PubMed Health⌕ Search

Biomedical subjects

H A Fozzard

Publications and source records attributed to H A Fozzard.

At least 19 recordsLinked to original sources
PubMed1992

A mutant of TTX-resistant cardiac sodium channels with TTX-sensitive properties.

The cardiac sodium channel alpha subunit (RHI) is less sensitive to tetrodotoxin (TTX) and saxitoxin (STX) and more sensitive to cadmium than brain and skeletal muscle (microliter) isoforms. An RHI mutant, with Tyr substituted for Cys at position 374 (as in microliter) confers three properties of TTX-sensitive channels: (i) greater sensitivity to TTX (730-fold); (ii) lower sensitivity to cadmium (28-fold); and (iii) altered additional block by toxin upon repetitive stimulation. Thus, the primary determinant of high-affinity TTX-STX binding is a critical aromatic residue at position 374, and the interaction may take place possibly through an ionized hydrogen bond. This finding requires revision of the sodium channel pore structure that has been previously suggested by homology with the potassium channel.

Amino Acid Sequence↗
PubMed1992

Mechanisms of pharmacologic intervention at the level of the calcium channel.

Calcium channels are large, complex membrane proteins that mediate transmembrane calcium currents. At least 2 kinds of calcium channels are found in heart muscle--the transient type and the long-lasting type. Calcium currents are modulated by diverse endogenous and exogenous factors including hormones, catecholamines, and calcium antagonists. Calcium antagonists act preferentially on vascular smooth muscle and have relatively less effect on the calcium channels of heart muscle. Compared with heart muscle, vascular smooth muscle is relatively depolarized, suggesting that vascular smooth muscle cells have predominantly the long-lasting type of calcium currents. The differential binding to different types of calcium channels underlies the clinical efficacy of the calcium antagonists. A drug such as bepridil, which acts preferentially on the coronary vasculature rather than on the peripheral vasculature, dilates the coronary vessels without depressing cardiac contraction, a putative clinical advantage.

Animals↗
PubMed1992

Afterdepolarizations and triggered activity.

One of the possible cellular mechanisms for certain types of ventricular arrhythmias is afterdepolarizations. There are two types of afterdepolarization. The delayed afterdepolarization (DAD) arises from the resting potential after full repolarization of an action potential and it may reach threshold for activation. It is favored by cellular Ca overload, and rapid preceding activation rates. The inward current generating the DAD is caused by one of two mechanisms: a Ca-dependent opening of non-specific cation channels, or Ca activation of a rheogenic Na/Ca exchange. The early afterdepolarization (EAD) arises on the shoulder of a preceding action potential plateau and it is favored by slow preceding activation rate and prolonged action potentials. Ca channels are usually responsible for the inward current for EAD's, and cellular Ca overload is not related. These afterdepolarizations have characteristics that suggest their etiological role in certain arrhythmias found in heart failure.

Animals↗
PubMed1992

The cloned cardiac Na channel alpha-subunit expressed in Xenopus oocytes show gating and blocking properties of native channels.

The neonatal rat cardiac Na channel alpha-subunit directed currents in oocytes show characteristic cardiac relative resistance to tetrodotoxin (TTX) block. TTX-sensitive currents obtained by expression in Xenopus oocytes of the alpha-subunits of the rat brain (BrnIIa) and adult skeletal muscle (microI) Na channels show abnormally slow decay kinetics. In order to determine if currents directed by the cardiac alpha-subunit (RHI) exhibit kinetics in oocytes like native currents, we compared RHI-directed currents in oocytes to Na currents in freshly isolated neonatal rat myocytes. The decay rate of RHI currents approached that of neonatal myocytes and was faster than BrnIIa and microI currents in oocytes. The voltage dependence of availability and activation was the same as that in the rat myocytes except for a 12-19 mV shift in the depolarizing direction. The RHI Na currents were sensitive to Cd2+ block, and they showed use dependence of TTX and lidocaine block similar to native currents. The current expressed in oocytes following injection of the cRNA encoding for the alpha-subunit of the cardiac Na channel possesses most of the characteristic kinetic and pharmacological properties of the native cardiac Na current.

Animals↗
PubMed1992

The ECG and the single channel.

