Calcium transients caused by calcium entry are influenced by the sarcoplasmic reticulum in guinea-pig atrial myocytes

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

Temperature dependence of cardiac sarcoplasmic reticulum function in rainbow trout myocytes

To explore how the cardiac sarcoplasmic reticulum (SR) functions over a range of temperatures, we used whole-cell voltage clamp combined with rapid caffeine application to study SR Ca2+ accumulation, release and steady-state content in atrial myocytes from rainbow trout. Myocytes were isolated from rainbow trout acclimated to 14°C, and the effect of varying stimulation pulse number, frequency and experimental temperature (7°C,14°C and 21°C) on SR function was studied. To add physiological relevance, in addition to 200 ms square (SQ) voltage pulses, myocytes were stimulated with temperature-specific action potentials (AP) applied at relevant frequencies for each test temperature. We found that the SR accumulated Ca2+ more rapidly and to a greater concentration(1043±189 μmol l-1 Ca2+, 1138±173μmol l-1 Ca2+, and 1095±142 μmol l-1 Ca2+ at 7°C, 14°C and 21°C,respectively) when stimulated with physiological AP waveforms at physiological frequencies compared with 200 ms SQ pulses at the same frequencies(664±180 μmol l-1 Ca2+, 474±75 μmol l-1 Ca2+ and 367±42 μmol l-1Ca2+ at 7°C, 14°C and 21°C, respectively). Also, and in contrast to 200 ms SQ pulse stimulation, temperature had little effect on steady-state SR Ca2+ accumulation during AP stimulation. Furthermore, we observed SR-Ca2+-dependent inactivation of the L-type Ca2+ channel current (ICa) at 7°C, 14°C and 21°C, providing additional evidence of maintained SR function in fish hearts over an acute range of temperatures. We conclude that the waveform of the AP may be critical in ensuring adequate SR Ca2+ cycling during temperature change in rainbow trout in vivo.

Cl- current blockade reduces triggered activity based on delayed afterdepolarisations

OBJECTIVES

Increasing evidence suggests that a Ca²⁺-activated Cl^- current (I_Cl(Ca)) contributes to the transient inward current (I_ti), the current responsible for proarrhythmic delayed after-depolarisations (DADs). Because the equilibrium potential for Cl^- ions (E_Cl) in myocytes is around - 50 mV, activation of the I_Cl(Ca) results in an inward depolarising current at resting membrane potential and I_Cl(Ca) may thus be responsible for a part of the depolarisation during a DAD. In this study, we investigated the ionic nature of I_ti and the effects of Cl^- current blockade on DADs.

METHODS AND RESULTS

The ionic mechanisms of I_ti and underlying DADs were studied in sheep ventricular myocytes using the patch-clamp methodology. The DADs were induced in the myocytes by exposure to 1 μM noradrenaline and the I_ti were elicited by repetitive depolarisations from -93 mV to +37 mV in the presence of the drug. The current-voltage relation of I_ti reversed in sign around -20 mV. The outward I_ti was completely blocked by the anion current blocker 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS), whereas the inward I_ti was only slightly affected. The DIDS-sensitive component of I_ti was outwardly rectifying with a reversal potential close to E_Cl. The DIDS-insensitive component of I_ti was abolished by blockade of the Na⁺-Ca²⁺ exchanger by substitution of extracellular Na⁺ by equimolar Li⁺. Interestingly, DIDS reduced the DAD amplitude and triggered activity based on DADs.

CONCLUSION

In sheep ventricular myocytes, I_ti consists of two ionic mechanisms: a Cl^- current and a Na⁺-Ca²⁺ exchange current. Blockade of the Cl^- current may be potentially antiarrhythmic by lowering DAD amplitude and triggered activity based on DADs.

