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A Peskoff

Publications and source records attributed to A Peskoff.

16 recordsLinked to original sources
PubMed1999

Measurement of action potential-induced presynaptic calcium domains at a cultured neuromuscular junction.

Spatially localized Ca(2+) domains are thought to play a key role in action potential (AP)-evoked neurotransmitter release at fast synapses. We used a stage-scan confocal spot-detection method and the low-affinity Ca(2+) indicator Oregon Green 488 BAPTA-5N to study the spatiotemporal profile of presynaptic AP-induced Ca(2+) domains. Families of scanned AP-induced fluorescence transients were detected from spot locations separated by 200-300 nm, within the vicinity of Ca(2+) entry sites. Typically, the largest transient in a particular scan peaked within approximately 1 msec and decayed with rapid (tau(1) of 1.7 msec) and slow components (tau(2) of 16 msec, tau(3) of 78 msec). As the spot was incrementally displaced, transients progressively exhibited a slowing in their time-to-peak and a loss of the fast decay component. Three-dimensional graphs of fluorescence versus time and spot displacement revealed the presence of AP-induced fluorescence domains that dissipated within approximately 7 msec. The size of fluorescence domains were estimated from the full-width at half-maximum of gaussian fits to isochronal DeltaF/F plots and ranged from 0.6 to 3.0 micrometer, with a mean +/- SD of 1.6 +/- 0.6 micrometer. Model simulations of a localized Ca(2+) entry site predicted the major features of the fluorescence transients and suggested that, within approximately 1 msec of the initiation of the Ca(2+) current, both the fluorescence domain and the underlying Ca(2+) domain do not extend significantly beyond the site of entry. Consistent with this prediction, the intracellular addition of EGTA (up to 2 mM) accelerated the decay of the measured transients but did not affect the domain size.

Action Potentials↗
PubMed1998

Calcium concentration and movement in the ventricular cardiac cell during an excitation-contraction cycle.

This paper extends the model for Ca movement in the cardiac ventricular cell from the diadic cleft space to the entire sarcomere. The model predicts the following: 1) Shortly after SR release there is a [Ca] gradient >3 orders of magnitude from cleft center to M-line which, 50 ms after release, is still >30. Outside the cleft, 40 ms after cessation of release, the axial gradient from Z to M-line is >3. 2) At the end of SR release, >50% of the total Ca released is bound to low-affinity inner sarcolemmal phospholipid binding sites within the cleft. 3) Halving the SR release almost doubles the fraction of release removed from the cell via Na/Ca exchange and reduces average sarcomeric free [Ca] by 70%. 4) Adding 100 microM fluo-3, which doubles the buffering capacity of the cytoplasm, reduces peak average sarcomeric [Ca] by >50% and increases the initial half-time for [Ca] decrease by approximately twofold. 5) A typical Ca "spark" can be generated by an SR release 20% of maximum (4 x 10(-20) moles) over 2 ms. Fluo-3 (100 microM) significantly "shrinks" the spark. 6) The "spark" is a consequence of elementary events within the diadic cleft space. For example, removal of cleft binding sites would cause average sarcomeric Ca to increase by >10 fold, fall 10 times more rapidly, decrease latency for appearance of the spark by >20 times, and reduce spark duration by 85%. 7) Dividing SR Ca release between cleft and corbular SR produces a secondary [Ca] peak and a "flattening" of the sarcomeric [Ca] transient. These changes probably could not be resolved with current confocal microscopic techniques.

Aniline Compounds↗
PubMed1996

Calcium concentration and movement in the diadic cleft space of the cardiac ventricular cell.

