The conducted action potential. Models and comparison to experiments.

Propagation of the action potential is a complex process, and the relationships among the various factors involved in conduction have not been clear. We use three mathematical models of uniform conduction in a cable to clarify some of these relationships. One model is newly derived here, and two have been previously derived by Hunter et al. (1975, Prog. Biophys. Mol. Biol., 30:99-144). These models were able to simulate individual experimental action potential upstrokes previously obtained (Walton and Fozzard, 1983, Biophys. J., 44:1-8). The models were then utilized to provide relationships between measures of conduction. Among these were that maximal upstroke velocity (Vmax) is directly proportional to peak inward ionic current normalized by capacitance that is filled during the upstroke (I/Cf), and that conduction velocity was directly related to the square root of either Vmax or I/Cf. These relationships were shown to be model independent and to predict the experimental results, thus providing validated quantitative relationships that were not discernible through use of experimental data alone. The concept of safety factor was considered and a parameter was proposed that may be related to safety factor.     

The relation of Vmax to INa, GNa, and h infinity in a model of the cardiac Purkinje fiber.

The inward sodium current in cardiac muscle is difficult to study by voltage clamp methods, so various indirect experimental measures have been used to obtain insight into its characteristics. These methods depend on the relationship between maximal upstroke velocity of the action potential (Vmax) and the sodium current (INa), usually defined in terms of the Hodgkin-Huxley model. These relationships were explored using an adaptation of this model to cardiac Purkinje fibers. In general Vmax corresponded to INa, and it could be used to determine the relationship of membrane potential to GNa, and h infinity. The results, however, depended on the method of stimulation of the action potential, and an optimal stimulation method was determined. A commonly used experimental technique called "membrane responsiveness" was shown to distort seriously the properties of steady-state gating inactivation that is supposed to measure. Estimation of the changes in maximal sodium conductance, such as those produced by tetrodotoxin (TTX), would be accurately measured. Some experimental results have indicated a voltage-dependent effect of TTX. Characteristics of the measures of TTX effect under those conditions were illustrated. In summary, calculations with a model of the cardiac Purkinje fiber action potential provide insight into the accuracy of certain experimental methods using maximal upstroke velocity as a measure of INa, and cast doubt on other experimental methods, such as membrane responsiveness.     

Passive properties and membrane currents of canine ventricular myocytes

The membrane potential and membrane currents of single canine ventricular myocytes were studied using either single microelectrodes or suction pipettes. The myocytes displayed passive membrane properties and an action potential configuration similar to those described for multicellular dog ventricular tissue. As for other cardiac cells, in canine ventricular myocytes: (a) an inward rectifier current plays an important role in determining the resting membrane potential and repolarization rate; (b) a tetrodotoxin-sensitive Na current helps maintain the action potential plateau; and (c) the Ca current has fast kinetics and a large amplitude. Unexpected findings were the following: (a) in approximately half of the myocytes, there is a transient outward current composed of two components, one blocked by 4-aminopyridine and the other by Mn or caffeine; (b) there is clearly a time-dependent outward current (delayed rectifier current) that contributes to repolarization; and (c) the relationship of maximum upstroke velocity of phase 0 to membrane potential is more positive and steeper than that observed in cardiac tissues from Purkinje fibers.     

Membrane Characteristics of the Canine Papillary Muscle Fiber

Passive and active responses to intracellular and extracellular stimulation were studied in the canine papillary muscle. The electrotonic potential produced by extracellular polarization with the partition chamber method fitted the time course and the spatial decay expected from the cable theory (the time constant, 3.3 msec; the space constant, 1.2 mm). Contrariwise, spatial decay of the electrotonic potentials produced by intracellular polarization was very short and did not fit the decay curve expected for a simple cable, although only a small difference of time course in the electrotonic potentials produced by intracellular and extracellular polarizations was observed. A similar time course might result from the fact that when current flow results from intracellular polarization, the input resistance is less dependent on the membrane resistance. The foot of the propagated action potential rose exponentially with a time constant of 1.1 msec and a conduction velocity of 0.68 m/sec. The membrane capacity was calculated from the time constant of the foot potential and the conduction velocity to be 0.76 µF/cm². The responses of the papillary muscle membrane to intracellular stimulation differed from those to extracellular stimulation applied with the partition method in the following ways: higher threshold potential, shorter latency for the active response, linearity of the current-voltage relationship, and no reduction in the membrane resistance at the crest of the action potential during current flow.     

