Ronald W. Joyner

Electrotonic influences on action potentials from isolated ventricular cells

This work combines a theoretical study of electrical interactions between two excitable heart cells, using a variable coupling resistance, with experimental studies on isolated rabbit ventricular cells coupled with a variable coupling resistance to a passive resistance and capacitance circuit. The theoretical results show that the response of an isolated cell to an increased frequency of stimulation is strongly altered by the presence of a coupling resistance to another cell. As the coupling resistance gradually is decreased, the stimulated cell becomes able to respond successfully to more rapid stimulation, and then, at levels of coupling resistance that allow conduction between the two cells, the coupled pair of cells exhibits arrhythmic interactions not predicted by the intrinsic properties of either cell. The experimental results show that the isolated rabbit ventricular cell is extremely sensitive to even a very small electrical load, with shortening of the action potential by 50% with electrical coupling to a model cell (of similar input resistance and capacitance to the ventricular cell) as high as 1,000 M omega, even though the action potential amplitude and current threshold are very insensitive to the electrical load.

Purkinje and ventricular activation sequences of canine papillary muscle. Effects of quinidine and calcium on the Purkinje-ventricular conduction delay.

We have studied in vitro preparations of canine right and left papillary muscles to determine the excitation sequences of the Purkinje and ventricular cells, using both monopolar surface electrodes and intracellular microelectrodes. Our results show, for right papillary muscles, that the Purkinje layer covers the basal part of the muscle, and that activation from the right bundle branch propagates over all of the Purkinje layer, but directly activates the underlying ventricular layer only at specific junctional sites. Left papillary muscles have attachments to both apical and basal Purkinje strands and the Purkinje layer covers the entire muscle, but, as for right papillary muscles, activation from the Purkinje layer to the ventricular layer occurs only at basal junctional sites. Antidromic conduction in papillary muscles (propagation from the ventricular layer to the Purkinje layer) can occur at regions other than the specific sites through which the Purkinje layer activates the ventricular layer. At the identified junctional sites, the Purkinje cell action potential duration is significantly shorter than in the free-running strand, but it remains longer than that of the ventricular cells. The time delay at the junctional sites is increased by quinidine, increased calcium concentration, and increased pacing frequency.

Developmental changes in calcium currents of rabbit ventricular cells.

We investigated the postnatal development of L-type Ca2+ current (ICa) in enzymatically isolated adult (AD) and newborn (NB) (1-3-day-old) rabbit ventricular cells using the whole-cell, patch-clamp method. ICa was recorded with Cs(+)-rich pipettes and a Na(+)- and K(+)-free bath solution at 36 degrees C to eliminate other currents. ICa density (obtained by normalizing ICa to the cell capacitance) was significantly higher in AD cells than in NB cells at potential levels between 0 and +50 mV with 1.8 mM Ca2+ as the charge carrier. There was no shift in the current-voltage relation between AD and NB cells. The maximum ICa density was 9.9 +/- 2.0 pA/pF at 14 +/- 5 mV in AD cells (n = 11) compared with 5.6 +/- 2.0 pA/pF at 13 +/- 5 mV in NB cells (n = 7) (mean +/- SD). Time to half inactivation (T 1/2) showed a nearly U-shaped relation to membrane potentials from -10 to +30 mV with the shortest T 1/2 at the potential giving the maximum ICa density in both groups. T 1/2 at 0 and +10 mV was slightly but significantly longer in NB cells (16.8 +/- 4.6 and 13.5 +/- 2.4 msec, respectively) than in AD cells (12.6 +/- 3.0 and 10.6 +/0 1.5 msec).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of anisotropy on the development of cardiac arrhythmias associated with focal activity

Abstract. The anisotropy that normally exists in the myocardium may be either enhanced in peri-infarction zones by loss of lateral cell connections or reduced by redistribution of gap junctions. To test how the degree of anisotropy affects the development of ectopic focal activity, we carried out computer simulations in which a model of an ectopic focus is incorporated as the central element of a two-dimensional sheet of ventricular cells. At low values of intercellular coupling conductance (Gc), the focus region is spontaneously active, but the limited intercellular current flow inhibits propagation. At high Gc, automaticity is suppressed by the loading effects of the surrounding cells. At intermediate Gc, the ectopic activity may propagate into the sheet. In the case of isotropic coupling, the minimum size of the focus region for propagation to occur (in terms of number of collaborating cells within the focus) is as small as approximately ten cells, and this number decreases with increasing anisotropy. Thus, the presence of anisotropy facilitates the development of ectopic focal activity. We conclude that the remodeling that occurs in peri-infarction zones may create a substrate that either facilitates (enhanced anisotropy) or inhibits (reduced anisotropy) the development of cardiac arrhythmias associated with ectopic focal activity.