Our ability to understand and use the ECG has increased with the availability of tools to study the heart's electrical system. Great advances were achieved with direct electrical recordings from intact animal and human hearts. The microelectrode opened the way to recording of the cellular action potentials and their underlying currents. We now have two new and powerful methods to study cardiac electrophysiology--the patch clamp and molecular biology. We have begun to characterize the behavior of the elementary unit of membrane current, the single channel. The single channel can best be visualized as existing in a finite set of states related sequentially as in a Markov chain. The primary structures of several channels have now been determined by cloning, and the structural determinants of channel function can be explored. We have entered the molecular age of electrophysiology and can expect a greater understanding of the basic physiological and pathophysiological processes underlying the ECG. We can also expect powerful drugs to be designed based on the structures of their channel targets. The ECG remains a rich area for study and an ever better clinical tool.

Animals↗
PubMed1992

Phosphorylation restores activity of L-type calcium channels after rundown in inside-out patches from rabbit cardiac cells.

1. Rundown of L-type calcium channels was studied in inside-out patches made from single isolated rabbit ventricular myocytes, using barium as the charge carrier. 2. In the cell-attached patches single-channel activity was stable for more than 15 min after the patch pipette sealed. beta-Receptor stimulation by isoprenaline caused a characteristic increase in opening probability and the appearance of prolonged openings. When the patch was excised to the inside-out configuration and exposed to a simple ionic solution, channel activity disappeared within 1-2 min and never reappeared spontaneously. 3. After rundown of L-type channel activity in the excised patch, exposure of the inside face of the patch to MgATP and the catalytic subunit of the cyclic AMP-dependent protein kinase (PKAc) resulted in recovery of Ca2+ channel activity. Under these conditions channel activity could be even greater than under control cell-attached conditions, resembling channel activity after exposure to isoprenaline. This recovery of activity persisted many minutes, usually until the patch was lost. Addition of MgATP alone caused a small transient increase in channel activity in some patches. 4. Recovery of activity by MgATP and PKAc could be prevented by prior exposure of the excised patch to protein kinase inhibitor (PKI), or it could be abruptly terminated by exposure to PKI after recovery of activity. Addition to the pipette solution of okadaic acid, a protein phosphatase inhibitor, greatly slowed rundown. These findings support the proposal that dephosphorylation is an important component of rundown, and that phosphorylation is needed for channel opening activity. 5. Single-channel conductance was not altered by patch excision, but it was reduced after exposure of the excised patch to MgATP and PKAc. Mg2+ was responsible for this effect, probably by direct channel block from the inside, and Mg2+ also caused a negative shift in the channel activation, as expected from shielding of inside fixed negative charges.

Adenosine Triphosphate↗
PubMed1992

Dose-dependent modulation of the cardiac sodium channel by sea anemone toxin ATXII.

The effects of sea anemone toxin ATXII on single sodium channels were studied in cell-attached patches on rabbit ventricular myocytes at 20-22 degrees C. Exposure of patches to 1,000 nM ATXII induced long-lasting bursts of openings, which were more dramatically different from control at -20 mV than at -50 mV. Mean open duration, which had a biphasic dependence on voltage in control patches, was monotonically dependent on voltage in toxin-exposed patches, being 3.5 times longer than control at -20 mV and 4.5 times longer at -10 mV. Multiple mean open durations were detected at depolarized potentials. To test whether the multiple mean open durations resulted from a mixture of modified and unmodified openings, histograms of late openings (when unmodified channels would be inactivated) were constructed. Because in most cases these fit a single exponential with a mean open duration like that of modified channels, we conclude that voltage-dependent toxin unbinding produced a mixed population of unmodified and modified openings. Consistent with this hypothesis, lower concentrations of toxin most often produced open-duration histograms best fit with two exponentials. Ensembles revealed complex decay kinetics, which could be interpreted within the context of the toxin-induced increase in mean open duration and burst duration and the summation of modified and unmodified events. Analysis of the numbers of early versus late events at -20 mV for patches exposed to 20 nM, 100 nM, and 1,000 nM ATXII predicted the ED50 for ATXII block to be 285 nM at this potential. Using a five-state Markovian model, the action of ATXII could be explained as a reduction of the open-to-inactivated rate constant without effect on inactivation from closed states or other rate transitions.

Animals↗
PubMed1991

Excitation-contraction coupling in the heart.