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).     

Effects of phlorizin and phloretin on passive and dynamic electrical properties in muscle membrane

The effects of phlorizin and phloretin on the cable properties were investigated in frog sartorius muscle by conventional cable analysis. Actions of phloretin on voltage-dependent ionic conductances were also studied by analysis of the phase plane trajectories. Both drugs evoked a significant decrease in specific membrane resistance (Rm) in chloride-containing Ringer's solution. The linear membrane capacitance increased by about 30%. On the contrary, in the presence of the non-penetrating anion, glutamate, a slight increase in Rm was induced by phlorizin. It is suggested that these drugs may increase the chloride conductance in the muscle membrane. Under the effect of phloretin the resting membrane potential remained unchanged but the amplitude of the action potential was lowered and the rate of repolarization was significantly reduced. The rate of depolarization during the "foot" of the action potential and the conduction velocity calculated from the rate constant of depolarization decreased. The maximum Na conductance was not altered by phloretin but K conductance was reduced. The time constant (tau K) reflecting the kinetic properties of K conductance was increased about seven-fold. It is suggested that great importance may be attributed to the dipole properties of these drugs in the actions presented above.     

Bistable nerve conduction

It has been demonstrated experimentally that slow and fast conduction waves with distinct conduction velocities can occur in the same nerve system depending on the strength or the form of the stimulus, which give rise to two modes of nerve functions. However, the mechanisms remain to be elucidated. In this study, we use computer simulations of the cable equation with modified Hodgkin-Huxley kinetics and analytical solutions of a simplified model to show that stimulus-dependent slow and fast waves recapitulating the experimental observations can occur in the cable, which are the two stable conduction states of a bistable conduction behavior. The bistable conduction is caused by a positive feedback loop of the wavefront upstroke speed, mediated by the sodium channel inactivation properties. Although the occurrence of bistable conduction only requires the presence of the sodium current, adding a calcium current to the model further promotes bistable conduction by potentiating the slow wave. We also show that the bistable conduction is robust, occurring for sodium and calcium activation thresholds well within the experimentally determined ones of the known sodium and calcium channel families. Since bistable conduction can occur in the cable equation of Hodgkin-Huxley kinetics with a single inward current, i.e., the sodium current, it can be a generic mechanism applicable to stimulus-dependent fast and slow conduction not only in the nerve systems but also in other electrically excitable systems, such as cardiac muscles.     

A model of slow conduction in bullfrog atrial trabeculae.

The success or failure of the propagation of electrical activity in cardiac tissue is dependent on both cellular membrane characteristics and intercellular coupling properties. This paper considers a linear arrangement of individual bullfrog atrial cells that are resistively coupled end to end to form a cylindrical strand. The strand, in turn, is encased by an endothelial sheath that provides a restricted extracellular space and an ion diffusion barrier to the outer bathing medium. This encased strand serves as an idealized model of an atrial trabeculum. Excitable membrane characteristics of the atrial cell are specified in terms of a Hodgkin-Huxley type of model that is quantitatively based on single-microelectrode voltage clamp data from bullfrog atrial myocytes. This membrane model can simulate the behavior of normal cells as well as of ischemic cells that exhibit depressed electrophysiological behavior (e.g., decreased resting potential, upstroke velocity, peak height, and action potential duration). Depressed activity can be easily simulated with variation of a single model parameter, the gain of the Na+/K+ pump current (INaK). Intercellular coupling properties are specified in terms of a lumped resistive T-type network between adjacent cells. The atrial strand model provides a means for studying the theoretical aspects of slow conduction in a "hybrid" strand that consists of a central region of cells having abnormal membrane or coupling properties, flanked on either side by normal atrial cells. Both uniform and discontinuous conduction are simulated by means of appropriate changes in the coupling resistance between cells. In addition, by varying either the degree of depressed electrical activity or the intercalated disc resistance in the central zone of the strand, slow conduction or complete conduction block in that region is demonstrated. Since the cellular model used in this study is based on experimental data and closely mimics both the atrial action potential and the underlying membrane currents, it has the potential to (1) accurately represent the current and voltage wave-forms occurring in the region of intercalated discs and (2) provide detailed information regarding the mechanisms in intercellular current spread in the region of slow conduction.     