We model the space between the junctional sarcoplasmic reticulum (JSR) membrane and the inner leaflet of the transverse tubular ("T") sarcolemmal (SL) membrane, the diadic cleft, with respect to calcium (Ca) concentration and movement. The model predicts the following: 1) Ca influx via the "L" channel increases [Ca] to 1 microM within a distance of 50 nm from the channel mouth in < 500 microseconds. This is sufficient to trigger Ca release from a domain of 9 "feet." 2) By contrast, "reverse" Na/Ca exchange will increase [Ca] to approximately 0.5 microM throughout the cleft space in 10 ms, sufficient to trigger Ca release, but clearly to a lesser extent and more slowly than the channel. 3) After a 20-ms JSR release into the cleft via the "feet" [Ca] peaks at 600 microM (cleft center) to 100 microM (cleft periphery) and then declines to diastolic level (100 nM) within 150 ms throughout the cleft. 4) The ratio of flux out of the cleft via Na/Ca exchange to flux out of the cleft to the cytosol varies inversely as JSR Ca release. 5) Removal of SL anionic Ca-binding sites from the model will cause [Ca] to fall to 100 nM throughout the cleft in < 1 ms after JSR release ceases. This markedly reduces Na/Ca exchange. 6) Removal from or decreased concentration of Na/Ca exchangers in the cleft will cause [Ca] to fall too slowly after JSR release to permit triggered release upon subsequent excitation.

Animals↗
PubMed1996

Inner sarcolemmal leaflet Ca(2+) binding: its role in cardiac Na/Ca exchange.

A recently completed model of Ca concentration and movements in the cardiac cell diadic cleft space predicts that removal or neutralization of inner sarcolemmal (SL) leaflet anionic Ca-binding sites at the sarcolemmal border of this space will greatly diminish Na/Ca exchange-mediated Ca efflux. The present study tests this prediction using the local anesthetic dibucaine as a probe. It is shown, in isolated SL, that dibucaine competitively displaces Ca specifically from anionic phospholipid headgroups. Dibucaine also displaces Ca from the SL when applied to intact cells. It does not affect the content or release of Ca from sarcoplasmic reticulum (SR) in these cells. This eliminates a primary effect on SR Ca as a contributing factor to dibucaine's effect on Na/Ca exchange-mediated Ca efflux. Measurement of this efflux from whole cells shows a highly significant reduction of 58% (p < 0.001) by 0.5 mM dibucaine. The inhibiting effect of dibucaine on Na/Ca exchange-mediated Ca efflux can be significantly reversed by augmentation of Ca release from SR by caffeine at the time of activation of Na/Ca exchange. This supports the contention that the dibucaine-SL interaction is a competitive one vis-a-vis Ca. The results are supportive of the model in which inner SL leaflet Ca-binding sites account for the delay of Ca diffusion from the diadic cleft, thereby prolonging the time for which [Ca] remains elevated in the cleft. The prolonged increased [Ca] significantly enhances the ability of Na/Ca exchange to remove Ca from the cell during the excitation-contraction cycle.

Animals↗
PubMed1992

Sarcolemmal calcium binding sites in heart: II. Mathematical model for diffusion of calcium released from the sarcoplasmic reticulum into the diadic region.

We present a model for predicting the temporal and spatial dependence of [Ca] in the cardiac subsarcolemmal diadic region (cleft), following Ca release from the "feet" of the sarcoplasmic reticulum. This region is modeled as a disc 10 nm thick, 430 nm in radius, with or without Ca binding sites and open at its periphery to the cytosol. [Ca] is computed for three diffusion coefficients (100, 20 and 4% of aqueous diffusion), following release of a 20-msec square pulse sufficient to produce 50% maximal contractile force, or repetitive release (400/min) of such pulses. Numerical solutions are obtained for the general diffusion/binding problem and analytic solutions for the case of no binding sites. For the middle value of diffusion coefficient, and in the absence of binding sites, [Ca] rises to approximately 1.5 mM in 20-msec and then falls to approximately 0.1 microM in less than 3 msec. Adding binding sites reduces peak [Ca] to approximately 0.6 mM but prolongs its decline, requiring approximately 200 msec to reach 20 microM. For repetitive release [Ca] is greater than 100 microM for roughly half of each cycle. Two major implications of the predicted [Ca] are: (i) The effect of Ca binding sites on [Ca] will cause Ca efflux from the cleft via the Na-Ca exchanger (Km(Ca) approximately 20 microM) to continue at a significant level for greater than 200 msec. (ii) The time constant for inactivation of release from the "feet" must be much greater than for activation if Ca-induced Ca release is to continue for greater than 1-2 msec.

