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C F Starmer

Publications and source records attributed to C F Starmer.

At least 55 records · Page 3Linked to original sources
PubMed1990

Channel specificity in antiarrhythmic drug action. Mechanism of potassium channel block and its role in suppressing and aggravating cardiac arrhythmias.

Although work on class III antiarrhythmics remains at an early stage, these agents still appear to possess greater efficacy and less proarrhythmia than conventional class I agents in those experimental arrhythmia models considered to be most representative of the clinical situation. Although prolongation of repolarization carries with its own tendency for pause-dependent arrhythmogenesis (i.e., torsade de pointes), available data suggest that this may be a function of nonspecificity in potassium channel block rather than a general characteristic of class III activity. The availability of new and more selective blockers of specific cardiac potassium channels under development as class III agents have already helped to clarify basic questions about the ionic mechanism of repolarization in the heart, and one hopes that a growing clinical data base will eventually determine the relative safety and efficacy of these agents in preventing symptomatic and life-threatening arrhythmias.

Animals↗
PubMed1990

Managing clinical research data: software tools for hypothesis exploration.

Data representation, data file specification, and the communication of data between software systems are playing increasingly important roles in clinical data management. This paper describes the concept of a self-documenting file that contains annotations or comments that aid visual inspection of the data file. We describe access of data from annotated files and illustrate data analysis with a few examples derived from the UNIX operating environment. Use of annotated files provides the investigator with both a useful representation of the primary data and a repository of comments that describe some of the context surrounding data capture.

Data Interpretation, Statistical↗
PubMed1990

State dependence of ethacizin and ethmozin block of sodium current in voltage clamped and internally perfused cardiac Purkinje cells.

Ethacizin, a positively charged analog of ethmozin, reduces the cardiac action potential upstroke and blocks peak sodium current (INa). We investigated ethacizin block of INa in 11 cells and ethmozin block in 4 cells at 20 degrees C. Rest block measured as the relative INa decrease for the first pulse in drug after 3 to 6 min at the holding potential was negligible for ethacizin but substantial (16% at 5 microM) for ethmozin. Use-dependent block developed exponentially; the time course of block and relative INa remaining were concentration-dependent. Frequency dependence of block between 0.5 and 4 cps was weak for ethacizin. Varying the depolarization duration from 5 to 100 msec, while keeping the recovery interval constant, did not alter the block by ethacizin. In contrast, prolonging the clamp step in ethmozin from 5 to 100 msec increased the rate and depth of block. Apparent binding rates for each drug were calculated using the assumptions of the guarded receptor model. We conclude that ethacizin blocks INa in a use-dependent manner by binding to a transiently available state such as the open state. In contrast, ethmozin block of INa exhibits both rest block and use-dependent block. Use-dependent block can be attributed to binding to a state (or states) maintained during depolarization such as the inactivated state. With these similar drugs, charge appears to be an important determinant of state-dependent binding.

Animals↗
PubMed1989

Characterization of the surface topography and putative tertiary structure of the human CD7 molecule.

The CD7 gp40 molecule is a member of the Ig gene superfamily and is expressed on T cell precursors before their entry into the thymus during fetal development. N-terminal amino acids 1-107 of CD7 are highly homologous to Ig kappa-L chains whereas the carboxyl-terminal region of the extracellular domain of CD7 is proline-rich and has been postulated to form a stalk from which the Ig domain projects. To define potential functional regions of CD7, we have studied the surface topography of the CD7 Ag by synthesizing peptides corresponding to linear sequences within the CD7 extracellular domains, by raising polyclonal anti-CD7 rabbit sera against these peptides, and by computer analysis of the primary CD7 amino acid sequence. Polyclonal anti-CD7 sera were studied using indirect immunofluorescence, RIA, radioimmunoprecipitation, and Western blot assays. Computer analysis was performed comparing the CD7 sequence with all other known protein sequences. We found that three CD7 epitopes defined by peptides CD7-1A (AA 1-38), CD7-4 (AA 48-74), and CD7-7 (AA 129-146) were available for binding antibody on the surface of the CD7 molecule. Using computer analysis, we transposed the amino acid sequence of the CD7 Ig kappa-like N-terminal domain of CD7 onto the spatial coordinates of REI, a previously reported Ig kappa-molecule highly homologous (48%) to the CD7 N-terminal Ig-like region. Based on computer analysis of this putative CD7 three-dimensional structure, both the CD7-1A and CD7-4 regions protruded from the surface of the N-terminal domain of the CD7 molecule. Finally, comparison of the CD7 transmembrane sequence with CD4 and HIV transmembrane sequences and with respiratory syncytial virus fusion sequences demonstrated similar sequence motifs among these molecules.

