Extra- and intracellular lanthanum: modified calcium distribution, inward currents and contractility in guinea pig ventricular preparations

Effect of the ostreolysin A/pleurotolysin B pore-forming complex on intracellular Ca2+ activity in the vascular smooth muscle cell line A10

Ostreolysin A/pleurotolysin B (OlyA/PlyB) is a binary pore-forming protein complex that produces a rapid cardiorespiratory arrest. Increased tonus of the coronary vascular wall produced by OlyA/PlyB may lead to ischemia, arrhythmias, the hypoxic injury of cardiomyocytes and cardiotoxicity. We evaluated the effects of OlyA/PlyB in cultured vascular smooth muscle A10 cells. Fluorometric measurements using the Ca(2+) indicator Fluo-4 AM and Fura-2 AM revealed that nanomolar concentrations of OlyA/PlyB increased the intracellular Ca(2+) activity [Ca(2+)]i in A10 cells. This effect was absent in a Ca(2+)-free medium, indicating that OlyA/PlyB-induced [Ca(2+)]i increase was dependent on Ca(2+) influx into cells. The increase in [Ca(2+)]i by OlyA/PlyB was partially prevented by: i) the calcium channel blockers verapamil and La(3+), ii) the inhibitor of the sodium-calcium exchanger (NCX) benzamil, and iii) the iso-osmotic replacement of NaCl by sucrose. The pre-treatment of cells with the Ca(2+)-ATPase inhibitor thapsigargin reduced the [Ca(2+)]i increase evoked by OlyA/PlyB, whereas the plasma membrane depolarization with high K(+) in the medium did not prevent OlyA/PlyB-induced [Ca(2+)]i. In summary, our data could suggest that the OlyA/PlyB-induced increase in [Ca(2+)]i is due to an influx of Ca(2+) through a variety of co-existing plasma membrane Ca(2+)-permeable channels, Ca(2+) entry through non-selective ion permeable pores formed de novo by OlyA/PlyB in the plasma membrane and calcium-induced intracellular Ca(2+) release, altogether leading to disturbed Ca(2+) homeostasis in A10 cells.

Mitochondrial Na+ overload is caused by oxidative stress and leads to activation of the caspase 3- dependent apoptotic machinery

Oxidative stress is one of the major causes of cell death. Using time-lapse confocal recording of live cardiomyocytes, we showed that H2O2 (OH*) caused a marked increase in Na+ and Ca2+ levels in both the cytosol ([Na]cyt, [Ca]cyt) and mitochondria ([Na]m, [Ca]m). The H2O2-induced intracellular Na+ ([Na]i) overload contributed to the H2O2-induced [Ca]cyt/[Ca]m overload via activation of the reverse mode of the Na-Ca exchanger. When myocytes were treated for 40 min with 100 microM H2O2 in normal medium, then returned to H2O2-free medium, the percentage of apoptotic cells increased from 4% at 0 h to 55 and 85% at 4.5 and 16 h, respectively. H2O2-induced apoptosis was completely prevented by using Na-free, but not Ca-free, medium. When a Na+ ionophore cocktail in Ca-free medium was used instead of H2O2 to increase the [Na]i by more than 30 mM without any change in the [Ca]i, cytochrome c release and caspase 3-dependent apoptosis occurred, showing that [Na]i overload per se induced apoptosis. We also showed that the increase in the mitochondrial, but not the cytosolic, Na+ levels resulted in the opening of the permeation transition pore, followed by cytochrome c release. Our findings therefore suggest that H2O2-induced [Na]m overload is an important upstream signal for the apoptotic machinery, and the prevention of [Na]m overload thus represents a particularly attractive target for strategies aimed at preventing oxidative stress-induced cell death.