Temperature effects on the Na and Ca currents in rat and hedgehog ventricular muscle

Cardiac transmembrane potentials and Na and Ca currents were recorded at different temperatures in rat and hedgehog ventricular muscle. At 35 degrees C in both species resting potential was about -80 mV and upstroke velocity (Vmax) of the action potential above 100 V/s. The shape of the action potential in hedgehog ventricular cells at 35 degrees C was similar to that in the rat showing a fast repolarization phase. When temperature was decreased, the membrane resting potential depolarized and action potential amplitude and Vmax declined. In rat ventricular cells at 10 degrees C, the resting potential was about -40 to -50 mV and Vmax was reduced to about 5 V/s. In hedgehog ventricular cells, however, the transmembrane potentials and Vmax were better maintained at low temperature. Phase 3 of the action potential was markedly prolonged below 20 degrees C in hedgehog but not in rat ventricular cells. When temperature was decreased to 10 degrees C the availability curve of the Na current shifted toward more negative potentials and ICa.peak declined in rat ventricular cells. In hedgehog cardiac preparations, the Na current was less influenced by the cooling and ICa.peak did not change very much at low temperatures. A transient inward current usually considered to induce cardiac arrhythmias could be recorded in rat ventricular cells below 20 degrees C but not in hedgehog preparations. These features of hedgehog cardiac membranes may contribute to the cold tolerance and the resistance to ventricular fibrillation during the hypothermia in mammalian hibernators.     

Voltage-activated currents in guinea pig pancreatic alpha 2 cells. Evidence for Ca2+-dependent action potentials

Glucagon-secreting alpha 2 cells were isolated from guinea pig pancreatic islets and used for electrophysiological studies of voltage- activated ionic conductances using the patch-clamp technique. The alpha 2 cells differed from beta cells in producing action potentials in the absence of glucose. The frequency of these potentials increased after addition of 10 mM arginine but remained unaffected in the presence of 5- 20 mM glucose. When studying the conductances underlying the action potentials, we identified a delayed rectifying K+ current, an Na+ current, and a Ca2+ current. The K+ current activated above -20 mV and then increased with the applied voltage. The Na+ current developed at potentials above -50 mV and reached a maximal peak amplitude of 550 pA during depolarizing pulses to -15 mV. The Na+ current inactivated rapidly (tau h approximately 0.7 ms at 0 mV). Half-maximal steady state inactivation was attained at -58 mV, and currents could no longer be elicited after conditioning pulses to potentials above -40 mV. The Ca2+ current first became detectable at -50 mV and reached a maximal amplitude of 90 pA (in extracellular [Ca2+] = 2.6 mM) at about -10 mV. Unlike the Na+ current, it inactivated little or not at all. Membrane potential measurements demonstrated that both the Ca2+ and Na+ currents contribute to the generation of the action potential. Whereas there was an absolute requirement of extracellular Ca2+ for action potentials to be elicited at all, suppression of the much larger Na+ current only reduced the upstroke velocity of the spikes. It is suggested that this behavior reflects the participation of a low-threshold Ca2+ conductance in the pacemaking of alpha 2 cells.     

Effect of nonuniform interstitial space properties on impulse propagation: a discrete multidomain model

This work presents a discrete multidomain model that describes ionic diffusion pathways between connected cells and within the interstitium. Unlike classical models of impulse propagation, the intracellular and extracellular spaces are represented as spatially distinct volumes with dynamic/static boundary conditions that electrically couple neighboring spaces. The model is used to investigate the impact of nonuniform geometrical and electrical properties of the interstitial space surrounding a fiber on conduction velocity and action potential waveshape. Comparison of the multidomain and bidomain models shows that although the conduction velocity is relatively insensitive to cases that confine 50% of the membrane surface by narrow extracellular depths (≥2 nm), the action potential morphology varies greatly around the fiber perimeter, resulting in changes in the magnitude of extracellular potential in the tight spaces. Results also show that when the conductivity of the tight spaces is sufficiently reduced, the membrane adjacent to the tight space is eliminated from participating in propagation, and the conduction velocity increases. Owing to its ability to describe the spatial discontinuity of cardiac microstructure, the discrete multidomain can be used to determine appropriate tissue properties for use in classical macroscopic models such as the bidomain during normal and pathophysiological conditions.     

