Amplification of human platelet activation by surface pannexin-1 channels

BACKGROUND

Pannexin-1 (Panx1) forms an anion-selective channel with a permeability up to ~1 kDa and represents a non-lytic, non-vesicular ATP release pathway in erythrocytes, leukocytes and neurons. Related connexin gap junction proteins have been reported in platelets; however, the expression and function of the pannexins remain unknown.

OBJECTIVE

To determine the expression and function of pannexins in human plate-lets, using molecular, cellular and functional techniques.

METHODS

Panx1 expression in human platelets was det-ermined using qPCR and antibody-based techniques. Contributions of Panx1 to agonist-evoked efflux of cytoplasmic calcein, Ca(2+) influx, ATP release and aggregation were assessed in washed platelets under conditions where the P2X1 receptor response was preserved (0.32 U mL(-1) apyrase). Thrombus formation in whole blood was assessed in vitro using a shear chamber assay. Two structurally unrelated and widely used Panx1 inhibitors, probenecid and carbenoxolone, were used throughout this study, at concentrations that do not affect connexin channels.

RESULTS

PANX1, but not PANX2 or PANX3, mRNA was detected in human platelets. Furthermore, Panx1 protein is glycosylated and present on the plasma membrane of platelets, and displays weak physical association with P2X1 receptors. Panx1 inhibition blocked thrombin-evoked efflux of calcein, and reduced Ca(2+) influx, ATP release, platelet aggregation and thrombus formation under arterial shear rates in vitro. The Panx1-dependent contribution was not additive to that of P2X1 receptors.

CONCLUSIONS

Panx1 is expressed on human platelets and amplifies Ca(2+) influx, ATP release and aggregation through the secondary activation of P2X1 receptors. We propose that Panx1 represents a novel target for the management of arterial thrombosis.

The unique contribution of ion channels to platelet and megakaryocyte function

Ion channels are transmembrane proteins that play ubiquitous roles in cellular homeostasis and activation. In addition to their recognized role in the regulation of ionic permeability and thus membrane potential, some channel proteins possess intrinsic kinase activity, directly interact with integrins or are permeable to molecules up to ≈1000 Da. The small size and anuclear nature of the platelet has often hindered progress in understanding the role of specific ion channels in hemostasis, thrombosis and other platelet-dependent events. However, with the aid of transgenic mice and 'surrogate' patch clamp recordings from primary megakaryocytes, important unique contributions to platelet function have been identified for several classes of ion channel. Examples include ATP-gated P2X1 channels, Orai1 store-operated Ca2+ channels, voltage-gated Kv1.3 channels, AMPA and kainate glutamate receptors and connexin gap junction channels. Furthermore, evidence exists that some ion channels, such as NMDA glutamate receptors, contribute to megakaryocyte development. This review examines the evidence for expression of a range of ion channels in the platelet and its progenitor cell, and highlights the distinct roles that these proteins may play in health and disease.

Novel consequences of voltage-dependence to G-protein-coupled P2Y1 receptors

BACKGROUND AND PURPOSE

Emerging evidence suggests that activation of G-protein-coupled receptors (GPCRs) can be directly regulated by membrane voltage. However, the physiological and pharmacological relevance of this effect remains unclear. We have further examined this phenomenon for P2Y1 receptors in the non-excitable megakaryocyte using a range of agonists and antagonists.

EXPERIMENTAL APPROACH

Simultaneous whole-cell patch clamp and fura-2 fluorescence recordings of rat megakaryocytes, which lack voltage-gated Ca2+ influx, were used to examine the voltage-dependence of P2Y1 receptor-evoked IP3-dependent Ca2+ mobilization.

RESULTS

Depolarization transiently and repeatedly enhanced P2Y1 receptor-evoked Ca2+ mobilization across a wide concentration range of both weak, partial and full, potent agonists. Moreover, the amplitude of the depolarization-evoked [Ca2+]i increase displayed an inverse relationship with agonist concentration, such that the greatest potentiating effect of voltage was observed at near-threshold levels of agonist. Unexpectedly, depolarization also stimulated an [Ca2+]i increase in the absence of agonist during exposure to the competitive antagonists A3P5PS and MRS2179, or the allosteric enhancer 2,2'-pyridylisatogen tosylate. A further effect of some antagonists, particularly suramin, was to enhance the depolarization-evoked Ca2+ responses during co-application of an agonist. Of several P2Y1 receptor inhibitors, only SCH202676, which has a proposed allosteric mechanism of action, could block ADP-induced voltage-dependent Ca2+ release.

