Subcellular restricted spaces: significance for cell signalling and excitation-contraction coupling

Adding a new dimension to cardiac nano-architecture using electron microscopy: coupling membrane excitation to calcium signaling

Advances in microscopic imaging technologies and associated computational methods now allow descriptions of cellular anatomy to go beyond 2-dimensions, revealing new micro-domain dynamics at unprecedented resolutions. In cardiomyocytes, electron microscopy (EM) first described junctional membrane complexes between the sarcolemma and sarcoplasmic reticulum over a half-century ago. Since then, 3-dimensional EM technologies such as electron tomography have become successful in determining the realistic nano-geometry of membrane junctions (dyads and peripheral junctions) and associated structures such as transverse tubules (T-tubules, aka. T-system). Concomitantly, super-resolution light microscopy has gone beyond the diffraction-limit to determine the distribution of molecules, such as ryanodine receptors, with 10(-8) meter (10nm) order accuracy. This review provides the current structural perspective and functional interpretation of membrane junction complexes, which are the central machinery controlling cardiac excitation-contraction coupling via calcium signaling.

Extracellular chloride regulation of Kv2.1, contributor to the major outward Kv current in mammalian outer hair cells

Outer hair cells (OHC) function as both receptors and effectors in providing a boost to auditory reception. Amplification is driven by the motor protein prestin, which is under anionic control. Interestingly, we now find that the major, 4-AP-sensitive, outward K(+) current of the OHC (I(K)) is also sensitive to Cl(-), although, in contrast to prestin, extracellularly. I(K) is inhibited by reducing extracellular Cl(-) levels, with a linear dependence of 0.4%/mM. Other voltage-dependent K(+) (Kv) channel conductances in supporting cells, such as Hensen and Deiters' cells, are not affected by reduced extracellular Cl(-). To elucidate the molecular basis of this Cl(-)-sensitive I(K), we looked at potential molecular candidates based on Cl(-) sensitivity and/or similarities in kinetics. For I(K), we identified three different Ca(2+)-independent components of I(K) based on the time constant of inactivation: a fast, transient outward current, a rapidly activating, slowly inactivating current (Ik(1)), and a slowly inactivating current (Ik(2)). Extracellular Cl(-) differentially affects these components. Because the inactivation time constants of Ik(1) and Ik(2) are similar to those of Kv1.5 and Kv2.1, we transiently transfected these constructs into CHO cells and found that low extracellular Cl(-) inhibited both channels with linear current reductions of 0.38%/mM and 0.49%/mM, respectively. We also tested heterologously expressed Slick and Slack conductances, two intracellularly Cl(-)-sensitive K(+) channels, but found no extracellular Cl(-) sensitivity. The Cl(-) sensitivity of Kv2.1 and its robust expression within OHCs verified by single-cell RT-PCR indicate that these channels underlie the OHC's extracellular Cl(-) sensitivity.

Ryanodine receptor-mediated arrhythmias and sudden cardiac death

The cardiac ryanodine receptor-Ca²⁺ release channel (RyR2) is an essential sarcoplasmic reticulum (SR) transmembrane protein that plays a central role in excitation–contraction coupling (ECC) in cardiomyocytes. Aberrant spontaneous, diastolic Ca²⁺ leak from the SR due to dysfunctional RyR2 contributes to the formation of delayed after-depolarisations, which are thought to underlie the fatal arrhythmia that occurs in both heart failure (HF) and in catecholaminergic polymorphic ventricular tachycardia (CPVT). CPVT is an inherited disorder associated with mutations in either the RyR2 or a SR luminal protein, calsequestrin. RyR2 shows normal function at rest in CPVT but the RyR2 dysfunction is unmasked by physical exercise or emotional stress, suggesting abnormal RyR2 activation as an underlying mechanism. Several potential mechanisms have been advanced to explain the dysfunctional RyR2 observed in HF and CPVT, including enhanced RyR2 phosphorylation status, altered RyR2 regulation at luminal/cytoplasmic sites and perturbed RyR2 intra/inter-molecular interactions. This review considers RyR2 dysfunction in the context of the structural and functional modulation of the channel, and potential therapeutic strategies to stabilise RyR2 function in cardiac pathology.