Effect of diameter on membrane capacity and conductance of sheep cardiac Purkinje fibers

Membrane electrical properties were measured in sheep cardiac Purkinje fibers, having diameters ranging from 50 to 300 mum. Both membrane capacitance and conductance per unit area of apparent fiber surface varied fourfold over this range. Membrane time constant, and capacitance per unit apparent surface area calculated from the foot of the action potential were independent of fiber diameter, having average values of 18.8 +/- 0.7 ms, and 3.4 +/- 0.25 muF/cm2, respectively (mean +/- SEM). The conduction velocity and time constant of the foot of the action potential also appeared independent of diameter, having values of 3.0 +/- 0.1 m/s and 0.10 +/- 0.007 ms. These findings are consistent with earlier suggestions that in addition to membrane on the surface of the fiber, there exists a large fraction of membrane in continuity with the extracellular space but not directly on the surface of the fiber. Combining the electrical and morphological information, it was possible to predict a passive length constant for the internal membranes of about 100 mum and a time constant for chaning these membranes in a passive 100-mum fiber of 1.7 ms.     

Gap junction uncoupling and discontinuous propagation in the heart. A comparison of experimental data with computer simulations.

The effects of octanol on longitudinal propagation in guinea pig papillary muscles were measured by intracellular microelectrodes. These data were compared with alterations in conduction induced by stepwise removal of gap junction channels in computer simulations of propagation based on a discontinuous cable model. Octanol reduced the velocity (theta) of propagating action potentials (APs) from 53.2 +/- 3.5 to less than 6.6 +/- 2.1 cm/s before block occurred. The maximal rate of rise (Vmax) changed in a biphasic manner, increasing from 133.1 +/- 5.4 in controls to 201.7 +/- 11.0 V/s when theta was 20.5 +/- 2.8 cm/s, and then declining to less than 58.6 +/- 15.2 V/s just before block. The input resistance and time constant of the AP foot increased, and the ascending limb of phase-plane loops became increasingly nonlinear and notched during octanol treatment. All effects of octanol reversed upon washout. A strand of cardiac tissue was modeled as a discontinuous cable composed of 40 cells, each with 10 isopotential membrane segments described by Beeler-Reuter kinetics, and coupled by a variable number of gap junction channels (156 pS). Decreasing the number of channels from 40,000 to 400 to 60 slowed conduction from 62.6 to 16.4 to 3.1 cm/s. As noted in the experimental data, Vmax increased from 103 to 130 and then fell to less than 96 V/s. The AP foot increased and became nonexponential. Distinct notches developed during phase 1 of the APs at slower propagation velocities in the experiments and simulations. The close similarities between the experimental and theoretical data obtained in this study supports the applicability of a discontinuous cable model for describing longitudinal propagation in the heart.     

The Na+/K+ pump is an important modulator of refractoriness and rotor dynamics in human atrial tissue

Pharmacological treatment of atrial fibrillation (AF) exhibits limited efficacy. Further developments require a comprehensive characterization of ionic modulators of electrophysiology in human atria. Our aim is to systematically investigate the relative importance of ionic properties in modulating excitability, refractoriness, and rotor dynamics in human atria before and after AF-related electrical remodeling (AFER). Computer simulations of single cell and tissue atrial electrophysiology were conducted using two human atrial action potential (AP) models. Changes in AP, refractory period (RP), conduction velocity (CV), and rotor dynamics caused by alterations in key properties of all atrial ionic currents were characterized before and after AFER. Results show that the investigated human atrial electrophysiological properties are primarily modulated by maximal value of Na(+)/K(+) pump current (G(NaK)) as well as conductances of inward rectifier potassium current (G(K1)) and fast inward sodium current (G(Na)). G(NaK) plays a fundamental role through both electrogenic and homeostatic modulation of AP duration (APD), APD restitution, RP, and reentrant dominant frequency (DF). G(K1) controls DF through modulation of AP, APD restitution, RP, and CV. G(Na) is key in determining DF through alteration of CV and RP, particularly in AFER. Changes in ionic currents have qualitatively similar effects in control and AFER, but effects are smaller in AFER. The systematic analysis conducted in this study unravels the important role of the Na(+)/K(+) pump current in determining human atrial electrophysiology.     

