细胞缺氧时膜电位变化与球细胞大小的关系

用细胞内记录法测定了８５个分离培养的大鼠颈动脉体球细胞的膜电位,并由显微照相法记录球细胞的形态进而以测微器测量球细胞的直径。由连二亚硫酸钠（Ｎａ２Ｓ２Ｏ４）造成缺氧（ＰＯ２,１．３－８．０ｋＰａ）。不同直径的球细胞对缺氧有两种不同反应：直径为８．０４±１．０９μｍ的球细胞对缺氧的反应均为去极化,直径为１４．３８±４．２１μｍ的球细胞对缺氧反应为超极化。因此似可认为,球细胞存在功能不同的亚型。缺氧程度不同对球细胞膜电位的改变也有一定影响,缺氧程度严重可使小型球细胞的去极化程度增加,但缺氧程度的高低不能改变两型球细胞对缺氧反应的固有型式。     

Slowly adapting cutaneous mechanoreceptor afferent units associated with Merkel cells in frogs and effects of direct currents.

In the bullfrog, two types of slowly adapting (SA) cutaneous mechanoreceptor afferent units have been identified physiologically: irregularly discharging frog type I (Ft I) units in both warty and nonwarty skin, and regularly discharging frog type II (Ft II) units in the nonwarty skin. In the present study, mechanosensitive spots of Ft I units were located around the skin warts in the warty skin. The quinacrine technique (Crowe and Whitear, 1978) revealed that quinacrine-accumulating Merkel cells were present around the skin warts and near the orifice of skin glands that also surrounded the skin warts. Thus, a significant correlation was found between the location of Merkel cells and the receptive fields (RFs) of Ft I units in the warty skin. Direct current (DC) stimulation was applied for 1 sec to the skin inside and outside the mechanical RFs of the two types of SA units. RFs for DC stimulation were located on those for mechanical stimulation in both types of SA units. The current threshold required to produce a single spike was lower in cathodal than in anodal pulses in both types of SA units. Greater current intensity elicited an increased number of spikes, but the effective polarity of currents was anodal for Ft I units and cathodal for Ft II units. The optimal current intensity for producing prolonged discharges ranged from +60 to +100 microA in Ft I units and - from -50 to -80 microA in Ft II units. The sequence of impulses evoked was irregular in Ft I units and regular in Ft II units, as seen in mechanical responses.(ABSTRACT TRUNCATED AT 250 WORDS)     

Vascular smooth muscle cell membrane depolarization after NOS inhibition hypertension

Nitric oxide (NO) synthase (NOS) inhibition with N(omega)-nitro-L-arginine (L-NNA) produces L-NNA hypertensive rats (LHR), which exhibit increased sensitivity to voltage-dependent Ca(2+) channel-mediated vasoconstriction. We hypothesized that enhanced contractile responsiveness after NOS inhibition is mediated by depolarization of membrane potential (E(m)) through attenuated K(+) channel conductance. E(m) measurements demonstrated that LHR vascular smooth muscle cells (VSMCs) are depolarized in open, nonpressurized (-44.5 +/- 1.0 mV in control vs. -36.8 +/- 0.8 mV in LHR) and pressurized mesenteric artery segments (-41.8 +/- 1.0 mV in control vs. -32.6 +/- 1.4 mV in LHR). Endothelium removal or exogenous L-NNA depolarized control VSMCs but not LHR VSMCs. Superfused L-arginine hyperpolarized VSMCs from both the control and LHR groups and reversed L-NNA-induced depolarization (-44.5 +/- 1.0 vs. -45.8 +/- 2.1 mV). A Ca(2+)-activated K(+) channel agonist, NS-1619 (10 microM), hyperpolarized both groups of arteries to a similar extent (from -50.8 +/- 1.0 to -62.5 +/- 1.2 mV in control and from -43.7 +/- 1.1 to -55.6 +/- 1.2 mV in LHR), although E(m) was still different in the presence of NS-1619. In addition, superfused iberiotoxin (50 nM) depolarized both groups similarly. Increasing the extracellular K(+) concentration from 1.2 to 45 mM depolarized E(m), as predicted by the Goldman-Hodgkin-Katz equation. These data support the hypothesis that loss of NO activation of K(+) channels contributes to VSMC depolarization in L-NNA-induced hypertension without a change in the number of functional large conductance Ca(2+)-activated K(+) channels.     

Electromechanical properties of the single cell-layered heart of tunicate Boltenia ovifera (sea potato)

The tubular heart of the sea potato is composed of a single layer of myoepithelial cells interconnected near the extraluminal surface by specialized junctions. If these junctions are used as the border which separates the luminal from extraluminal membrane, the surface area ratio, luminal:extraluminal, is approximately 12:1. A single myofibril is located near the luminal surface in each cell. Current passed across the heart wall in the direction that depolarizes the luminal membrane and hyperpolarizes the extraluminal membrane immediately produces "all- or-none" action potentials and contractions. Current passed in the opposite direction fails to produce action potentials until after the break of the stimulus, suggesting anodal break excitation of the hyperpolarized luminal membrane. High potassium solutions depolarized the myoepithelium and produced contractions only when applied to the luminal surface of the heart. [Ca]0 increases and [Mg]0 decreases twitch tension only on the luminal surface of the heart. The transwall resistivity is low (50-100 omega/cm2) due to an extracellular shunt. Because of this shunt and the larger surface area of the luminal membrane, the extraluminal membrane is effectively clamped to the potential of the luminal membrane and is not capable of directly influencing excitation-contraction coupling. These findings suggest that only the luminal membrane of the sea potato myoepithelium is capable of generating an action potential and triggering contraction.