Calcium-activated Cl(-) current contributes to delayed afterdepolarizations in single Purkinje and ventricular myocytes

BACKGROUND

The ionic mechanism underlying the transient inward current (I(ti)), the current responsible for delayed afterdepolarizations (DADs), appears to be different in ventricular myocytes and Purkinje fibers. In ventricular myocytes, I(ti) was ascribed to a Na(+)-Ca(2+) exchange current, whereas in Purkinje fibers, it was additionally ascribed to a Cl(-) current and a nonselective cation current. If Cl(-) current contributes to I(ti) and thus to DADs, Cl(-) current blockade may be potentially antiarrhythmogenic. In this study, we investigated the ionic nature of I(ti) in single sheep Purkinje and ventricular myocytes and the effects of Cl(-) current blockade on DADs.

METHODS AND RESULTS

In whole-cell patch-clamp experiments, I(ti) was induced by repetitive depolarizations from -93 to +37 mV in the presence of 1 micromol/L norepinephrine. In both Purkinje and ventricular myocytes, I(ti) was inward at negative potentials and outward at positive potentials. The anion blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) blocked outward I(ti) completely but inward I(ti) only slightly. The DIDS-sensitive component of I(ti) was outwardly rectifying, with a reversal close to the reversal potential of Cl(-) currents. Blockade of Na(+)-Ca(2+) exchange by substitution of extracellular Na(+) by equimolar Li(+) abolished the DIDS-insensitive component of I(ti). DIDS reduced both DAD amplitude and triggered activity based on DADs. Conclusions-In both Purkinje and ventricular myocytes, I(ti) consists of 2 ionic mechanisms: a Cl(-) current and a Na(+)-Ca(2+) exchange current. Blockade of the Cl(-) current may be potentially antiarrhythmogenic by lowering DAD amplitude and triggered activity based on DADs.

Ca(2+) transients and Ca(2+) waves in purkinje cells : role in action potential initiation

Purkinje cells contain sarcoplasmic reticulum (SR) directly under the surface membrane, are devoid of t-tubuli, and are packed with myofibrils surrounded by central SR. Several studies have reported that electrical excitation induces a biphasic Ca(2+) transient in Purkinje fiber bundles. We determined the nature of the biphasic Ca(2+) transient in aggregates of Purkinje cells. Aggregates (n=12) were dispersed from the subendocardial Purkinje fiber network of normal canine left ventricle, loaded with Fluo-3/AM, and studied in normal Tyrode's solution (24 degrees C). Membrane action potentials were recorded with fine-tipped microelectrodes, and spatial and temporal changes in [Ca(2+)](i) were obtained from fluorescent images with an epifluorescent microscope (x20; Nikon). Electrical stimulation elicited an action potential as well as a sudden increase in fluorescence (L(0)) compared with resting levels. This was followed by a further increase in fluorescence (L(1)) along the edges of the cells. Fluorescence then progressed toward the Purkinje cell core (velocity of propagation 180 to 313 microm/s). In 62% of the aggregates, initial fluorescent changes of L(0) were followed by focally arising Ca(2+) waves (L(2)), which propagated at 158+/-14 microm/s (n=13). Spontaneous Ca(2+) waves (L(2)*) propagated like L(2) (164+/-10 microm/s) occurred between stimuli and caused slow membrane depolarization; 28% of L(2)* elicited action potentials. Both spontaneous Ca(2+) wave propagation and resulting membrane depolarization were thapsigargin sensitive. Early afterdepolarizations were not accompanied by Ca(2+) waves. Action potentials in Purkinje aggregates induced a rapid rise of Ca(2+) through I(CaL) and release from a subsarcolemmal compartment (L(0)). Ca(2+) release during L(0) either induced further Ca(2+) release, which propagated toward the cell core (L(1)), or initiated Ca(2+) release from small regions and caused L(2) Ca(2+) waves, which propagated throughout the aggregate. Spontaneous Ca(2+) waves (L(2)*) induce action potentials.