A 3D Monte Carlo analysis of the role of dyadic space geometry in spark generation

In multiple biological systems, vital intracellular signaling processes occur locally in minute periplasmic subspaces often referred to as signaling microdomains. The number of signaling molecules in these microdomains is small enough to render the notion of continuous concentration changes invalid, such that signaling events are better described using stochastic rather than deterministic methods. Of particular interest is the dyadic cleft in the cardiac myocyte, where short-lived, local increases in intracellular Ca2+ known as Ca2+ sparks regulate excitation-contraction coupling. The geometry of dyadic spaces can alter in disease and development and display significant interspecies variability. We created and studied a 3D Monte Carlo model of the dyadic cleft, specifying the spatial localization of L-type Ca2+ channels and ryanodine receptors. Our analysis revealed how reaction specificity and efficiency are regulated by microdomain geometry as well as the physical separation of signaling molecules into functional complexes. The spark amplitude and rise time were found to be highly dependent on the concentration of activated channels per dyadic cleft and on the intermembrane separation, but not very sensitive to other cleft dimensions. The role of L-type Ca2+ channel and ryanodine receptor phosphorylation was also examined. We anticipate that this modeling approach may be applied to other systems (e.g., neuronal growth cones and chemotactic cells) to create a general description of stochastic events in Ca2+ signaling.

Model study of ATP and ADP buffering, transport of Ca(2+) and Mg(2+), and regulation of ion pumps in ventricular myocyte

We extended the model of the ventricular myocyte by Winslow et al. (Circ. Res 1999, 84:571-586) by incorporating equations for Ca(2+) and Mg(2+) buffering and transport by ATP and ADP and equations for MgATP regulation of ion transporters (Na(+)-K(+) pump, sarcolemmal and sarcoplasmic Ca(2+) pumps). The results indicate that, under normal conditions, Ca(2+) binding by low-affinity ATP and diffusion of CaATP may affect the amplitude and time course of intracellular Ca(2+) signals. The model also suggests that a fall in ATP/ADP ratio significantly reduces sarcoplasmic Ca(2+) content, increases diastolic Ca(2+), lowers systolic Ca(2+), increases Ca(2+) influx through L-type channels, and decreases the efficiency of the Na(+)/Ca(2+) exchanger in extruding Ca(2+) during periodic voltage-clamp stimulation. The analysis suggests that the most important reason for these changes during metabolic inhibition is the down-regulation of the sarcoplasmic Ca(2+)-ATPase pump by reduced diastolic MgATP levels. High Ca(2+) concentrations developed near the membrane might have a greater influence on Mg(2+), ATP, and ADP concentrations than that of the lower Ca(2+) concentrations in the bulk myoplasm. The model predictions are in general agreement with experimental observations measured under normal and pathological conditions.

Localized intracellular calcium signaling in muscle: calcium sparks and calcium quarks

Subcellularly localized Ca2+ signals in cardiac and skeletal muscle have recently been identified as elementary Ca2+ signaling events. The signals, termed Ca2+ sparks and Ca2+ quarks, represent openings of Ca2+ release channels located in the membrane of the sarcoplasmic reticulum (SR). In cardiac muscle, the revolutionary discovery of Ca2+ sparks has allowed the development of a fundamentally different concept for the amplification of Ca2+ signals by Ca(2+)-induced Ca2+ release. In such a system, a graded amplification of the triggering Ca2+ signal entering the myocyte via L-type Ca2+ channels is accomplished by a recruitment process whereby individual SR Ca2+ release units are locally controlled by L-type Ca2+ channels. In skeletal muscle, the initial SR Ca2+ release is governed by voltage-sensors but subsequently activates additional Ca2+ sparks by Ca(2+)-induced Ca2+ release from the SR. Results from studies on elementary Ca2+ release events will improve our knowledge of muscle Ca2+ signaling at all levels of complexity, from the molecule to normal cellular function, and from the regulation of cardiac and skeletal muscle force to the pathophysiology of excitation-contraction coupling.