Animals↗
PubMed1991

Diffusion around a cardiac calcium channel and the role of surface bound calcium.

The diffusion of Ca as it converges to the external mouth of a Ca channel is examined. Diffusional limitation on Ca ions entering Ca channels during current flow, cause local extracellular Ca depletions. Such extracellular Ca depletions have been reported in cardiac muscle. The cardiac sarcolemma has a large number of low-affinity Ca binding sites that can buffer these local Ca depletions. For a hemisphere of extracellular space (of radius less than 0.33 microns) centered on the external mouth of a Ca channel the amount of Ca bound at the membrane surface exceeds that which is free within the associated hemisphere. The ratio of bound Ca/free Ca increases as r decreases, such that the [Ca] nearest the Ca channel is the most strongly buffered by sarcolemmal bound Ca. It is demonstrated that Ca ions coming from these sarcolemmal Ca binding sites contribute quantitatively to the integrated Ca current. The electric field generated by the local depletion of Ca near the channel mouth has little impact on the extent of Ca depletion, but if an additional electric field exists at the mouth of the channel, Ca depletion can be significantly altered. Other low-affinity Ca binding sites in the interstitium may also contribute to the buffering of extracellular Ca. The complex geometry of the extracellular space in cardiac muscle (e.g., transverse tubules and restrictions of extracellular space between cells) increases both the predicted Ca depletions (in the absence of binding) and the bound/free ratio. Thus, the impact of this surface Ca binding is greatly increased. By considering arrays of Ca channels in transverse tubules or in parallel planes (e.g., membranes of neighboring cells), extracellular Ca depletions are predicted which agree with those measured experimentally. Membrane Ca binding may also be expected to buffer increases in [Ca] around the inner mouth of Ca channels. It is demonstrated that in the absence of other intracellular systems most of the Ca entering the cell via Ca channels might be expected to be bound to the inner sarcolemmal surface. It is concluded that surface Ca binding may have a substantial impact on the processes of extracellular Ca depletion (and intracellular Ca accumulation).

Animals↗
PubMed1988

Electrodiffusion of ions approaching the mouth of a conducting membrane channel.

The movement of ions in the aqueous medium as they approach the mouth (radius a) of a conducting membrane channel is analyzed. Starting with the Nernst-Planck and Poisson equations, we derive a nonlinear integrodifferential equation for the electric potential, phi(r), a less than or equal to r less than infinity. The formulation allows deviations from charge neutrality and dependence of phi(r) on ion flux. A numerical solution is obtained by converting the equation to an integral equation that is solved by an iterative method for an assumed mouth potential, combined with a shooting method to adjust the mouth potential until the numerical solution agrees with an asymptotic expansion of the potential at r-a much greater than lambda (lambda = Debye length). Approximate analytic solutions are obtained by assuming charge neutrality (Läuger, 1976) and by linearizing. The linear approximation agrees with the exact solution under most physiological conditions, but the charge-neutrality solution is only valid for r much greater than lambda and thus cannot be used unless a much greater than lambda. Families of curves of ion flux vs. potential drop across the electrolyte, phi(infinity)-phi (a), and of permeant ion density at the channel mouth, n1(a), vs. flux are obtained for different values of a/lambda and S = a d phi/dr(a). If a much greater than lambda and S = O, the maximum flux (which is approached when n1(a)----0) is reduced by 50% compared to the value predicted by the charge-neutrality solution. Access resistance is shown to be a factor a/[2 (a + lambda)] times the published formula (Hille, 1968), which was derived without including deviations from charge neutrality and ion density gradients and hence does not apply when there is no counter-ion current. The results are applied to an idealized diffusion-limited channel with symmetric electrolytes. For S = O, the current/voltage curves saturate at a value dependent on a/lambda; for S greater than O, they increase linearly for large voltage.