Optical measurement of conduction in single demyelinated axons

Demyelination was initiated in Xenopus sciatic nerves by an intraneural injection of lysolecithin over a 2-3-mm region. During the next week macrophages and Schwann cells removed all remaining damaged myelin by phagocytosis. Proliferating Schwann cells then began to remyelinate the axons, with the first few lamellae appearing 13 d after surgery. Action potentials were recorded optically through the use of a potential-sensitive dye. Signals could be detected both at normal nodes of Ranvier and within demyelinated segments. Before remyelination, conduction through the lesion occurred in only a small fraction of the fibers. However, in these particular cases we could demonstrate continuous (nonsaltatory) conduction at very low velocities over long (greater than one internode) lengths of demyelinated axons. We have previously found through loose patch clamp experiments that the internodal axolemma contains voltage-dependent Na+ channels at a density approximately 4% of that at the nodes. These channels alone, however, are insufficient for successful conduction past the transition point between myelinated and demyelinated regions. Small improvements in the passive cable properties of the axon, adequate for propagation at this site, can be realized through the close apposition of macrophages and Schwann cells. As the initial lamellae of myelin appear, the probability of success at the transition zone increases rapidly, though the conduction velocity through the demyelinated segment is not appreciably changed. A detailed computational model is used to test the relative roles of the internodal Na+ channels and the new extracellular layer. The results suggest a possible mechanism that may contribute to the spontaneous recovery of function often seen in demyelinating disease.     

The Na conductance in the sarcolemma and the transverse tubular system membranes of mammalian skeletal muscle fibers

Na (and Li) currents and fluorescence transients were recorded simultaneously under voltage-clamp conditions from mouse flexor digitorum brevis fibers stained with the potentiometric dye di-8-ANEPPS to investigate the distribution of Na channels between the surface and transverse tubular system (TTS) membranes. In fibers rendered electrically passive, voltage pulses resulted in step-like fluorescence changes that were used to calibrate the dye response. The effects of Na channel activation on the TTS voltage were investigated using Li, instead of Na, because di-8-ANEPPS transients show anomalies in the presence of the latter. Na and Li inward currents (I(Na), I(Li); using half of the physiological ion concentration) showed very steep voltage dependences, with no reversal for depolarizations beyond the calculated equilibrium potential, suggesting that most of the current originates from a noncontrolled membrane compartment. Maximum peak I(Li) was ～ 30% smaller than for I(Na), suggesting a Li-blocking effect. I(Li) activation resulted in the appearance of overshoots in otherwise step-like di-8-ANEPPS transients. Overshoots had comparable durations and voltage dependence as those of I(Li). Simultaneously measured maximal overshoot and peak I(Li) were 54 ± 5% and 773 ± 53 μA/cm(2), respectively. Radial cable model simulations predicted the properties of I(Li) and di-8-ANEPPS transients when TTS access resistances of 10-20 Ω cm(2), and TTS-to-surface Na permeability density ratios in the range of 40:60 to 70:30, were used. Formamide-based osmotic shock resulted in incomplete detubulation. However, results from a subpopulation of treated fibers (low capacitance) provide confirmatory evidence that a significant proportion of I(Li), and the overshoot in the optical signals, arises from the TTS in normal fibers. The quantitative evaluation of the distribution of Na channels between the sarcolemma and the TTS membranes, as provided here, is crucial for the understanding of the radial and longitudinal propagation of the action potential, which ultimately govern the mechanical activation of muscle in normal and diseased conditions.     

A comparative study of collagenase- and trypsin-dissociated embryonic heart cells: reaggregation, electrophysiology, and pharmacology