CONCLUSIONS AND IMPLICATIONS

The ability of depolarization to potentiate GPCRs at near-threshold agonist concentrations represents a novel mechanism for coincidence detection. Furthermore, the induction and enhancement of voltage-dependent GPCR responses by antagonists has implications for the design of therapeutic compounds.

Primary and secondary agonists can use P2X(1) receptors as a major pathway to increase intracellular Ca(2+) in the human platelet

In the platelet, it is well established that many G-protein- and tyrosine kinase-coupled receptors stimulate phospholipase-C-dependent Ca(2+) mobilization; however, the extent to which secondary activation of adenosine 5'-triphosphate (ATP)-gated P2X(1) receptors contributes to intracellular Ca(2+) responses remains unclear. We now show that selective inhibition of P2X(1) receptors substantially reduces the [Ca(2+)](i) increase evoked by several important agonists in human platelets; for collagen, thromboxane A(2), thrombin, and adenosine 5'-diphoshate (ADP) the maximal effect was a reduction to 18%, 34%, 52%, and 69% of control, respectively. The direct contribution of P2X(1) to the secondary Ca(2+) response was far greater than that of either P2Y receptors activated by co-released ADP, or via synergistic P2X(1):P2Y interactions. The relative contribution of P2X(1) to the peak Ca(2+) increase varied with the strength of the initial stimulus, being greater at low compared to high levels of stimulation for both glycoprotein VI and PAR-1, whereas P2X(1) contributed equally at both low and high levels of stimulation of thromboxane A(2) receptors. In contrast, only strong stimulation of P2Y receptors resulted in significant P2X(1) receptor activation. ATP release was detected by soluble luciferin:luciferase in response to all agonists that stimulated secondary P2X(1) receptor activation. However, P2X(1) receptors were stimulated earlier and to a greater extent than predicted from the average ATP release, which can be accounted for by a predominantly autocrine mechanism of activation. Given the central role of [Ca(2+)](i) increases in platelet activation, these studies indicate that ATP should be considered alongside ADP and thromboxane A(2) as a significant secondary platelet agonist.

Arrhythmogenic mechanisms in the isolated perfused hypokalaemic murine heart

Aim

Hypokalaemia is associated with a lethal form of ventricular tachycardia (VT), torsade de pointes, through pathophysiological mechanisms requiring clarification.

Methods

Left ventricular endocardial and epicardial monophasic action potentials were compared in isolated mouse hearts paced from the right ventricular epicardium perfused with hypokalaemic (3 and 4 mm [K⁺]_o) solutions. Corresponding K⁺ currents were compared in whole-cell patch-clamped epicardial and endocardial myocytes.

Results

Hypokalaemia prolonged epicardial action potential durations (APD) from mean APD₉₀s of 37.2 ± 1.7 ms (n = 7) to 58.4 ± 4.1 ms (n =7) and 66.7 ± 2.1 ms (n = 11) at 5.2, 4 and 3 mm [K⁺]_o respectively. Endocardial APD₉₀s correspondingly increased from 51.6 ± 1.9 ms (n = 7) to 62.8 ± 2.8 ms (n = 7) and 62.9 ± 5.9 ms (n = 11) giving reductions in endocardial–epicardial differences, ΔAPD₉₀, from 14.4 ± 2.6 to 4.4 ± 5.0 and −3.4 ± 6.0 ms respectively. Early afterdepolarizations (EADs) occurred in epicardia in three of seven spontaneously beating hearts at 4 mm [K⁺]_o with triggered beats followed by episodes of non-sustained VT in nine of 11 preparations at 3 mm. Programmed electrical stimulation never induced arrhythmic events in preparations perfused with normokalemic solutions yet induced VT in two of seven and nine of 11 preparations at 4 and 3 mm [K⁺]_o respectively. Early outward K⁺ current correspondingly fell from 73.46 ± 8.45 to 61.16±6.14 pA/pF in isolated epicardial but not endocardial myocytes (n = 9) (3 mm [K⁺]_o).

Conclusions

Hypokalaemic mouse hearts recapitulate the clinical arrhythmogenic phenotype, demonstrating EADs and triggered beats that might initiate VT on the one hand and reduced transmural dispersion of repolarization reflected in ΔAPD₉₀ suggesting arrhythmogenic substrate on the other.