Confocal laser scanning microscopy used to monitor intracellular Ca2+ changes in hippocampal CA 1 neurons during energy deprivation

An increase in intracellular calcium during cerebral ischemia has been proposed as a common final pathway underlying the events leading to neuronal death. Intracellular calcium has been measured with ion selective electrodes during energy deprivation (ED) in hippocampal slices and with fluorescent techniques in neuronal cultures. In the present study, we describe a novel method to visualize and quantify changes in intracellular calcium in brain slices using Confocal Laser Scanning Microscopy (CLSM). CA 1 pyramidal neurons in hippocampal slices were filled by intracellular injection with a 1:2 mixture of the fluorescent dyes Fluo 3 and Fura Red. The neurons were then visualized using CLSM, and the ratio of the fluorescence from each probe used to quantify intracellular calcium concentrations before and during ED. The free intracellular calcium concentration was 60 nM prior to ED and increased to 24 microM during ED. These results demonstrates that CLSM and fluorescent probes can be used in functional neuronal networks in addition to cell cultures as previously described.

All-or-none Ca2+ release from intracellular stores triggered by Ca2+ influx through voltage-gated Ca2+ channels in rat sensory neurons

Ca2+-induced Ca2+ release (CICR) from intracellular stores amplifies the Ca2+ signal that results from depolarization. In neurons, the amplification has been described as a graded process. Here we show that regenerative CICR develops as an all-or-none event in cultured rat dorsal root ganglion neurons in which ryanodine receptors have been sensitized to Ca2+ by caffeine. We used indo-1-based microfluorimetry in combination with whole-cell patch-clamp recording to characterize the relationship between Ca2+ influx and Ca2+ release. Regenerative release of Ca2+ was triggered when action potential-induced Ca2+ influx increased the intracellular Ca2+ concentration ([Ca2+]i) above threshold. The threshold was modulated by caffeine and intraluminal Ca2+. A relative refractory period followed CICR. The pharmacological profile of the response was consistent with Ca2+ influx through voltage-gated Ca2+ channels triggering release from ryanodine-sensitive stores. The activation of a suprathreshold response increased more than fivefold the amplitude and duration of the [Ca2+]i transient. The switch to a suprathreshold response was regulated very precisely in that addition of a single action potential to the stimulus train was sufficient for this transformation. Confocal imaging experiments showed that CICR facilitated propagation of the Ca2+ signal from the plasmalemma to the nucleus. This all-or-none reaction may serve as a switch that determines whether a given electrical signal will be transduced into a local or widespread increase in [Ca2+]i.

All-or-none Ca2+ release from intracellular stores triggered by Ca2+ influx through voltage-gated Ca2+ channels in rat sensory neurons

Ca2+-induced Ca2+ release (CICR) from intracellular stores amplifies the Ca2+ signal that results from depolarization. In neurons, the amplification has been described as a graded process. Here we show that regenerative CICR develops as an all-or-none event in cultured rat dorsal root ganglion neurons in which ryanodine receptors have been sensitized to Ca2+ by caffeine. We used indo-1-based microfluorimetry in combination with whole-cell patch-clamp recording to characterize the relationship between Ca2+ influx and Ca2+ release. Regenerative release of Ca2+ was triggered when action potential-induced Ca2+ influx increased the intracellular Ca2+ concentration ([Ca2+]i) above threshold. The threshold was modulated by caffeine and intraluminal Ca2+. A relative refractory period followed CICR. The pharmacological profile of the response was consistent with Ca2+ influx through voltage-gated Ca2+ channels triggering release from ryanodine-sensitive stores. The activation of a suprathreshold response increased more than fivefold the amplitude and duration of the [Ca2+]i transient. The switch to a suprathreshold response was regulated very precisely in that addition of a single action potential to the stimulus train was sufficient for this transformation. Confocal imaging experiments showed that CICR facilitated propagation of the Ca2+ signal from the plasmalemma to the nucleus. This all-or-none reaction may serve as a switch that determines whether a given electrical signal will be transduced into a local or widespread increase in [Ca2+]i.

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.