Control of L-type calcium current during the action potential of guinea-pig ventricular myocytes

1. During an action potential the L-type Ca2+ current (ICa,L) activates rapidly, then partially declines leading to a sustained inward current during the plateau phase. The reason for the sustained part of ICa,L has been investigated here. 2. In the present study the mechanisms controlling the ICa,L during an action potential were investigated quantitatively in isolated guinea-pig ventricular myocytes by whole-cell patch clamp. To measure the actual time courses of ICa,L and the corresponding L-type channel inactivation (fAP) during an action potential, action potential-clamp protocols combined with square pulses were applied. 3. Within the first 10 ms of the action potential the ICa,L rapidly inactivated by about 50 %; during the plateau phase inactivation proceeded to 95 %. Later, during repolarization, the L-type channels recovered up to 25 %. 4. The voltage-dependent component of inactivation during an action potential was determined from measurements of L-type current carried by monovalent cations. This component of inactivation proceeded rather slowly and contributed only a little to fAP. ICa,L during an action potential is thus mainly controlled by Ca2+-dependent inactivation. 5. In order to investigate the source of the Ca2+ controlling fAP, internal Ca2+ homeostasis was manipulated by the use of Ca2+ buffers (EGTA, BAPTA), by blocking Na+-Ca2+ exchange, or by blocking Ca2+ release from the sarcoplasmic reticulum (SR). Internal BAPTA markedly reduced the L-type channel inactivation during the entire action potential, whereas EGTA affected fAP only during the middle and late plateau phases. Inhibition of Na+-Ca2+ exchange markedly increased inactivation of L-type channels. Although blocking SR Ca2+ release decreased the fura-2-measured cytoplasmic Ca2+ concentration ([Ca2+]i) transient by about 90 %, it reduced L-type channel inactivation only during the initial 50 ms of the action potential. Thus, it is Ca2+ entering the cell through the L-type channels that controls the inactivation process for the majority of the action potential. Nevertheless, SR Ca2+-release contributes 40-50 % to L-type channel inactivation during the initial period of the action potential. However, the maximum extent of inactivation reached during the plateau is independent of Ca2+ released from the SR. 6. For the first time, the actual time course of L-type channel inactivation has been directly determined during an action potential under various defined [Ca2+]i conditions. Thereby, the relative contribution to ICa,L inactivation of voltage, Ca2+ entering through L-type channels, and Ca2+ being released from the SR could be directly demonstrated.

Carbachol promotes Na+ entry and augments Na/Ca exchange current in guinea pig ventricular myocytes

The effect of carbachol (CCh) on the Na/Ca exchange current (I(Na/Ca)) was studied in voltage-clamped ventricular myocytes isolated from guinea pig hearts and superfused with Tyrode solution at 35 degrees C. CCh (100 microM) increased outward current during depolarizations (10-200 ms) from -45 mV and tail current amplitude on repolarization; CCh had no effect on the L-type Ca2+ current. Amplitudes of the outward and tail currents declined with increasing duration of the depolarizing clamp pulse. Ouabain produced similar current changes that are suppressed by intrapipette ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid and are characteristic of I(Na/Ca). Depolarization from -80 to -30 mV elicited the rapid Na+ current followed by a slowly decaying inward I(Na/Ca) (J. C. Gilbert, T. Shirayama, and A. J. Pappano. Circ. Res. 69: 1632-1639, 1991.) that was reversibly increased by CCh. Atropine (1-3 microM) prevented the CCh effect. All procedures that suppressed I(Na/Ca) also suppressed the CCh effect. Sarcoplasmic reticulum (SR) Ca2+ release participated in generating I(Na/Ca) because 10 mM caffeine or 1 microM ryanodine blocked I(Na/Ca) and the effect of CCh. Rapid superfusion of 10 mM caffeine induced inward I(Na/Ca) at -75 mV; a caffeine-induced charge transfer gives an SR Ca2+ content of 67 microM. CCh increased caffeine-induced current; SR Ca2+ content rose to 98 microM. CCh also augmented the amplitude of steady-state intracellular Ca2+ transients and contractions during a train of voltage-clamp pulses (-75 to 30 mV for 200 ms) at 1 Hz. CCh elevated intracellular Na+ (M. Korth and V. Kühlkamp. Pflügers Arch. 403: 266-272, 1985) by inducing a background Na+ current [K. Matsumoto and A. J. Pappano. J. Physiol. (Lond.) 415: 487-502, 1989]. Together with these data, the present results are consistent with the hypothesis that CCh, via muscarinic receptors, eventually promotes I(Na/Ca) at the sarcolemma through a mechanism that requires the SR and that this action accounts for the increased contractions.