Variations in the functional electrical coupling between the subendocardial Purkinje and ventricular layers of the canine left ventricle.

Action potential propagation from the subendocardial Purkinje network into the ventricular muscle is an essential link in cardiac activation. Studies of papillary muscles have indicated that ventricular muscle activation by the Purkinje network occurs only at discrete, localized regions near the papillary muscle base. Over the rest of the endocardial surface, however, the spatial distribution of these subendocardial Purkinje to ventricular muscle connections has been less well defined. We therefore studied in vitro 12 canine left ventricular preparations (eight from the septum, four from the lateral wall), using a high-density (1-mm spacings), high-resolution extracellular mapping technique to determine the subendocardial Purkinje and ventricular muscle activation sequences. These studies show that the distribution of subendocardial Purkinje to ventricular muscle electrical coupling is spatially inhomogeneous, and that the junctional regions themselves have variable degrees of electrical coupling. We also attempted to determine whether ventricular muscle coupling to the Purkinje network might influence Purkinje network conduction velocity. We found that on the papillary muscle apex, a region without direct Purkinje to ventricular muscle propagation, Purkinje network conduction velocity was slowed, suggesting that the Purkinje network might be electrically loaded by the underlying ventricular muscle. Finally, we performed numerical simulations using a model consisting of two layers of excitable cells to evaluate the effects that different electrical coupling patterns and/or different coupling resistivities between the two layers might have on activation of each layer. These simulation studies suggest that a coupling pattern having discrete junctional sites between the two layers (similar to our findings for subendocardial Purkinje to ventricular muscle coupling) is beneficial, as this arrangement allows more rapid activation of both layers by minimizing electrical loading of the thin Purkinje layer by the thicker ventricular muscle layer.

Propagation through electrically coupled cells. Effects of a resistive barrier

Action potential propagation through cardiac tissue occurs in a spatially inhomogeneous three-dimensional electrical syncytium composed of discrete cells with regional variations in membrane properties and intercellular resistance. In comparison with axons, cardiac tissue presents some differences in the application of core conductor cable theory. We have used analytical and numerical techniques to contrast the propagation of action potentials along nerve axons and along cardiac strands, including an explicit inclusion of cellular anatomical factors (the surface-to-volume ratio), the strand radius, and the regional distribution of longitudinal resistance. A localized decrease in the number of gap junctions will produce a functional resistive barrier, which can lead to unidirectional block of propagation if the tissue on two sides of the barrier in either excitability or passive electrical load. However, in some circumstances, a resistive barrier separating regions of different electrical load can actually facilitate propagation into the region of larger electrical load.

Propagation through electrically coupled cells. Effects of regional changes in membrane properties.

The normal process of excitation of the heart involves propagation of action potentials through cardiac regions of different anatomy and different intrinsic membrane properties. Although our understanding of these properties is still incomplete, it is well accepted that the parameters measured from a single cell penetration in an electrical syncytium (e.g., action potential duration, rate of rise, and velocity) reflect not only the properties of that cell but also the electrotonic interactions with other cells to which the recorded cell is electrically coupled. We have used simulation techniques to predict the spatial distribution of action potential parameters resulting from discretely localized alterations in the intrinsic membrane properties of some of the cells of an electrical syncytium. We have shown that the resulting spatial distribution is markedly different for alterations in plateau and pacemaker currents vs. rising phase currents, and that other factors, such as the site of stimulation and the underlying spatial pattern of cell-cell coupling resistance, also modify the spatial distribution of action potential properties resulting from a discrete regional change in intrinsic membrane properties.

Developmental changes in the beta-adrenergic modulation of calcium currents in rabbit ventricular cells.