Lanthanum is transported by the sodium/calcium exchanger and regulates its activity

La3+ uptake was measured in fura 2-loaded Chinese hamster ovary cells expressing the bovine cardiac Na+/Ca2+ exchanger (NCX1.1). La3+ was taken up by the cells after an initial lag phase of 50-60 s and achieved a steady state within 5-6 min. Neonatal cardiac myocytes accumulated La3+ in a similar manner. La3+ uptake was due to the activity of the exchanger, because no uptake was seen in nontransfected cells or in transfected cells that had been treated with gramicidin to remove cytosolic Na+. The low rate of La3+ uptake during the lag period resulted from insufficient cytosolic Ca2+ to activate the exchanger at its regulatory sites, as shown by the following observations. La3+ uptake occurred without a lag period in cells expressing a mutant of NCX1.1 that does not exhibit regulatory activation by cytosolic Ca2+. The rate of La3+ uptake by wild-type cells was increased, and the lag phase was reduced or eliminated, when the cytosolic Ca2+ concentration was increased before initiating La3+ uptake. La3+ could substitute for Ca2+ at very low concentrations to activate exchange activity. Thus preloading cells expressing NCX1.1 with a small quantity of La3+ increased the rate of exchange-mediated Ca2+ influx by 20-fold; in contrast, cytosolic La3+ partially inhibited Ca2+ uptake by the regulation-deficient mutant. With an estimated KD of 30 pM for the binding of La3+ to fura 2, we conclude that cytosolic La3+ activates exchange activity at picomolar concentrations. We speculatively suggest that endogenous trace metals might activate exchange activity under physiological conditions.

Tetrodotoxin-blockable calcium currents in rat ventricular myocytes; a third type of cardiac cell sodium current

1. Whole-cell patch clamp currents from freshly isolated adult rat ventricular cells, recorded in external Ca2+ (Ca2+o) but no external Na+ (Na+o), displayed two inward current components: a smaller component that activated over more negative potentials and a larger component (L-type Ca2+ current) that activated at more positive potentials. The smaller component was not generated by Ca2+ channels. It was insensitive to 50 microM Ni2+ and 10 microM La3+ but suppressed by 10 microM tetrodotoxin (TTX). We refer to this component as ICa(TTX). 2. The conductance-voltage, g(V), relation in Ca2+o only was well described by a single Boltzmann function (half-maximum potential, V1/2, of -44.5; slope factor, k, of -4.49 mV, means of 3 cells). g(V) in Ca2+o plus Na+o was better described as the sum of two Boltzmann functions, one nearly identical to that in Ca2+o only (mean V1/2 of -45.1 and k of -3.90 mV), and one clearly distinct (mean V1/2 of -35.6 and k of -2.31 mV). Mean maximum conductance for ICa(TTX) channels increased 23.7% on adding 1 mM Na+o to 3 mM Ca2+o. ICa(TTX) channels are permeable to Na+ ions, insensitive to Ni2+ and La3+ and blocked by TTX. They are Na+ channels. 3. ICa(TTX) channels are distinct from classical cardiac Na+ channels. They activate and inactivate over a more negative range of potentials and have a slower time constant of inactivation than the classical Na+ channels. They are also distinct from yet another rat ventricular Na+ current component characterized by a much higher TTX sensitivity and by a persistent, non-fast-inactivating fraction. That ICa(TTX) channels activate over a more negative range of potentials than classical cardiac Na+ channels suggests that they may be critical for triggering the ventricular action potential and so of importance for cardiac arrhythmias.

Cellular distribution patterns of lanthanum and morphometry of rat hearts exposed to different degrees of ischemic stress

BACKGROUND

The element lanthanum (La) can be used as a tracer for verification of membrane permeability. The aim of this study was to establish whether 1) distribution of La in the myocardium of rat hearts depends on the degree of ischemic stress and 2) morphometrically determined cell and mitochondrial swelling correlates with the La distribution.

MATERIALS AND METHODS

Isolated beating rat hearts were arrested by coronary perfusion with the cardioplegic solution Custodiol (controls) or by aortic cross clamping followed by exposure to different degrees of ischemic stress. The solutions for perfusion-and postfixation as well as for rinsing contained 1.1% La(NO3)3. Cellular and mitochondrial swelling were determined morphometrically and myocytes exhibiting intracellular La were quantified and stated as percentage of test fields.