Amino Acid Sequence↗
PubMed1989

Ethacizin blockade of calcium channels: a test of the guarded receptor hypothesis.

The effect on calcium channels of the sodium channel antagonist, ethacizin, was studied in isolated frog ventricular cells using the whole cell voltage-clamp methodology. Ethacizin was found to block inward calcium current in a frequency-, voltage-, and concentration-dependent manner. The frequency-dependent blocking properties were modeled by considering the drug interaction with a voltage-dependent mixture of calcium channels harboring either an accessible or an inaccessible binding site. With repetitive stimulation, the pulse-to-pulse reduction in peak current is shown to be exponential, with a rate linearly related to the interstimulus interval and the drug concentration. Observed frequency- and concentration-dependent blocks were consistent with the predictions of the model, and mixture-specific rate constants were estimated from these data. The negligible shift in channel inactivation and the reduction of apparent binding and unbinding rates with more polarized membrane potentials imply the active moiety of ethacizin blocks open channels and is trapped within the channel at resting membrane potentials. The binding rate at 0 mV is similar to that observed in studies of interactions of other open channel blocking agents with voltage- and ligand-gated channels.

Animals↗
PubMed1989

Blockade of rabbit atrial sodium channels by lidocaine. Characterization of continuous and frequency-dependent blocking.

Lidocaine block of the cardiac sodium channel is believed to be primarily a function of channel state. For subthreshold potentials, block is limited to the inactivated state, whereas above threshold, block results from the combination of open- and inactivated-state block. Since, in the absence of drug, inactivation develops with time constants that vary from several hundred milliseconds to a few milliseconds as potential is varied from subthreshold to strongly depolarized levels, we would predict a similar voltage dependence of at least a fraction of block. Prior theoretical analyses from our laboratory suggest that there should be a direct parallel between blockade determined with a single pulse and trains of pulses. We tested these predictions by measuring the blockade of sodium current in cultured atrial myocytes during exposure to 80 microM lidocaine. We selected two test potentials for most of our studies, -80 mV, which was clearly in the subthreshold range of potentials, and -20 mV, which was close to the peak of the current-voltage curve. With single pulses of increasing duration, block developed with a single exponential time course and with time constants that decreased from 694 +/- 117 msec at -80 mV to 373 +/- 54 msec at -20 mV. In the absence of drug, inactivation developed with a time constant 176 +/- 17 at -80 mV and 2.9 +/- .5 msec at -20 mV. Despite the much slower onset of inactivation at -80 mV, no second-order delay in block development was observed. This suggests that at -80 mV block is occurring to a channel conformation that is accessed without delay rather than the classical inactivated state. We compared the kinetics of block during a single continuous pulse with trains of pulses at -20 mV. The rate of block onset was faster during the pulse trains, suggesting an element of "activated state" block. We computed shifts in apparent inactivation from observed steady-state blockade. The computed shifts agree well with those observed, indicating that shifts in apparent inactivation result largely from voltage-sensitive equilibrium blockade. The classical states described in the Hodgkin-Huxley formalism may be too restrictive to fully describe the voltage- and time-dependent block of cardiac sodium channels.

Animals↗
PubMed1989

Blockade of cardiac sodium channels by lidocaine. Single-channel analysis.