Na(+)-Ca2+ exchange currents in cortical neurons: concomitant forward and reverse operation and effect of glutamate

Na(+)-Ca2+ exchanger-associated membrane currents were studied in cultured murine neocortical neurons, using whole-cell recording combined with intracellular perfusion. A net inward current specifically associated with forward (Na+(o)-Ca2+(i)) exchange was evoked at -40 mV by switching external 140 mM Li+ to 140 mM Na+. The voltage dependence of this current was consistent with that predicted for 3Na+:1Ca2+ exchange. As expected, the current depended on internal Ca2+, and could be blocked by intracellular application of the exchanger inhibitory peptide, XIP. Raising internal Na+ from 3 to 20 mM or switching the external solution from 140 mM Li+ to 30 mM Na+ activated outward currents, consistent with reverse (Na+(i)-Ca2+(o)) exchange. An external Ca2(+)-sensitive current was also identified as associated with reverse Na(+)-Ca2+ exchange based on its internal Na+ dependence and sensitivity to XIP. Combined application of external Na+ and Ca2+ in the absence of internal Na+ triggered a 3.3-fold larger inward current than the current activated in the presence of 3 mM internal Na+, raising the intriguing possibility that Na(+)-Ca2+ exchangers might concurrently operate in both the forward and the reverse direction, perhaps in different subcellular locations. With this idea in mind, we examined the effect of excitotoxic glutamate receptor activation on exchanger operation. After 3-5 min of exposure to 100-200 microM glutamate, the forward exchanger current was significantly increased even when external Na+ was reduced to 100 mM, and the external Ca2(+)-activated reverse exchanger current was eliminated.

Subcellular properties of triggered Ca2+ waves in isolated citrate-loaded guinea-pig atrial myocytes characterized by ratiometric confocal microscopy

1. Spatiotemporal aspects of subcellular Ca2+ signalling were studied in cultured adult guinea-pig atrial myocytes. A mixture of the Ca2+ indicators fluo-3 and Fura Red in combination with laser-scanning confocal microscopy was used for [Ca2+]i measurements while membrane currents were recorded simultaneously. 2. In citrate-loaded atrial myocytes not every Ca2+ current (ICa) could trigger Ca2+ release from the sarcoplasmic reticulum (SR). Two types of Ca2+ signals could be observed: Ca2+ transients resulting from (i) Ca2+ influx alone and (ii) additional Ca2+ release. 3. Ca2+ release elicited by voltage steps of 100-150 ms duration was either apparently homogeneous or propagated as Ca2+ waves through the entire cell. With brief ICa (50-75 ms), Ca2+ waves with limited subcellular propagation were observed frequently. These waves always originated from either end of the myocyte. 4. The time course of changes in Na(+)-Ca2+ exchange current (INaCa) depended on the subcellular properties of the underlying Ca2+ transient and on the particular cell geometry. Apparently homogeneous Ca2+ release was accompanied by an inward change of INaCa the onset phase of which was fused with ICa. Changes in INaCa caused by a Ca2+ wave propagating through the entire cell showed a W shape, which could be attributed to differences of the fractional surface-to-volume ratio in different cell segments during propagation of the Ca2+ wavefront. Those waves with limited spreading only activated a small component of INaCa. 5. The different subcellular patterns of Ca2+ release signals can be explained by spatial inhomogeneities in the positive feedback of the SR. This depends on the local SR Ca2+ loading state under the control of the local Ca2+ influx during activation of ICa. Due to the higher surface-to-volume ratio at the two ends of the myocyte, SR loading and therefore the positive feedback in Ca(2+)-induced Ca2+ release may be higher at the ends, locations where Ca2+ waves are preferentially triggered. 6. We conclude that the individual cell geometry may be an important determinant of subcellular Ca2+ signalling not only in cardiac muscle cells but presumably also in other types of cells that depend on Ca2+ signalling. In addition, the cell geometry in combination with varying subcellular Ca2+ release patterns can greatly affect the time course of Ca(2+)-activated membrane currents.