RESULTS

Immediately after cardiac arrest La was present as precipitates only in a few myocytes adjacent to the outer mitochondrial membrane as seen by cTEM and ESI. In such cells La was also detected by EELS in mitochondrial matrix and myofibrils. Advanced ischemic stress led to an increase of the percentage of myocytes containing detectable intracellular La. After 45 min ischemia at 30 degrees C, myocytes and mitochondria showed a remarkable edema and different intracellular distribution patterns of La. After 90 min of ischemia at 20 degrees C interruptions of sarcolemma could only be detected in a few of the swollen myocytes. Roundish La granules were seen in the myofibrils. The percentage of myocytes containing intracellular La and the extent of cellular and mitochondrial swelling showed a significant correlation.

CONCLUSIONS

Patterns of intracellular La distribution depend on the degree of ischemic stress and correspond to the degree of cellular as well as mitochondrial edema. These results point at a direct relation between alterations of membrane permeability and development of edema.

Ion microscopy evidence that La3+ releases Ca2+ from Golgi complex in LLC-PK1 cells

The effect of La3+ on LLC-PK1 cells was investigated by ion microscopy, a mass spectrometry-based technique with a spatial resolution of approximately 0.5 micron. Cells were incubated with LaCl3 for 10 min. (1 mM) or 30 min (0.1 mM), and intracellular calcium distributions were measured with a Cameca IMS-3f ion microscope in cryogenically prepared cells. Compared with control cells, La3+ reduced total calcium in the Golgi complex by > 100 microM in both treatments, whereas other cellular regions, such as the nucleus and cytoplasm, remained largely unchanged. These two treatments were repeated on cells that were preincubated with 1 mM ouabain. The presence of ouabain in the medium increased the loss of calcium from the Golgi by about fourfold compared with the treatments without ouabain. The La3+ effect, therefore, was amplified by ouabain-induced Na+ loading, indicating a possible involvement of a Na+/La3+ exchanger. La3+ was detected within cells and its influx was facilitated by Na+ loading. These results suggest that La3+ may affect cellular calcium homeostasis by actions other than as a simple Ca2+ antagonist.

The effects of terbium on the accumulation of cisplatin in human ovarian cancer cells

In this investigation, we report a relationship between the terbium (Tb3+) binding protein and the accumulation of cisplatin in human ovarian cancer cells. The number of Tb3+ binding sites in cisplatin-resistant C13+ cells is significantly greater by 79% than those in cisplatin-sensitive 2008 cells. Exposure to Tb3+ also increased the cellular accumulation of cisplatin. The accumulation of cisplatin as a function of the Tb3+ concentration in the C13+ cells (0.85%/microM Tb3+) was significantly greater than the accumulation of cisplatin in 2008 cells with respect to Tb3+ (0.46%/microM Tb3+). The number of Tb3+ binding sites in revertant RH4 cells was similar to that in 2008 cells. The RH4 cells were less sensitive to the stimulatory effects of Tb3+ than the C13+ cells. Our results show that the Tb3+ binding protein correlates with cisplatin resistance, and the receptor binding of Tb3+ increases the accumulation of cisplatin in cisplatin-resistant cells.

Contractions in guinea-pig ventricular myocytes triggered by a calcium-release mechanism separate from Na+ and L-currents