The mechanism of interaction of lidocaine with cardiac sodium channels during use-dependent block is not well defined. We examined the blockade of single cardiac sodium channels by lidocaine and its hydrophobic derivative RAD-242 in rabbit ventricular myocytes. Experiments were performed in cell-attached and inside-out patches. Use-dependent block was assessed with trains of ten 200-msec pulses with interpulse intervals of 500 msec and test potentials of -60 to -40 mV. Single-channel kinetics sometimes showed time-dependent change in the absence of drug. During exposure to 80 microM lidocaine, use-dependent block during the trains was associated with a decrease in the average number of openings per step. At -60 mV, mean open time was not significantly changed (control, 1.4 +/- 0.6 msec; lidocaine, 1.2 +/- 0.3 msec, p greater than 0.05). Greater block developed during trains of 200-msec pulses compared with trains of 20-msec pulses at the same interpulse interval at test potentials during which openings were uncommon later than 20 msec (-50 and -40 mV). Prolonged bursts of channels showing slow-gating kinetics were observed both in control and the presence of 80 microM lidocaine. However, lidocaine may decrease the late sodium current by altering the kinetics of slow gating. The hydrophobic lidocaine derivative RAD-242, which has a 10-fold greater lipid solubility than lidocaine, decreased the peak averaged current during pulse train stimulation by 60% without a change in the mean open time. Our results suggest that the major effect of lidocaine during use-dependent block involves the interaction with a nonconducting state of the sodium channel followed by a failure to open during subsequent depolarization.

Animals↗
PubMed1989

Characterization of concentration- and use-dependent effects of quinidine from conduction delay and declining conduction velocity in canine Purkinje fibers.

The dynamic response of squared conduction velocity, theta 2, to repetitive stimulation in canine Purkinje fibers with quinidine was studied using a double-microelectrode technique. With stimulation, a frequency-dependent monoexponential increase in conduction delay (CD) and a decline in theta 2 were observed. The exponential rates and changes in steady-state CD and theta 2 were frequency- and concentration-dependent. The overall drug uptake rates describing blockade and the interpulse recovery interval were linearly related and steady-state values of theta 2 were linearly related to an exponential function of the stimulus intervals. Based on first-order binding, the frequency- and concentration-dependent properties of quinidine were characterized by the apparent binding and unbinding rates of 14.2 +/- 5.7 X 10(6) mol-1.s-1 and 63 +/- 12 s-1 for activated and 14.8 +/- 1.0 X 10(2) mol-1.s-1 and 0.16 +/- 0.03 s-1 for resting states. The recovery time constant extracted from the pulse train interpulse interval was 5.8 +/- 1.5 s compared with 5.1 +/- 0.6 s determined from a posttrain test pulse protocol. This study demonstrates that the kinetics of drug action can be derived from measures of impulse propagation. This provides a basis for characterizing frequency-dependent properties of antiarrhythmic agents in vivo and suggests the plausibility of a quantitative assessment of drug binding and recovery rates in man.

Action Potentials↗
PubMed1989

Marked QRS complex abnormalities and sodium channel blockade by propoxyphene reversed with lidocaine.

The opiate analgesic propoxyphene produces cardiac toxicity when taken in overdose. We recently observed a patient with propoxyphene overdose in whom marked QRS widening was reversed by lidocaine. The reversal is apparently paradoxical as both agents block the inward sodium current (INa). We examined possible mechanisms of the reversal by measuring INa in rabbit atrial myocytes during exposure to propoxyphene and the combination of propoxyphene and lidocaine (60 and 80 microM, respectively). Propoxyphene caused use-dependent block of INa during pulse train stimulation. Block recovered slowly with time constants of 20.8 +/- 3.9 s. Block during lidocaine exposure recovered with time constants of 2-3 s. During exposure to the mixture, block recovered as a double exponential. The half time for recovery during exposure to the mixture was 1.6 +/- .9 s compared with a half-time of 14.3 +/- 2.9 s during exposure to propoxyphene alone. During pulse train stimulation, less steady-state block was observed during exposure to the mixture than during exposure to propoxyphene alone when the interval between pulses was greater than 0.95 s. Both drugs compete for a common receptor during the polarizing phase. The more rapid dissociation of lidocaine during the recovery period leads to less block during the mixture than during exposure to propoxyphene alone. The experiments suggest a mechanism for reversal of the cardiac toxicity of drugs which have slow unbinding kinetics.

Adult↗
PubMed1988

Characterizing activity-dependent processes with a piecewise exponential model.

The response of some biological processes is dependent on the frequency of stimulation. With first-order processes, the response is driven exponentially to an equilibrium determined by the value of the driving function. When the stimulus or driving function is viewed as switching between constant values the resulting response is piecewise exponential. With periodic excitation, the time course of a point fixed in time relative to the initiation time of each stimulus is shown to be exponential with a rate and steady state that are linearly dependent on the rates and equilibria associated with each component exponential. This linearity can be exploited and leads to a simple estimation procedure for the apparent state-dependent rates.