Calcium gradients during excitation-contraction coupling in cat atrial myocytes

1. Confocal microscopy in combination with the calcium-sensitive fluorescent probe fluo-3 was used to study spatial aspects of intracellular Ca2+ signals during excitation-contraction coupling in isolated atrial myocytes from cat heart. 2. Imaging of [Ca2+]i transients evoked by electrical stimulation revealed that Ca2+ release started at the periphery and subsequently spread towards the centre of the myocyte. 3. Blocking sarcoplasmic reticulum (SR) Ca2+ release with 50 microM ryanodine unmasked spatial inhomogeneities in the [Ca2+]i was higher in the periphery than in central regions of the myocyte. 4. Positive (or negative) staircase or postrest potentiation of the 'whole-cell' [Ca2+] transients were paralleled by characteristic changes in the spatial profile of the [Ca2+]i signal. With low SR Ca2+ load [Ca2+]i transients in the subsarcolemmal space were small and no Ca2+ release in the centre of the cell was observed. Loading of the SR increased subsarcolemmal [Ca2+]i transient amplitude and subsequently triggered further release in more central regions of the cell. 5. Spontaneous Ca2+ release from functional SR units, i.e. Ca2+ sparks, occurred at higher frequency in the subsarcolemmal space than in more central regions of the myocyte. 6. Visualization of the surface membrane using the membrane-selective dye Di-8-ANEPPS demonstrated that transverse tubules (t-tubules) were absent in atrial cells. 7. It is concluded that in atrial myocytes voltage-dependent Ca2+ entry triggers Ca2+ release from peripheral coupling SR that subsequently induces further Ca2+ release from stores in more central regions of the myocyte. Spreading of Ca2+ release from the cell periphery to the centre accounts for [Ca2+]i gradients underlying the whole-cell [Ca2+]i transient. The finding that cat atrial myocytes lack t-tubules demonstrates the functional importance of Ca2+ release from extended junctional (corbular) SR in these cells.

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.

Down-regulation of A1 adenosine receptors coupled to muscarinic K+ current in cultured guinea-pig atrial myocytes

1. Muscarinic K+ current (IK(ACh)) was measured in cultured atrial myocytes from hearts of adult guinea-pigs using whole-cell voltage clamp. IK(ACh) was activated by superfusion with solutions containing either acetylcholine (ACh) or adenosine (Ado), in saturating concentrations of 2 microM (ACh) and 1 mM (Ado), respectively. 2. In freshly isolated cells the amplitude of the current activated by Ado (IK(Ado)) was 58% (mean) of the current that was induced by ACh. In serum-free culture this relation, but also the absolute density of IK(ACh), remained fairly constant for up to 8 days. 3. If the culture medium was supplemented with fetal calf serum (FCS, 5%) the relation IK(Ado)/IK(ACh) gradually decayed, reaching a value of less than 0.1 on days 7-8, whereas the response to ACh remained stable over this period of time. 4. After treatment of cells with FCS-containing medium, no recovery was observed upon FCS withdrawal for up to 4 days. 5. The effect of FCS on responsiveness to Ado was half-maximal at about 1% (v/v). The active principle can be dialysed (mol. mass exclusion: 10 kDa). It is not identical with an albumin-associated factor that has been shown to be a potent activator of atrial IK(ACh) upon acute superfusion. Loss of responsiveness to Ado was paralleled by a reduction of binding sites to the A1 adenosine receptor-specific radioligand 8-cyclopentyl-1,3-dipropylxanthine ([3H]CPX). 6. It is concluded that FCS contains a factor that causes down-regulation of A1 Ado receptors. The signalling pathway that leads to an increased opening activity of IK(ACh) channels and other receptors, such as the M2 muscarinic receptor, linked to this signalling pathway are not affected by this factor.

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.

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)

Ca2+ and Sr2+ entry induced Ca2+ release from the intracellular Ca2+ store in smooth muscle cells of rat portal vein