1. Unloaded cell shortening and membrane currents were examined in isolated guinea-pig ventricular myocytes at 37 degrees C using video edge detection and single-electrode voltage clamp. 2. Inward Na+ currents were eliminated by lidocaine, tetrodotoxin, replacement of extracellular Na+ with choline chloride or sucrose, or by voltage inactivation of Na+ channels. In the absence of Na+ current, the threshold for contraction was approximately -50 or -55 mV. 3. Verapamil (5 microM) and nifedipine (2 microM) failed to inhibit contractions at negative membrane potentials when positive conditioning pulses were used to maintain intracellular Ca2+ stores via Na(+)-Ca2+ exchange. In contrast, 200 microM Ni2+ inhibited these contractions. 4. Contractions were abolished when the extracellular solution was nominally Ca2+ free. However, contractions were restored by as little as 50 microM extracellular Ca2+. 5. Ryanodine (30 nM) completely abolished contractions initiated by depolarizing steps from -65 to -40 mV, but had minimal effects on contractions initiated by depolarizing steps from -40 to +5 mV. Subtraction of contraction-voltage relations determined in the presence of ryanodine from control relations revealed a ryanodine-sensitive component of contraction. This component activated at -55 mV and reached a plateau near -25 mV. 6. The amplitudes of contractions initiated by depolarizing steps from -40 mV were directly proportional to the magnitude of Ca2+ current (ICa). In contrast, contractions initiated by steps from either -55 or -65 mV were not proportional to ICa. These contractions appeared at potentials negative to the threshold for L-type Ca2+ current, increased to a plateau at more positive potentials and did not decrease at potentials at which ICa decreased. 7. Subtraction of the contraction-voltage relationship determined from a membrane potential of -40 mV from that at -55 mV revealed a component of contraction with a negative activation threshold whose amplitude was not proportional to inward current. The shape of this relationship was virtually identical to that of the ryanodine-sensitive component of contraction. 8. This study identifies a component of contraction associated with Ca2+ release from sarcoplasmic reticulum (SR) which can be separated from other mechanisms of contraction on the basis of membrane potential. Our observations suggest that this voltage-dependent release mechanism is a true trigger mechanism which activates a portion of cardiac contraction which is attributable to SR Ca2+ release.

Propafenone preferentially blocks the rapidly activating component of delayed rectifier K+ current in guinea pig ventricular myocytes. Voltage-independent and time-dependent block of the slowly activating component

The effects of propafenone on the delayed rectifier K+ current were studied in guinea pig ventricular myocytes by using the patch-clamp technique. In these myocytes, this current consists of at least two components: a La(3+)-sensitive component activating rapidly with moderate depolarizations and a La(3+)-resistant current slowly activating at more positive potentials. In the absence of La3+ (when both components are present), propafenone inhibited the delayed outward current, its effects being more marked after weak than after strong depolarizations. Propafenone-induced block of the tail currents elicited on return to -30 mV was more marked after short than after long depolarizing pulses. In the presence of 1 mumol/L propafenone, the envelope-of-tails test was satisfied, thus indicating that at this concentration propafenone completely blocks the rapidly activating component. In the presence of La3+ (when only the slow component is present), the steady state inhibition induced by 5 mumol/L propafenone on both the maximum activated and the tail currents was independent of the test pulse voltage. Development of propafenone-induced block on the slowly activating component was very fast and linked to channel opening. In addition, the blockade appeared to be use dependent, with the rate constant of the onset kinetics at 2 Hz being 0.44 +/- 0.1 pulse-1. The recovery process from propafenone-induced block exhibited a time constant of 2.5 +/- 0.4 s. These results indicated that propafenone preferentially inhibits the rapidly activating component of the delayed rectifier and that it blocks in a voltage-independent and time-dependent manner the slow component of this current.

Evaluation of lanthanide tracer methods in the study of mammalian pulmonary parenchyma and cardiac muscle by electron energy-loss spectroscopy