High cytoplasmic Ca2+ levels reached during Ca(2+)-induced Ca2+ release in single smooth muscle cell as reported by a low affinity Ca2+ indicator Mag-Indo-1

The low-affinity Ca2+ indicator Mag-Indo-1 was used to measure increments of ionised Ca2+ concentration in the cytoplasm of single smooth muscle cells isolated from guinea-pig urinary bladder. With 3.6 mM [Ca2+]o, depolarization steps to 0 mV were associated with a transient increase of fluorescence ratio (F410/F470) only when Ica triggered a Ca(2+)-induced Ca2+ release (CICR). [Ca2+]i transiently peaked to 3-5 microM and despite continuous Ca2+ influx the [Ca2+]i signal fell close to the baseline. Rapidly applied caffeine (10 mM) increased [Ca2+]l by 16 microM, the response was completely blocked by intracellular ryanodine (20 microM). With ryanodine intracellularly, Ica produced very small [Ca2+]i signals unless it was augmented by elevation of [Ca2+]0 to 10 mM and addition of 1 microM Bay K8644. Under these conditions, [Ca2+]l responded with a tonic elevation lasting as long as the depolarizing pulse. It is concluded that the low-affinity indicator Mag-Indo-1 reports predominantly Ca2+ release from SR in cytoplasm.

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.

Photolysis of caged compounds characterized by ratiometric confocal microscopy: a new approach to homogeneously control and measure the calcium concentration in cardiac myocytes

Here we describe the subcellularly uniform control of the intracellular Ca2+ concentration ([Ca2+]i) by flash photolysis of caged Ca2+ or a caged Ca2+ buffer. A mixture of the two Ca2+ indicators Fluo-3 and Fura-red was used together with a laser-scanning confocal microscope to reveal spatial aspects of intracellular Ca2+ signals. The patch clamp technique in the whole-cell variant was applied to load the cells with the indicator mixture together with either DM-nitrophen or diazo-2 and to measure changes in the membrane current. An in vivo calibration was performed to convert the Fluo-3/Fura-red fluorescence ratios to [Ca2+] values. The resulting calibration curve suggested an apparent KD of 1.6 microM, Rmax of 2.15, Rmin of 0.08 and a Hill-coefficient of 0.75 for the indicator mixture. Controlled rupture of the cell membrane revealed a large fraction of immobile intracellular Fura-red fluorescence that may account for the reduced in vivo Rmax value when compared to the in vitro value of 3.1. In cardiac myocytes, flash photolytic release of Ca2+ from DM-nitrophen generated inwardly directed Na+/Ca2+ exchange currents and Ca2+ signals that were graded with the discharged flash-energy. Rapid line-scans revealed subcellularly homogeneous [Ca2+] jumps regardless of the discharged flash energy. Ca2+ signals evoked by L-type Ca2+ currents (ICa) could be terminated rapidly in a spatially homogeneous manner by UV flash photolysis of diazo-2. No side-effects of the photolytic products of DM-nitrophen or diazo-2 with the mixture of Fluo-3/Fura-red were detectable in our experiments. The combination of UV flash photolysis and laser scanning confocal microscopy enabled us to control [Ca2+]i homogeneously on the subcellular level. This approach may improve our understanding of the subcellular properties of cardiac Ca2+ signalling. The technique can also be applied in other cell types and with other signalling systems for which caged compounds are available.