1. Changes in cytosolic free Ca2+ concentration ([Ca2+]i) induced by membrane depolarizations were investigated using indo-1 microspectrofluorimetry in single patch-clamped smooth muscle cells of rat portal vein at room temperature (20-21 degrees C) and in the presence of 2 mM Ca2+. 2. During a 1 s depolarization from -50 to -30 mV [Ca2+]i rose, but, although the Ca2+ current was terminated by repolarization to -50 mV, [Ca2+]i continued to increase in a regenerative manner. The delay between the end of the voltage step and the peak of the [Ca2+]i rise was reduced by increasing the depolarization. 3. When a second identical depolarization was rapidly applied (8-13s) after the first one, it induced an identical Ca2+ current but a smaller increase in [Ca2+]i which started to decay upon repolarization. 4. A low concentration of caffeine (0.05 mM), applied to cells showing a small depolarization-induced [Ca2+]i transient which reached a peak at the end of the voltage step, produced an increase in amplitude and in duration of the [Ca2+]i rise without changing the amplitude of the depolarization-induced Ca2+ current. 5. The depolarization-induced [Ca2+]i rise was shortened and reduced in amplitude after noradrenaline- (NA 10 microM) or caffeine- (5 mM) induced release of Ca2+ store and when the patch pipette solution contained ryanodine (100 microM). Under these conditions, the depolarization-induced [Ca2+]i transient was maximal at the end of the voltage step and declined immediately when the membrane was repolarized at -50 mV. 6. Experiments were done by replacing extracellular Ca2+ by Sr2+. Depolarization-induced Sr2+ entry through voltage-dependent Ca2+ channels could evoke an increase in indo-1 fluorescence which occurred after the termination of the voltage step. This delayed component of fluorescence increase displayed properties similar to those of the regenerative [Ca2+]i rise recorded in the Ca(2+)-containing solution. 7. The inefficiency of the second of two successive depolarizations to produce the delayed component of [Ca2+]i rise was not due to the emptiness of the intracellular Ca2+ store since, under these conditions, caffeine was still able to induce a Ca2+ release. 8. It is concluded that depolarization-evoked Ca2+ or Sr2+ entry through voltage-dependent Ca2+ channels induced the release of Ca2+ from an intracellular store, which could occur in a regenerative manner, independent of the termination of the triggering current.

Effects of thapsigargin and cyclopiazonic acid on twitch force and sarcoplasmic reticulum Ca2+ content of rabbit ventricular muscle

Thapsigargin (TG) and cyclopiazonic acid (CPA) are reported to be specific high-affinity inhibitors of the sarcoplasmic reticulum (SR) Ca2+ pump in isolated membranes and cells, with TG causing complete pump inhibition at nanomolar concentrations. To evaluate the effectiveness of TG and CPA in small multicellular cardiac preparations, we used rapid cooling contractures (RCCs) to assess the SR Ca2+ load. In contrast to observations in single myocytes, TG caused remarkably slow and incomplete SR Ca2+ depletion in multicellular preparations. A 45-minute exposure to 500 microM TG at 30 degrees C and 0.5-Hz stimulation only decreased RCCs by 76 +/- 5% (and 100 microM CPA reduced RCCs by 59 +/- 10% [mean +/- SEM]). In contrast, 10 minutes with 20 mM caffeine completely abolished RCCs. This confirms that there was still a caffeine-sensitive pool of Ca2+ in the TG-treated muscle. The time constant of rest decay of RCCs was accelerated by both TG (from 83 +/- 18 to 26 +/- 6 seconds) and CPA (from 68 +/- 11 to 10 +/- 5 seconds). This might be expected since Ca2+ leaking from the SR during rest cannot be taken back up as efficiently, favoring Ca2+ extrusion by the sarcolemmal Na(+)-Ca2+ exchanger. TG and CPA decreased twitch force (by 44 +/- 7% and 40 +/- 11%, respectively) and increased twitch duration, presumably because of the SR effects. We conclude that complete blockade of SR Ca2+ uptake by TG or CPA in multicellular preparations cannot be assumed, even at high [TG] or [CPA], unless evaluated (eg, by RCC).