Lanthanum (La) has widely been used as a tracer to study the integrity of plasma membranes. With conventional transmission electron microscopy (cTEM), the absence of electron scattering deposits from the cytoplasm has generally been assumed to reflect an intact cell membrane. However, the application of electron spectroscopic imaging (ESI) and electron energy-loss spectroscopy (EELS) reveals that electron scattering deposits may be present which do not contain La. However, La could be detected in regions of pulmonary parenchyma and cardiac muscle that were devoid of electron scattering deposits. Therefore, to exclude misinterpretations based on cTEM the application of microanalytical techniques is strongly recommended for the study of the integrity of plasma membranes by means of La tracers. In addition, ESI and EELS are shown to distinguish between different tracers in simultaneous applications of La and terbium (Tb) which were used at the different faces of the pulmonary air-blood barrier. The analysis of the distribution of both tracers which form electron scattering deposits, indistinguishable by cTEM, may help us to understand the different functional significances of cellular alterations of both cellular borders of the barrier. As was shown for La, however, strictly controlled conditions are mandatory during the fixation procedure because an increase in the incubation time to more than 1 h in samples of pulmonary parenchyma may result in the occurrence of La deposits within the cytoplasm. In the absence of electron scattering deposits, the presence of La in glycogen granules and ribosome-containing areas of various types of alveolar septal cells even after 15 min incubation indicates that the absence of deposits does not necessarily correspond to the absence of the tracer.

Lanthanum influx into cultured human keratinocytes: Effect on calcium flux and terminal differentiation

Trivalent cation lanthanum (La) binds to calcium binding sites of cells and either mimics the properties of calcium or inhibits the effects of calcium by displacing calcium from its binding sites. Extracellular calcium induces differentiation of human epidermal keratinocytes in culture, in part by increasing the intracellular calcium levels (Cai). Therefore, in this study we determined the effect of La on differentiation and intracellular calcium levels of keratinocytes. We observed that La inhibited the production of cornified envelopes, a marker for terminal differentiation of keratinocytes. La inhibited the calcium requiring envelope cross-linking enzyme, transglutaminase, in a direct manner, presumably, by displacing calcium from its binding site on the enzyme. La inhibited the influx and the efflux of 45Ca from keratinocytes. Paradoxically, extracellular La appeared to increase the Cai levels of keratinocytes as measured by the fluorescent probe indo-1. However, subsequent experiments revealed that indo-1 bound La with a higher affinity than Ca and emitted fluorescence in the same wavelength as the Ca bound form. Using this probe, we observed that La enters keratinocytes in a dose-dependent fashion and achieves concentrations exceeding 80 nM when the external La concentration is raised to 300 microM. This fully accounted for the apparent increase in Cai when La was added to the cells. Treatment of cells with ionomycin increased indo-1 fluorescence maximally in the presence of La indicating influx of La via this Ca specific ionophore. Our results indicate that La enters cells and inhibits calcium mediated keratinocyte differentiation both by blocking Ca influx and by blocking calcium regulated intracellular processes such as transglutaminase directed cornified envelope formation.

Angiotensin II-induced increase in slowly exchanging 45Ca2+ in relation to contractile responses of rat and guinea-pig aorta

To gain more information about sources of activator Ca2+ involved in the contraction of rat and guinea-pig aorta evoked by angiotensin II and their sensitivity to Ca2+ entry blockers, measurement of slowly exchanging 45Ca2+ was established. A more physiological procedure was used, replacing La(3+)- and EGTA-containing solutions by a normal Ca(2+)-containing buffer. It was demonstrated that the angiotensin II-induced increase in slowly exchanging 45Ca2+ in rat aorta was incompletely (by approximately 60%-70%) inhibited by the organic Ca2+ entry blockers nifedipine, verapamil and diltiazem and by other Ca2+ entry blocking compounds like CoCl2 and chlorpromazine. 8-(N,N-diethylamino)octyl 3,4,5-trimethoxybenzoate hydrochloride (TMB-8) was able to inhibit the angiotensin II-induced increase in 45Ca2+ content completely, but this may be an intracellular storage effects. By contrast, the organic Ca2+ entry blockers completely inhibited that part of the angiotensin II-induced contraction of rat aorta which was dependent upon extracellular Ca2+. In guinea-pig aorta, the increase in 45Ca2+ content elicited by angiotensin II could be completely suppressed by all compounds under study. The results of these experiments correlated well with data from the functional experiments in guinea-pig aorta. In both preparations the release of Ca2+ from a rapidly as well as a slowly exchanging intracellular pool appears to contribute to the contractile response elicited by angiotensin II.