Diffusion↗
PubMed1985

Calcium at the surface of cardiac plasma membrane vesicles: cation binding, surface charge screening, and Na-Ca exchange.

Calcium binding and Na-Ca exchange activity were measured in isolated cardiac plasma membrane vesicles under various ionic conditions. A model was developed to describe the Ca binding characteristics of cardiac sarcolemmal vesicles using the Gouy-Chapman theory of the diffuse double layer with specific cation binding to phospholipid carboxyl and phosphate groups. The surface association constants used for Ca, Na, K and H binding to both of these groups were 7,0.63, 0.3 and 3800 M-1, respectively. This model allows the estimation of surface [Ca] under any specific ionic conditions. The effects of the divalent screening cation, dimethonium, on Ca binding and Na-Ca exchange were compared. Dimethonium had no significant effect on Ca binding at high ionic strength (150 mM KCl), but strongly depressed Ca binding at low ionic strength. Dimethonium had no significant effect on Na-Ca exchange (Na-inside dependent Ca influx) at either high or low ionic strength. These results suggest that the Ca sites of the Na-Ca exchanger are in a physical environment where they are either not exposed to or not sensitive to surface [Ca].

Animals↗
PubMed1985

Reappraisal of diffusion, solubility, and consumption of oxygen in frog skeletal muscle, with applications to muscle energy balance.

Previously we tested the validity of the one-dimensional diffusion equation for O2 in the excised frog sartorius muscle and used it to measure the diffusion coefficient (D) for O2 in this muscle and the time course of its rate of O2 consumption (Qo2) after a tetanus (Mahler, 1978, 1979, J. Gen. Physiol., 71:533-557, 559-580, 73:159-174). A transverse section of the frog sartorius is in fact well fit by a hemi-ellipse with width divided by maximum thickness averaging 5.1 +/- 0.2. Using the previous techniques with the two-dimensional diffusion equation and this hemi-elliptical boundary yields a value for D that is 30% smaller than reported previously; the revised values at 0, 10, and 22.8 degrees C are 6.2, 7.9, and 10.8 X 10(-6) cm2/s, respectively. After a tetanus at 20 degrees C, Qo2 rose quickly to a peak and then declined exponentially, with a time constant (tau) approximately 15% faster than that reported previously; tau averaged 2.1 min in Rana temporaria and 2.6 min in Rana pipiens. A technique was devised to measure the solubility (alpha) of O2 in intact, respiring muscles, and yielded alpha (muscle)/alpha (H2O) = 1.26 +/- 0.04. With these modifications, the values for O2 consumption obtained with the diffusion method were in agreement with those measured by the direct method of Kushmerick and Paul (1976, J. Physiol. [Lond.]., 254:693-709). Using results from both methods, at 20 degrees C the ratio of phosphorylcreatine split during a tetanus to O2 consumption during recovery ranged from 5.2 to 6.2 mumol/mumol, and postcontractile ATP hydrolysis was estimated to be 13.6 +/- 4.1 (n = 3) nmol/mumol total creatine.

Adenosine Triphosphate↗
PubMed1984

Analysis of hydrogen ion concentration in the gastric gel mucus layer.

Secretion of HCO-3 by the gastric epithelium has been thought to lower the concentration of H+ in the gastric mucus layer. This has been analyzed mathematically to include the HCO-3-H+ reaction, bulk water flow, diffusion, ion-ion electrical interaction, and ion-fixed charge interaction. The reaction-electrodiffusion problem is solved by use of singular perturbation theory. We show that there is a very thin layer for the reaction, equivalent to a sink of H+. In this layer there is negligible HCO-3 accumulation. A steady-state model is satisfactory if gastric mixing motions are more frequent than every 3 min. H+ concentration at the epithelium decreases with increased bicarbonate secretion, increased volume flow associated with bicarbonate secretion, increased thickness of the mucus layer, increased fixed negative charge of the mucus, and decreased cation flux into the lumen. The resultant lowering of H+ concentration may be as small as 5 mM but is probably considerably larger. Determining the actual drop will depend on more precise experimental measurements of the parameters of the problem.

Electrophysiology↗