The electrophysiological and pharmacological properties of aggregates prepared from cells of 7-day-old chick embryo heart ventricles depend on the enzyme used for cell dissociation. The mean beat rate of aggregates formed from trypsin-dissociated cells was about 53 beats/min whereas aggregates formed from collagenase-dissociated cells had a mean beat rate of more than twice this value. Spontaneous activity of most aggregates formed from trypsin-dissociated cells was inhibited by elevating external potassium or by adding tetrodotoxin to the medium. A similar response to potassium was seen in all aggregates formed from collagenase-dissociated cells. However, approximately half of the aggregates formed from collagenase-dissociated cells were tetrodotoxin insensitive. Intracellular microelectrode recordings demonstrated that aggregates formed from collagenase-dissociated cells typically had reduced action potential maximal upstroke velocities and depolarized threshold potentials in comparison to those recorded from aggregates formed from trypsin-dissociated cells. In the presence of tetrodotoxin the maximal upstroke velocity of aggregates formed from either collagenase- or trypsin-dissociated cells decreased markedly. In the case of the collagenase-treated cells, the spontaneous activity which persisted in the presence of tetrodotoxin was abolished by the slow channel blocker D-600. Computer simulation of membrane depolarization supports the view that aggregates formed from collagenase-treated cells have a reduced fast inward sodium current and a significant leakage current. Aggregates prepared from trypsin-dissociated cells display properties which more closely resemble those of intact 7-day embryonic ventricular tissue. We therefore conclude that, contrary to previous reports, collagenase is not the enzyme necessarily best suited for cell dissociation in all tissue culture studies.     

Model Studies of the Role of Mechano-sensitive Currents in the Generation of Cardiac Arrhythmias

Mechano-electrical feedback is studied by incorporating linear, instantaneously activating mechano-sensitive conductances into single cardiac cell models, as well as one- and two-dimensional cardiac network models. The models qualitatively reproduce effects of maintained mechanical stretch on experimentally measured action potential characteristics such as amplitude, maximum diastolic potential, peak upstroke velocity, and conduction velocity. Models are also used to simulate stretch-induced depolarizations, action potentials, and arrhythmias produced by pulsatile volume changes in left ventricle of dog. The mechano-sensitive conductance threshold for a stretch-induced action potential is closely related to the magnitude of the time-independent K⁺current,I_K1, which offsets inward mechano-sensitive current. Activation of mechano-sensitive conductances in small, spatially localized region of cells can evoke graded depolarizations, propagating ectopic beats, and if timed appropriately, spiral reentrant waves. Mechano-sensitive conductance changes required to evoke these responses are well within the physiologically plausible range. Results therefore indicate that many mechano-electrical feedback effects can be modeled using linear, instantaneously activating mechano-sensitive conductances. As an example of how stretch can occur in real human hearts, magnetic resonance images with saturation tagging are used to reconstruct the three-dimensional left ventricular wall motion. In patients with infarcts or recent ischemic events, “paradoxical deformation” is observed in that regions of myocardium are stretched rather than contracted during systole. In contrast, normal hearts contract uniformly with no stretch during systole. Paradoxical deformations in ischemic hearts may therefore present one possible substrate for the mechanically induced arrhythmias modeled above.     

Sodium nitroprusside increases pacemaker rhythm of sinoatrial nodes via nitric oxide-cGMP pathway

Effects of sodium nitroprusside (SNP), a nitric oxide donor, on the action potential in isolated guinea-pig sinoatrial nodes and ventricular papillary muscles were investigated. In the driven ventricular papillary muscle, SNP (10(-10)-10(-3) M) decreased the twitch tension in a concentration-dependent manner without significantly changing the configuration of action potential and the maximal velocity of depolarizing upstroke. In isolated sinoatrial nodes, SNP (10(-8)-10(-3) M) increased the pacemaker rhythm in a concentration-dependent manner. At 10(-5) M SNP, the pacemaker activity increased from 197.2+/-6.1 to 221.4+/-9.7 bpm. Changes of configuration of the action potential included a decrease of the duration of repolarization, i.e., from peak to the maximal diastolic potential (MDP), from 141.4+/-6.4 to 130.0+/-7.0 ms and an increase of the slope of the diastolic membrane potential from 101.6+/-5.3 to 116.5+/-7.3 mV/s (n=6, p<0.05). However, MDP and threshold potential were not significantly changed. Methylene blue (MB, 10(-5) M), a guanylate cyclase inhibitor, significantly decreased the pacemaker activity of the sinoatrial node by increasing the durations of repolarization and diastolic depolarization. After pretreatment with 10(-5) M MB, the effect of SNP was inhibited. The results indicate that nitric oxide, released from SNP, increases the pacemaker activity by enhancing the rates of repolarization and diastolic depolarization. These effects are possibly due to increases in delayed-rectifier K+ and diastolic slow inward currents, which are involved in a mechanism associated with the NO-cGMP pathway.