Physicochemical characteristics of the terbium-adriamycin complex and its effects on the sinus node automaticity

The physicochemical characteristics of the terbium-adriamycin complex (terbomycin) were studied. Perturbations in the visible absorption spectrum of adriamycin by terbium (Tb3+) was indicative of formation of the terbomycin complex. The absorption maximum of free adriamycin at 479 nm shifted towards the absorption maximum of terbomycin at 539 nm. The binding of Tb3+ to adriamycin was negligible at acidic pH. At alkaline pH, the affinity of Tb3+ for adriamycin increased. The stoichiometry of binding was estimated to be 0.5; one Tb3+ ion per two adriamycin molecules. Thermodynamic analysis revealed that the spontaneous formation of terbomycin was due to an increase in the entropy of the system. The effects of adriamycin, Tb3+ and terbomycin on sinus node automaticity were studied using sinus node from rats, superfused with modified mammalian Tris-Tyrode's solution (37 degrees C). The sinus node rate was monitored with intracellular microelectrodes. 25 microM Tb3+ increased the sinus node rate. Adriamycin (50 microM) depressed sinus node automaticity. Terbomycin also reduced the sinus node rate. There was no difference between the effects of adriamycin and terbomycin. The chronotropic effect of terbomycin persisted in the presence of atropine.

Lanthanum probing of cell membrane permeability in the rat heart: pathological versus artefactual alterations

The influence of fixation on membrane permeability has been examined in the isolated rat heart using lanthanum as a permeability probe. Normal and ischaemic hearts were probed at various stages during a conventional fixation programme with either ionic or colloidal lanthanum and compared with lanthanum saline administered prior to fixation. Fixation of the myocardium coincident with or followed by lanthanum probing resulted in an influx of the probe into most myocytes in normal tissue. Alterations in permeability after ischaemic episodes could not be distinguished from the artefact. However, lanthanum saline prior to fixation showed exclusion of the probe from normal tissue, while the increased permeability demonstrated after ischaemia was associated with declining myocardial performance during subsequent reperfusion. These results illustrate the need for caution in the application and evaluation of methods determining permeability in fixed tissue. Probes of differing size and charge permeated fixed tissue to varying degrees thereby implicating the formation of specific lesions during chemical fixation.

Lanthanum mimicks the trp photoreceptor mutant of Drosophila in the blowfly Calliphora

The effect of lanthanum on the light response of blowfly (Calliphora erythrocephala) photoreceptors was studied. The electrophysiological behaviour of the photoreceptors in the presence of La can be summarized as follows: 1. Upon long stimulation the photoreceptors responded with a 'transient receptor potential', i.e. the cells depolarized at the onset of the stimulus and then repolarized to (or below) the resting potential. This effect was dependent on stimulus intensity and occurred only at high intensities. During illumination membrane noise was reduced. 2. The light-induced changes in membrane potential were paralleled by changes in membrane resistance. 3. The time course of the receptor response was slowed down. 4. Light adaptation led to an increase in response latency. 5. The recovery of the receptor response after light adaptation was slowed down. 6. The sensitivity of the receptor cells measured by the response to short light stimuli was reduced. In summary, the electrophysiological behaviour of Calliphora photoreceptors in the presence of La was very similar to that of the photoreceptors of the trp (transient receptor potential) mutant of Drosophila melanogaster. This result suggests that La and trp mutation affect the same cellular processes in the photoreceptors.