Intracellular citrate induces regenerative calcium release from sarcoplasmic reticulum in guinea-pig atrial myocytes

Ca2+ release from the sarcoplasmic reticulum was studied in voltage-clamped guinea-pig atrial myocytes. Cells were dialysed with a pipette solution containing the Ca2+ indicator 1- [2-amino-5-(6-carboxyindol-2-yl) phenoxy]-2-(2'-amino-5'-methylphenoxy) ethane-N,N,N',N'-tetraacetic acid] (Indo-1, 100 microM) and as main anion either chloride or the low-affinity Ca2+ buffer citrate. Intracellular Ca2+ transients (Cai transients) were elicited by depolarizations from a holding potential of -50 mV. In chloride-dialysed cells, Cai transients showed a bell-shaped dependence on the amplitude of the depolarizing pulse. In citrate-dialysed cells, membrane depolarizations were associated with a small rise in [Ca2+]i. These small changes in [Ca2+]i were either followed by a large Cai transient or failed to induce large changes in [Ca2+]i. The peak amplitude of the large Cai transient did not vary with the amplitude of the depolarizing pulse. These results demonstrate that in the presence of intracellular chloride, Ca2+ release in atrial cells is a graded process triggered by Ca2+ influx. Using citrate as the main intracellular anoin, Ca2+ release triggered by Ca2+ entry was no longer graded but occurred in a regenerative manner. The results are discussed in terms of two models in which citrate, affects the spatial distribution of [Ca2+]i or the loading state of the sarcoplasmic reticulum.

Voltage dependence of Na-Ca exchanger conformational currents

Properties of a transient current (Icont) believed to reflect a conformational change of the Na-Ca exchanger molecules after Ca2+ binding were investigated. Intracellular Ca2+ concentration jumps in isolated cardiac myocytes were generated with flash photolysis of caged Ca2+ dimethoxynitrophenamine, and membrane currents were simultaneously measured using the whole-cell variant of the patch-clamp technique. A previously unresolved shallow voltage dependence of Icont was revealed after developing an experimental protocol designed to compensate for the photoconsumption of the caged compound. This voltage dependence can be interpreted to reflect the distribution of Na-Ca exchanger conformational states with the Ca2+ binding site exposed to the inside of the cell immediately before the flash. Analysis performed by fitting a Boltzmann distribution to the observed data suggests that under control conditions most exchanger molecules reside in states with the Ca2+ binding site facing the outside of the cell. Dialysis of the cytosol with 3',4'-dichlorobenzamil, an organic inhibitor of the Na-Ca exchange, increased the magnitude of Icont and changed the voltage dependence, consistent with a parallel shift of the charge/voltage curve. This shift may result from intracellular DCB interfering with an Na(+)-binding or Na(+)-translocating step. These observations are consistent with Icont arising from a charge movement mediated by the Na-Ca exchanger molecules after binding of Ca2+.

Modulation of Ca2+ release in cultured neonatal rat cardiac myocytes. Insight from subcellular release patterns revealed by confocal microscopy

It is well established that in heart muscle the influx of Ca2+ through Ca2+ channels during the action potential is the main trigger for Ca2+ release from the sarcoplasmic reticulum (SR), but intact cardiac tissue and single myocytes are also known to exhibit spontaneous Ca2+ release from the SR under a variety of circumstances. Although conditions favoring spontaneous activity have been examined extensively, mechanisms modulating or regulating spontaneous as well as triggered Ca2+ release are still largely unknown. Using the high spatial and temporal resolution of laser-scanning confocal microscopy, we investigated subcellular aspects of spontaneous and triggered Ca2+ release in isolated rat neonatal myocytes loaded with the Ca(2+)-sensitive fluorescent dye fluo 3. Three distinct patterns of spontaneous Ca2+ release were identified: (1) a homogeneous Ca2+ release, presumably corresponding to Ca2+ release during a spontaneous action potential, (2) a focal or spatially restricted Ca2+ release with no or only limited subcellular propagation, and (3) a Ca2+ release propagating as a wave throughout the entire cell. Pharmacologic tools that interfere with the SR revealed that all release types were critically dependent on the Ca2+ release and uptake function of the SR. From our results we conclude that the probability, extent, and pattern of Ca2+ release are modulated on the subcellular level. The observed spectrum of release patterns can be explained by a space- and time-dependent variability in the positive feedback of the Ca(2+)-induced Ca(2+)-release mechanism within an individual myocyte. Presumably, this variability depends on the existence of subcellular functional elements of the SR. The actual degree of positive feedback may be modulated locally by the Ca(2+)-loading state of each SR element.