There has been dramatic progress in our understanding of normal cardiac excitation-contraction coupling and in control of contraction strength, as the result of the new patch pipette method of voltage clamping of single cells and the new methods for monitoring Cai. Several abnormalities have been shown to exist in hypertrophied muscle; the action potential is changed and the contraction is slower. A kinetic change appears to exist in the L-type Ca current, associated with a slower decay of Cai. The next few years should bring a much improved understanding of the molecular and cellular basis for the changes of hypertrophy.

Animals↗
PubMed1991

External site for local anesthetic block of cardiac Na+ channels.

We report patch clamp studies of single Na+ channels from cardiac ventricular and Purkinje cells that support the hypothesis that local anesthetics can act from the outside of the membrane, and that demonstrate some aspects of their mechanism of action. Inclusion of lidocaine (0.1 mM) or QX-314 (0.5 mM), a membrane-impermeant, quaternary ammonium derivative of lidocaine, in the pipette solution for on-cell single channel recording demonstrated four important findings. (1) The open probability of the channel is reduced by drug in a use-dependent way. (2) Late openings are preferentially reduced. (3) Mean open time is shortened. (4) Hyperpolarization enhances recovery of the drug-bound channels. These findings are consistent with a hyperpolarizing shift of the transition rates for drug-bound channel. Further, we postulate that there is a drug-bound channel conformation which conducts current. At least some of the properties of local anesthetic interaction with the cardiac Na+ channels may be the result of kinetic effects mediated by binding to an external site.

Anesthetics, Local↗
PubMed1979

Suppression of ventricular arrhythmias with intravenous disopyramide and lidocaine: efficacy comparison in a randomized trial.

Twenty-six patients with clinically significant ventricular arrhythmias were randomly assigned to treatment with either intravenous disopyramide or lidocaine; crossover to the other agent was permitted in nine cases of primary drug failure. In addition, disopyramide was administered nonrandomly to seven patients with ventricular arrhythmias not controlled by lidocaine in standard doses. Arrhythmia control (greater than 50 percent reduction of premature ventricular complexes) was achieved in all 22 trials with disopyramide and in 9 of 13 trails with lidocaine in the random study, whereas clinical efficacy (arrhythmia control with absence of side effects) occurred respectively in 15 of 22, and 8 of 13 trials. In all 11 patients (7 nonrandom, 4 random) whose arrhythmia was not controlled with lidocaine the arrhythmia was controlled with disopyramide. Thus, the clinical efficacy of intravenous disopyramide ran parallel to that of lidocaine in patients with ventricular arrhythmias. Furthermore, intravenous disopyramide was an effective alternative agent for patients with arrhythmia not controlled by lidocaine.

Acute Disease↗
PubMed1979

The influence of intercellular clefts on the electrical properties of sheep cardiac Purkinje fibers.

A model of a 100 micrometers diameter Purkinje fiber with intercellular clefts was studied under voltage clamp conditions to examine the consequences of radial nonuniformity. Sodium and potassium conductances were distributed so that the surface and cleft membranes had similar channel density. Assuming that the model is appropriate, sodium current (and conductance) measured in the voltage clamp is grossly underestimated because of loss of voltage control of the cleft membrane. Under these conditions a value for g Na of about 15-20 mmho/cm2 of actual membrane is consistent with the experimental measurements of Dudel and Rüdel (1970. Pfluegers Arch. Eur. J. Physiol. 315:136-158.). Intermediate and slow currents (slow inward current and potassium current) appear to be accurately measured under the model conditions, despite some voltage nonuniformity within the cleft. This result depended on the presence of a residual sodium current, and experimental removal of sodium may alter this result. All effects of nonuniformity would be accentuated in fibers of larger diameter.

Animals↗
PubMed1979

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.

Action Potentials↗
PubMed1979

Ca and Na selectivity of the active membrane of rabbit AV nodal cells.