Detection of La3+ influx in ventricular cells by indo-1 fluorescence

We exposed indo-1-loaded cultured embryonic chick ventricular cells to 0.03-1.0 mM extracellular lanthanum concentration ([La3+]o) and simultaneously measured cell contractile motion and the 410/480 nm fluorescence intensity ratio. After exposure to La3+, ventricular cells stopped contracting and relaxed within seconds, and the 410/480 fluorescence ratio increased. The increase in the 410/480 signal was related to [La3+]o but was not affected by short exposures to zero extracellular calcium concentration ([Ca2+]o) or caffeine, suggesting that the fluorescence was not caused by a La3+-induced increase in intracellular calcium concentration ([Ca2+]i) but rather to increased intracellular lanthanum concentration ([La3+]i). In vitro studies confirmed that indo-1 fluorescence was sensitive to La3+. The increase in [La3+]i in 0.1 mM [La3+]o was directly related to intracellular sodium concentration ([Na+]i), suggesting that La3+ entered cells via Na+-La3+ exchange. In contrast to ventricular cells, which have a functionally distinct Na+-Ca2+ exchange system, exposure of indo-1-loaded cultured bovine endothelial cells to La3+ failed to produce an increase in [La3+]i. These results indicate that exposure of ventricular cells to 0.1-1.0 mM [La3+]o results in a [La3+]i greater than 250 nM within 1 min. Therefore, changes in myocardial 45Ca2+ fluxes and contents induced by La3+ cannot be ascribed solely to extracellular La3+ effects.

Selective block of calcium current by lanthanum in single bullfrog atrial cells

A single suction microelectrode voltage-clamp technique was used to study the actions of lanthanum ions (La3+) on ionic currents in single cells isolated from bullfrog right atrium. La3+, added as LaCl3, blocked the "slow" inward Ca2+ current (ICa) in a dose-dependent fashion; 10(-5) M produced complete inhibition. This effect was best fitted by a dose-response curve that was calculated assuming 1:1 binding of La3+ to a site having a dissociation constant of 7.5 x 10(-7) M. La3+ block was reversed (to 90% of control ICa) following washout and, in the presence of 10(-5) M La3+, was antagonized by raising the Ca2+ concentration from 2.5 to 7.5 mM (ICa recovered to 56% of the control). However, the latter effect took approximately 1 h to develop. Concentrations of La3+ that reduced ICa by 12-67%, 0.1-1.5 x 10(-6) M, had no measurable effect upon the voltage dependence of steady state ICa inactivation, which suggest that at these concentrations there are no significant surface-charge effects of La3+ on this gating mechanism. Three additional findings indicate that doses of La3+ that blocked ICa failed to produce nonspecific effects: (a) 10(-5) M La3+ had no measurable effect on the time-independent inwardly rectifying current, IK1; (b) the same concentration had no effect on the kinetics, amplitude, or voltage dependence of a time- and voltage-dependent K+ current, IK; and (c) 10(-4) M La3+ did not alter the size of the tetrodotoxin-sensitive inward Na+ current, INa, or the voltage dependence of its steady state inactivation. Higher concentrations (0.5-1.0 mM) reduced both IK1 and IK, and shifted the steady state activation curve for IK toward more positive potentials, presumably by reducing the external surface potential. Our results suggest that at a concentration of less than or equal to 10(-5) M, La3+ inhibits ICa selectively by direct blockade of Ca channels rather than by altering the external surface potential. At higher concentrations, La3+ exhibits nonspecific effects, including neutralization of negative external surface charge and inhibition of other time- and voltage-dependent ionic currents.

Membrane Ca2+ interactions and contraction in denervated rat soleus muscle

Under voltage clamp conditions contractile responses and ionic currents of single fibres isolated from rat soleus, denervated for more than 20 days, were recorded in Na-free TEA containing solutions. The relationship between membrane potential and contraction has been analysed under various conditions. The addition of trivalent cations (La3+, Gd3+) resulted in a dose dependent reduction of the contractile response and similar effects were produced by polymyxin B (0.05-0.5 mM). By contrast in the presence of phospholipase D (1-5 U/ml) contractions were significantly increased for all values of depolarization. The time course of the change of tension amplitude after the application of Ca-free medium, was dependent on the amplitude, the duration and the frequency of the depolarization. Upon depolarization glycerol-treated fibres generated contractile responses which were similar to those recorded in normal muscle and were also dependent on [Ca]o. It is proposed that in denervated soleus muscle the negatively charged phospholipids at the outside of the membrane were involved in the depolarization-contraction coupling by means of their Ca binding properties. The quantity of Ca binding sites would be dependent on [Ca]o and membrane potential and their binding properties modified during and/or following variation in membrane potential.