Buffering of calcium influx by sarcoplasmic reticulum during the action potential in guinea-pig ventricular myocytes

1. Intracellular [Ca2+] ([Ca2+]i) transients, monitored by the fluorescent Ca2+ indicator, indo-1, and twitch contractions elicited by action potentials, by voltage clamp pulses or by rapid, brief pulses of caffeine, were measured in guinea-pig single ventricular myocytes. Experiments were designed to determine whether and to what extent the trans-sarcolemmal Ca2+ influx is immediately sequestered by the sarcoplasmic reticulum (SR). 2. Rapid, brief (100-200 ms) pulses of caffeine onto a rested myocyte elicited a [Ca2+]i transient and a contraction. Following exposure to specific SR inhibitors, ryanodine (100 nM) or thapsigargin (200 nM), the rapid application of caffeine onto a rested myocyte failed to elicit changes in [Ca2+]i or in cell length, indicating that caffeine increases [Ca2+]i by specifically discharging Ca2+ from the SR. In the absence of these inhibitors, a second pulse of caffeine, within 3 min following a prior pulse, failed to elicit a [Ca2+]i transient or contraction, indicating that a caffeine pulse depletes the SR releasable Ca2+ pool. 3. Following Ca2+ depletion of the SR by double caffeine pulses at rest, an electrical stimulation elicited a slow increase in [Ca2+]i, and, after a delay, a small, slow twitch contraction. The simultaneous application of caffeine and electrical stimulation of cells in which the SR was Ca2+ depleted elicited [Ca2+]i transients with an increased rate of rise and a larger amplitude (53 +/- 8 and 63 +/- 9% respectively; mean +/- S.E.M., n = 21) than those elicited by electrical stimulation alone. 4. Whether caffeine affected the L-type calcium current (ICa) elicited by electrical stimulation was determined under whole-cell voltage clamp. A caffeine pulse delivered at the onset of a depolarizing voltage clamp step also increased the rates of rise and the amplitudes of the [Ca2+]i transients and twitch contractions in cells in which the SR was depleted of Ca2+. However, Ca2+ influx via ICa decreased when caffeine was pulsed in conjunction with the voltage clamp, as the peak ICa was either unchanged or decreased while its inactivation was consistently accelerated. 5. Because the stimulation-dependent trans-sarcolemmal Ca2+ influx via ICa is not increased by a caffeine pulse, the augmentation of the rates of rise and the amplitudes of the electrically stimulated [Ca2+]i transients by caffeine pulsed in conjunction with the electrical stimulation in cells in which the SR had been depleted of Ca2+ indicates that a portion of Ca2+ influx during depolarization in the absence of caffeine is rapidly buffered by the SR.(ABSTRACT TRUNCATED AT 400 WORDS)

Ratiometric confocal Ca(2+)-measurements with visible wavelength indicators in isolated cardiac myocytes

We present a new method for ratiometric Ca2+ measurements using indicators with excitation spectra in the visible range of wavelengths. Laser-scanning confocal microscopy was used to record intracellular Ca(2+)-signals with high temporal and spatial resolution in single cardiac myocytes. The patch-clamp technique was applied to load the cells with the fluorescent Ca(2+)-indicators and to follow the membrane currents with the fluorescence signals simultaneously. Intracellular free Ca(2+)-concentration ([Ca2+]i) was estimated with a ratiometric method. An in vitro calibration procedure was used to convert the fluorescence ratio obtained with two different Ca(2+)-indicators (Fluo-3 and Fura-Red) into Ca(2+)-concentrations. Fluo-3 showed an increase in fluorescence upon a rise in intracellular Ca(2+)-concentration, while the Fura-Red fluorescence decreased. Since the fluorescence of Fluo-3 was around 2-fold brighter than the Fura-Red signal the cells were loaded with a 1:2 mixture of the two indicators. The large increase of the fluorescence ratio during a rise in [Ca2+]i (up to 4-fold) allowed us to record time-resolved signals with this mixture even when monitored in a very small subcellular volume (around 1 micron3). Long lasting continuous recordings of the fluorescence were possible because the dye-mixture exhibited no detectable bleaching with illumination periods of up to 30 s. The use of the Fluo-3/Fura-Red ratio method should significantly facilitate and improve quantitative measurements of [Ca2+]i with high temporal and spatial resolution. Moreover, this approach is especially valuable when used with confocal microscopes which are usually equipped with lasers in the visible light range. Furthermore, it may be possible to use the same approach with mixtures of other indicators to estimate the concentration of other biologically important ions/compounds with a ratiometric calibration.