The atrioventricular (AV) node is thought to have a slow ionic channel. These experiments were designed to measure the relative contributions of Na and Ca ions to inward currents in the AV nodal cells of rabbit heart superfused with Tyrode solution. The effects of tetrodoxin (TTX), Mn2+, and verapamil observed in this study were in agreement with reports by others. The overshoot of AV nodal (N) cells was related to external Ca, with a slope of 12 mV/decade, unchanged by addition of TTX. Similar dependence of overshoot on external Na was seen, with a slope of 20 mV/decade. The slope did not change on addition of TTX. Total removal of either Na or Ca from the solution abolished excitability. Using a constant field equation, we estimated relative permeability (P) of the membrane at the time of maximal overshoot to be PCa/PNa congruent to 60 similar to or approximately 100 and PK/PNa congruent to 1. Relative contributions of these ions to the currents were estimated as ICa congruent to 17%, INa congruent to 33% (inward currents), and IK congruent to 50% (outward current). In conclusion, AV nodal cells have "slow inward-current channels" that are selective for Ca over Na ions.

Action Potentials↗
PubMed1979

Effect of stretch on conduction velocity and cable properties of cardiac Purkinje fibers.

Cardiac Purkinje fibers were studied before and after stretch to 30% and 50% in excess of their slack length, and membrane properties and conduction velocity were measured in relation to the stretch. Conduction velocity increased by 26% with 50% stretch. The resting potentials averaged -77 mV and did not change with stretch. In addition, the action potential height, maximal upstroke velocity, duration, and time constant of the foot did not change. These results suggested that the increase in conduction velocity was not due to a change in membrane excitability. Two geometric models of stretch, called unfolding and uncoiling, were considered. Cable analyses were performed to distinguish between these models. The effects were mixed, with about 2/3 of the length change resulting from unfolding and 1/3 from uncoiling. These results support the concept of redundancy of the surface membrane by folding. The changes are likely to be of importance in activation of the heart by the His-Purkinje system, especially in cardiac dilatation due to disease.

Action Potentials↗
PubMed1979

Membrane permeability during low potassium depolarization in sheep cardiac Purkinje fibers.

Exposure of sheep Purkinje fibers to low [K]o leads to marked depolarization to a stable potential of about -40 mV. This level is equivalent to the plateau of the Purkinje fiber action potential. The low [K]o depolarization could be prevented by removal of [Na]o and was modified by tetrodotoxin. The membrane potential in the depolarized state was unresponsive to changes in [Cl]o or [Ca]o and it was poorly responsive to changes in [K]o between 0 and 2 mM. Repolarization was induced by decrease in [Na]o with a slope response of 30 mV/10-fold change in [Na]o. Average internal K activity (aK) in the resting state with a [K]o of 5 mM was 121.4 mM for a membrane potential of -80 mV. During low K depolarization aK was 119.7 mM with a membrane potential of -34 mV. The depolarization was therefore due to a change in membrane permeability, with little change in aK. Upon restoration of [K]o the fiber repolarized to values transiently more negative than the prior resting potential. These transient potentials were more negative than the K equilibrium potential (VK), if it is calculated assuming a uniform [K]o. The hyperpolarization was reduced by ouabain [10(-6)] or by low [Ca]o.

Animals↗
PubMed1979

Surgical treatment of recurrent primary malignant tumor of the left atrium.

A young woman presented with a tumor in the left atrium resembling a left atrial myxoma. After simple excision of the tumor the diagnosis of primary malignant fibrous histiocytoma of the heart was made. A course of radiation therapy was given. Four subsequent recurrences were treated by cardiotomy and resection of the left atrial wall. At the third, fourth, and fifth operations fulguration of the left atrial wall was performed. Subsequent chemotherapy failed to control the tumor. The patient was admitted 6 weeks after the last resection and died. Postmortem examination revealed a large recurrent tumor obstructing the left atrium with no metastases. The clinical course, cardiac catherization data, and postmortem examination are presented. Palliation was achieved by repeated resection of a radiation-resistent primary sarcoma of the heart.

Adult↗
PubMed1979

Effects of imipramine on cellular electrophysiological properties of cardiac Purkinje fibers.

The electrophysiological effects of imipramine (Tofranil) on cardiac Purkinje fibers were studied in vitro. Imipramine has a direct membrane action. It reduces excitatory inward current in Purkinje fibers, at least as measured by Vmax and conduction velocity. The mechanism of action of imipramine is probably by both reduction in gNa and marked slowing of the time constants of recovery of the rapid inward sodium current. Imipramine also reduces the action protential duration, decreasing the absolute refractory period. This effect is not seen with antiarrhythmic agents of the local anesthetic class, but it is similar to lidocaine. This effect may be a result of the decrease in steady-state gNa, but effects of the drug on other ionic conductances could play a role.

Action Potentials↗