Metabolic compartmentation and substrate channelling in muscle cells. Role of coupled creatine kinases in in vivo regulation of cellular respiration--a synthesis

The published experimental data and existing concepts of cellular regulation of respiration are analyzed. Conventional, simplified considerations of regulatory mechanism by cytoplasmic ADP according to Michaelis-Menten kinetics or by derived parameters such as phosphate potential etc. do not explain relationships between oxygen consumption, workload and metabolic state of the cell. On the other hand, there are abundant data in literature showing microheterogeneity of cytoplasmic space in muscle cells, in particular with respect to ATP (and ADP) due to the structural organization of cell interior, existence of multienzyme complexes and structured water phase. Also very recent experimental data show that the intracellular diffusion of ADP is retarded in cardiomyocytes because of very low permeability of the mitochondrial outer membrane for adenine nucleotides in vivo. Most probably, permeability of the outer mitochondrial membrane porin channels is controlled in the cells in vivo by some intracellular factors which may be connected to cytoskeleton and lost during mitochondrial isolation. All these numerous data show convincingly that cellular metabolism cannot be understood if cell interior is considered as homogenous solution, and it is necessary to use the theories of organized metabolic systems and substrate-product channelling in multienzyme systems to understand metabolic regulation of respiration. One of these systems is the creatine kinase system, which channels high energy phosphates from mitochondria to sites of energy utilization. It is proposed that in muscle cells feed-back signal between contraction and mitochondrial respiration may be conducted by metabolic wave (propagation of oscillations of local concentration of ADP and creatine) through cytoplasmic equilibrium creatine and adenylate kinases and is amplified by coupled creatine kinase reaction in mitochondria. Mitochondrial creatine kinase has experimentally been shown to be a powerful amplifier of regulatory action of weak ADP fluxes due to its coupling to adenine nucleotide translocase. This phenomenon is also carefully analyzed.

Sodium current-induced calcium signals in isolated guinea-pig ventricular myocytes

1. Na+ current (INa)-induced Ca2+ transients were studied in ventricular myocytes isolated from adult guinea-pig hearts. The fluorescent Ca2+ indicator fluo-3 or a mixture of fluo-3 and fura-red were used in conjunction with confocal microscopy to follow the intracellular Ca2+ concentration while membrane currents were measured simultaneously with the whole-cell configuration of the patch-clamp technique. 2. Ca2+ release from the sarcoplasmic reticulum (SR) could be triggered either by Ca2+ current (ICa) or Na+ current (INa). Analysis of INa-induced Ca2+ signals at higher temporal resolution revealed a faster upstroke of these transients when compared with those triggered by ICa. 3. In the presence of 20 microM ryanodine to block SR Ca2+ release ICa elicited a verapamil-sensitive Ca2+ transient with a slow upstroke. INa also induced a residual Ca2+ transient that was insensitive to 10 microM verapamil and characterized by a rapid upstroke. 4. The existence of a residual Ca2+ transient in the absence of SR Ca2+ release and L-type ICa indicates that INa is indeed able to evoke an increase in [Ca2+]i without uncontrolled activation of Ca2+ channels. 5. Substitution of extracellular Na+ by Li+ suppressed INa-induced Ca2+ transients, suggesting that the Ca2+ release and the residual Ca2+ transient can only be elicited by influx of Na+ and not by Li+. This result supports the notion that both the residual Ca2+ transient as well as the INa-induced Ca2+ release are mediated by the Na(+)-Ca2+ exchange.(ABSTRACT TRUNCATED AT 250 WORDS)