Animals↗
PubMed1987

Ligand binding to transiently accessible sites: mechanisms for varying apparent binding rates.

Many biological processes are characterized by models where ligand has continuous access to binding sites. Use-dependent processes such as ion channel blockade appear to represent a class of processes where binding site access is transient and is dependent on channel conformation (state). (Alternatively, the existence of a proper conformation for binding might be transient.) We assume binding takes place only with a site that is accessible or in a bindable conformation. For ion channels, channel conformation responds to local chemical or electrical stimulation. The stimulus amplitude determines the mixture of channels with accessible and inaccessible sites. When conformation equilibrium is achieved rapidly in relation to ligand binding, then ligand binding can be considered the result of an apparent binding rate determined by the true binding rate as modified by the fraction of accessible sites. With repetitive stimulation, the continuous access model can be extended to a setting where apparent binding rates repetitively switch in response to the stimulus induced variation in mixture of site states. The resulting theoretical description provides a means for relating equilibrium binding parameters with those obtained under conditions of repetitive stimulation. In particular: (1) binding is piecewise exponential; (2) the envelope of binding measured at each pulse in a train varies exponentially with an apparent rate linearly dependent on the mixture specific rates; and (3) the steady state degree of binding site saturation is linearly dependent on the mixture specific equilibria.

Animals↗
PubMed1987

Managing research data with self-documenting files.

Processing biomedical research data is frequently complex due to the evolutionary nature of experiments and the requisite modification of analysis software. For the past several years we have been evolving a set of software tools designed to improve our ability to respond to evolving experimental designs. These tools allow the investigator to easily manipulate the research data and specify desired data transformations at run time. Coupling of analysis software to research data files is dependent on data files that are commented in a manner similar to that used in programming languages. The resulting annotated data files are self-documenting, and their use facilitates visual interpretation of displayed data as well as automatic processing of subsets of data. Here we present a formal description of a self-documenting file and describe several software tools that facilitate processing of biomedical research data.

Electronic Data Processing↗
PubMed1987

Theoretical characterization of ion channel blockade. Competitive binding to periodically accessible receptors.

Competitive ligand binding to periodically activated or accessible receptors is influenced by the interaction between ligand binding kinetics and the interval of time the binding site is accessible. This interaction produces a paradoxical reduction in bound receptors under certain conditions. A mathematical description of multi-ligand binding to a single binding site is presented for both the continuously and transiently accessible cases. The theoretical results predict paradoxical "agonism" and are consistent with the results of studies of lidocaine and bupivacaine binding to cardiac sodium channels.

Binding, Competitive↗
PubMed1987

Mechanisms of closure of cardiac sodium channels in rabbit ventricular myocytes: single-channel analysis.

We have examined the kinetics of closure of sodium channels using single-channel recordings in cell-attached and excised membrane patches of rabbit ventricular myocytes. Sodium-channel closure was dependent on membrane potential. The closing rate initially decreased with depolarization. The rate then passed through a minimum and increased at strongly depolarized potentials. We attempted to determine the separate voltage dependence of the deactivation and inactivation rate constants using the method of Aldrich, Corey, and Stevens. In a majority of experiments, the method did not give internally consistent results. As an alternative approach, batrachotoxin was used to remove inactivation and determine the voltage dependence of deactivation rate. The deactivation rate decreased with depolarization. To account for the increase in the closing rate at strongly depolarized test potentials, one must postulate voltage dependence of inactivation. The ensemble average current relaxed with a time course that was usually best described by the sum of two exponentials. The larger of the two rate constants that described the relaxation was strongly voltage-dependent, increasing with depolarization. The larger rate constant may reflect voltage-dependent inactivation. We found evidence of two possible mechanisms for the slow component of relaxation: 1) cardiac sodium channels may open repetitively during a given depolarizing epoch, and 2) channels may return from the inactivated state with low probability and burst for as much as 200 msec with open times that are longer than those during usual gating. The slow component appears to be more prominent in cardiac muscle than in nerve and may play an important role in the control of the action potential duration and the inotropic state of the heart.