TPA-induced alteration of actin organization in cultured human keratinocytes

Distribution of actin filaments of human epidermal keratinocyte in the primary culture was observed by immunofluorescence staining. In the cytoplasm, actin was distributed diffusely, and strong antiactin immunofluorescence was observed along the leading edge, showing ruffling and the contact zone to the neighboring cell. 12-O-Tetradecanoylphorbol-13-acetate (TPA) induced organization of actin filaments. Many short bundles of actin filaments appeared shortly after the addition of 16 nM TPA, and large actin-containing ribbons of crescent-shape, circular or gyrus-like form were sometimes observed. Phorbol-12-13-diacetate, a non-promoter phorbol ester, induced a similar change, but to a much lesser extent. Addition of 1 mM cycloheximide did not interfere with the organization of actin filaments by TPA. La3+ aborted it completely possibly by replacing Ca2+ at the binding site of the cell surface, and the cultivation in low Ca2+ environment suppressed the effect of TPA. These findings make a contrast to those reported in fibroblasts, and may be linked to the characteristic response of cultured human keratinocytes to TPA in the proliferation of cells and induction of ornithine decarboxylase.

Ca-pools involved in the regulation of cardiac contraction under positive inotropy. X-ray microanalysis on rapidly-frozen ventricular muscles of guinea-pig

Electron probe microanalysis of rapidly-frozen small ventricular trabeculae of guinea-pig demonstrates that the distribution of total intracellular calcium varies under positive inotropy depending on the type of inotropic intervention. The sarcoplasmic reticulum (SR) (or part of it) localized at the level of the z-lines reveals high calcium accumulation at the end of diastole whenever a stimulus is followed by a contraction with a short time to peak of force. After paired pulse stimulation, only this cell compartment accumulates calcium at the end of diastole. Since this cell compartment is "Ca-empty" in muscles frozen during contraction, SR is considered to be the source of activator Ca. In several cases of inotropy (after application of ARL, caffeine or after lowering the extracellular Na+ concentration), calcium is also detectable on the mitochondria, suggesting that these organelles participate in slow regulation of cytosolic calcium. In some cases, total calcium located on the sarcomeres is increased. The interpretation of this finding is intriguing and requires the assumption of supplementary cytosolic Ca-sinks as yet unknown.

Presystolic calcium-loading of the sarcoplasmic reticulum influences time to peak force of contraction. X-ray microanalysis on rapidly frozen guinea-pig ventricular muscle preparations

Guinea-pig ventricular small papillary muscles and trabeculae were rapidly frozen presystolically after prolonged rest following positive inotropic interventions which strongly influenced peak of force and time to peak force. The possible sources of activator calcium for the different types of contraction were investigated. After rest in the presence of noradrenaline (10(-5)mol/l) the first post-rest contraction showed a retarded activation and a "late" peak of force. Muscle strips frozen after a rest period of 5 min in a bath solution containing noradrenaline were cryosectioned and analyzed with X-ray microanalysis for elemental distribution: although at this time an applied stimulus would induce a potentiated contraction, intracellular membrane systems such as sarcoplasmic reticulum and mitochondria failed to reveal any accumulation of calcium. After rest in a low sodium Tyrode the first post-rest contraction showed an "early" peak of force. Muscles frozen after rest in a low sodium solution revealed intracellular Ca accumulation on the sarcoplasmic reticulum, in the network at the level of the Z-lines. The results support the hypothesis that 1. the sarcoplasmic reticulum (SR) accumulates calcium presystolically when "early" contractions follow stimulation; 2. the network of sarcoplasmic reticulum at the level of the Z-lines is a crucial source of activator calcium; 3. the activator calcium for late contractions is probably of extracellular origin.