We studied the developmental changes in the beta-adrenergic modulation of L-type calcium current (ICa) in enzymatically isolated adult (AD) and newborn (NB, 1-4-day-old) rabbit ventricular cells using the whole-cell patch-clamp method. ICa was measured as the peak inward current at a test potential of +15 mV by applying a 180-450-msec pulse from a holding potential of -40 mV with Cs(+)-rich pipettes and a K(+)-free bath solution at room temperature. In control, ICa density (obtained by normalizing ICa to the cell capacitance) was significantly higher in AD cells (5.5 +/- 0.2 [mean +/- SEM] pA/pF, n = 65) than in NB cells (2.6 +/- 0.1 pA/pF, n = 60). Isoproterenol (ISO, 1 nM-30 microM) increased ICa in a dose-dependent manner for both groups. The maximal effect (Emax) of ISO, expressed as percent increase in ICa over control levels, and the concentration for one half of the maximal effect (EC50) were 203% and 51 nM, respectively, for AD cells and 111% and 81 nM, respectively, for NB cells. The effect of ISO (1 microM) on ICa was decreased as the test potential was increased from -10 to +40 mV. However, the ratio of the percent increase in ICa for AD versus NB cells was almost constant (2.09-2.45) at each test potential. Dose-response curves of forskolin (FOR, 0.3-50 microM) gave Emax and EC50 of 268% and 0.74 microM, respectively, for AD cells and 380% and 1.15 microM, respectively, for NB cells. After stimulating ICa by 10 microM ISO, the addition of 10 microM FOR produced a further increase in ICa of only 12 +/- 2% in AD cells (n = 4) but a further increase of 140 +/- 41% in NB cells (n = 6). FOR (10 microM) did not produce any increase in ICa for AD and NB cells after stimulating ICa by intracellular application of 200 microM cAMP. ICa density stimulated by 10 microM ISO (17.8 +/- 1.1 pA/pF, n = 7), 10 microM FOR (21.0 +/- 1.3 pA/pF, n = 8), or 200 microM cAMP (18.0 +/- 1.3 pA/pF, n = 5) was equivalent in AD cells, whereas ICa density stimulated by 10 microM ISO (5.8 +/- 0.6 pA/pF, n = 9) was significantly lower than that stimulated by either 10 microM FOR (13.8 +/- 1.5 pA/pF, n = 7) or 200 microM cAMP (13.4 +/- 0.7 pA/pF, n = 7) in NB cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Determinants of action potential initiation in isolated rabbit atrial and ventricular myocytes

Action potential conduction through the atrium and the ventricle of the heart depends on the membrane properties of the atrial and ventricular cells, particularly with respect to the determinants of the initiation of action potentials in each cell type. We have utilized both current- and voltage-clamp techniques on isolated cells to examine biophysical properties of the two cell types at physiological temperature. The resting membrane potential, action potential amplitude, current threshold, voltage threshold, and maximum rate of rise measured from atrial cells (-80 ± 1 mV, 109 ± 3 mV, 0.69 ± 0.05 nA, -59 ± 1 mV, and 206 ± 17 V/s, respectively; means ± SE) differed significantly ( P < 0.05) from those values measured from ventricular cells (-82.7 ± 0.4 mV, 127 ± 1 mV, 2.45 ± 0.13 nA, -46 ± 2 mV, and 395 ± 21 V/s, respectively). Input impedance, capacitance, time constant, and critical depolarization for activation also were significantly different between atrial (341 ± 41 MΩ, 70 ± 4 pF, 23.8 ± 2.3 ms, and 19 ± 1 mV, respectively) and ventricular (16.5 ± 5.4 MΩ, 99 ± 4.3 pF, 1.56 ± 0.32 ms, and 36 ± 1 mV, respectively) cells. The major mechanism of these differences is the much greater magnitude of the inward rectifying potassium current in ventricular cells compared with that in atrial cells, with an additional difference of an apparently lower availability of inward Na current in atrial cells. These differences in the two cell types may be important in allowing the atrial cells to be driven successfully by normal regions of automaticity (e.g., the sinoatrial node), whereas ventricular cells would suppress action potential initiation from a region of automaticity (e.g., an ectopic focus).

Mechanisms of unidirectional block in cardiac tissues

We used numerical solutions for cable equations representing nonuniform cardiac strands to investigate possible mechanisms of unidirectional block (UB) of action potential propagation. Because the presence of UB implies spatial asymmetry in some property along the strand, we varied membrane properties (gNa or leakage conductance), cell diameter, or intercellular resistance as functions of distance such that a propagating action potential encountered the parameter changes either gradually or abruptly. For changes in membrane properties there was very little difference in the effects on propagation for the gradual or abrupt encounter; but, for changes in cell diameter or in intercellular resistance, there were large differences leading to the production of UB over a wide range of parameter values.