Animals↗
PubMed1986

Theoretical characterization of ion channel blockade: ligand binding to periodically accessible receptors.

With repetitive stimulation, the time course of use-dependent blockade as assessed by peak membrane ion currents can be described by a sequence of blocking relationships that have the form of recurrence equations. The equations of the sequence describe blockade acquired during each interval of a stimulus where the possibly different binding and unbinding rates are assumed constant during each interval. The solution predicts that use-dependent uptake follows an exponential time course. Furthermore, the exponential uptake rate is a linear function of uptake rates associated with the stimulus time intervals. Similarly, the fraction of blocked channels at steady state is a linear function of the interval dependent blockade equilibria. Several novel tests of consistency between the model and observations are derived from these theoretical results. It is also shown that as the stimulus interval increases to infinity, steady state dissociation constants measured by peak membrane currents are theoretically equivalent to those measured with true equilibrium methods such as radioligand binding studies.

Aminopyridines↗
PubMed1986

A quantitative description of QX222 blockade of sodium channels in squid axons.

The interaction of QX222, a quaternary ammonium derivative of lidocaine, with the Na channel was studied in internally perfused squid axons under voltage-clamped conditions. A use-dependent block was observed in response to repetitive depolarizing pulses. The time constant for block development and the steady state level of the block were increased with increasing frequency of stimulation from 0.1 to 10 Hz. Use-dependent block can be viewed as a net increase in the drug incorporation into Na channels with successive pulses. That is, net drug uptake by Na channels occurs during the depolarizing phase and net drug release occurs during the interpulse interval. The observed uptake rate of use-dependent block is shown to be a linear combination of the uptake rates associated with the depolarizing and resting potentials. Also, the steady state fraction of blocked channels is shown to be a linear combination of the state-dependent blockade equilibria. Drug-channel interactions are assumed to be dependent on gated control of the diffusion path between drug pool and the interior channel binding site. Drug ingress to the binding site can be inhibited by the channel gates (receptor guarding), while drug bound to the channel may become trapped by closure of the channel gates (trapping). On the basis of these assumptions, a simple procedure is proposed for estimating apparent rate constants governing the drug-channel binding reactions for two cases of channel blockade. The estimated forward (k) and backward (1) rate constants are: 2.45 x I05 M-1 s- and 0.23 x 103 s-1, respectively, for k and I for the case when the drug is trapped by both activation and inactivation gates, and 3.58 x 105 M-l s-l and 4.15 x 10-3 S-l for the case when the drug is not trapped. While these two schemes make a similar prediction with respect to the resulting uptake rates, their prediction of the steady state level of block differs. The observed steady state level of block could quantitatively be predicted by the trapped scheme but not by the untrapped scheme, thus providing a means for discriminating between these two schemes. In addition, the trapped scheme, but not the untrapped scheme, could provide an explanation for the observed voltage dependence of the slow recovery from use-dependent block.

Animals↗
PubMed1986

Modeling ion channel blockade at guarded binding sites: application to tertiary drugs.

Excitable membranes exposed to sodium channel blocking agents (D; local anesthetics and antiarrhythmic drugs) show a progressive reduction of peak sodium current when repetitively depolarized (use dependence). Thus, with repetitive excitation, use dependence reflects a net rightward shift in the balance between unblocked channels (U) and blocked channels (B): U + D in equilibrium with B. The modulated receptor hypothesis (a 7-parameter model) has been proposed to account for this shift and is based on a channel lumen binding site whose affinity varies with channel state and where drug-complexed channels exhibit modified inactivation gate kinetics. Alternatively, we consider use-dependent binding as the result of transient access to a constant-affinity binding site. In this setting, the channel gate conformation is viewed as controlling the flux of drug as it diffuses between drug pools and the binding site. Apparent variation in binding rates is therefore considered the result of variations in the fraction of accessible sites. This guarded receptor hypothesis, with three fewer parameters, is able to predict apparent changes in channel binding and apparent shifts in channel inactivation without incorporating modified gating parameters in drug-complexed channels. Furthermore, with this model one is able to characterize both relaxation kinetics and channel blockade associated with tertiary amines as well as hydrophobic and hydrophilic agents. The pH dependence of repriming rates is utilized to estimate several of the important parameters associated with this simplified hypothesis.

Amines↗