The interpretation of current-clamp recordings in the cell-attached patch-clamp configuration

In these experiments we have investigated the feasibility and accuracy of recording steady-state and dynamic changes in transmembrane potential noninvasively across an intact cell-attached patch using the current-clamp mode of a conventional patch-clamp amplifier. Using an equivalent circuit mimicking simultaneous whole-cell voltage-clamp and cell-attached current-clamp recordings we have defined both mathematically and experimentally the relationship between the membrane patch resistance, the seal resistance, and the fraction of the whole-cell potential recorded across an intact membrane patch. This analysis revealed a steep increase in the accuracy of recording of steady-state membrane potential as the seal/membrane ratio increases from 0. The recording accuracy approaches 100% as the seal/membrane ratio approaches infinity. Membrane potential measurements across intact cell-attached patches in rat basophilic leukemia cells and rat megakaryocytes revealed a surprisingly high degree of accuracy and demonstrated the ability of this noninvasive technique to follow dynamic changes in potential in nonexcitable cells.

Effects of premature stimulation on HERG K(+) channels

1. The unusual kinetics of human ether-à-go-go-related gene (HERG) K(+) channels are consistent with a role in the suppression of arrhythmias initiated by premature beats. Action potential clamp protocols were used to investigate the effect of premature stimulation on HERG K(+) channels, transfected in Chinese hamster ovary cells, at 37 degrees C. 2. HERG K(+) channel currents peaked during the terminal repolarization phase of normally paced action potential waveforms. However, the magnitude of the current and the time point at which conductance was maximal depended on the type of action potential waveform used (epicardial, endocardial, Purkinje fibre or atrial). 3. HERG K(+) channel currents recorded during premature action potentials consisted of an early transient outward current followed by a sustained outward current. The magnitude of the transient current component showed a biphasic dependence on the coupling interval between the normally paced and premature action potentials and was maximal at a coupling interval equivalent to 90 % repolarization (APD(90)) for ventricular action potentials. The largest transient current response occurred at shorter coupling intervals for Purkinje fibre (APD(90) - 20 ms) and atrial (APD(90) - 30 ms) action potentials. 4. The magnitude of the sustained current response following premature stimulation was similar to that recorded during the first action potential for ventricular action potential waveforms. However, for Purkinje and atrial action potentials the sustained current response was significantly larger during the premature action potential than during the normally paced action potential. 5. A Markov model that included three closed states, one open and one inactivated state with transitions permitted between the pre-open closed state and the inactivated state, successfully reproduced our results for the effects of premature stimuli, both during square pulse and action potential clamp waveforms. 6. These properties of HERG K(+) channels may help to suppress arrhythmias initiated by early afterdepolarizations and premature beats in the ventricles, Purkinje fibres or atria.

Depolarisation-evoked Ca2+ waves in the non-excitable rat megakaryocyte

1. A combination of patch clamp, confocal microscopy and immunohistochemistry was used to examine the spatial properties of Ca2+ signalling in the rat megakaryocyte, a non-excitable cell type in which membrane potential can markedly modulate agonist-evoked Ca2+ release. 2. Intracellular calcium ion concentration ([Ca2+]i) increases, stimulated by both ADP and depolarisation, frequently originated from a peripheral locus and spread as a wave throughout the cell. Spatially restricted [Ca2+]i increases, consistent with elementary Ca2+ release events, were occasionally observed prior to ADP-evoked waves. 3. ADP- and depolarisation-evoked Ca2+ waves travelled approximately twice as fast around the periphery of the cell compared to across its radius, leading to a curvilinear wavefront. There was no significant difference between wave velocities generated by the two stimuli. 4. Immunohistochemical staining of type III IP3 receptors, the endoplasmic reticulum-specific protein GRP78/BiP and calreticulin indicated a major peripheral location of the cellular Ca2+ stores which probably accounts for the accelerated wave velocity at the cell periphery. 5. These data demonstrate that [Ca2+]i increases, stimulated by depolarisation or the agonist ADP, have indistinguishable spatial properties, providing evidence that similar underlying mechanisms are responsible for their generation.

Voltage-dependent Ca2+ release in rat megakaryocytes requires functional IP3 receptors

Using simultaneous whole-cell patch-clamp and fluorescence measurements of [Ca2+]i in rat megakaryocytes we have investigated the requirement for functional inositol 1,4,5-trisphosphate (IP3) receptors in Ca2+ release induced by membrane depolarization during agonist stimulation. Voltage-dependent Ca2+ release was observed during application of the IP3-generating agonists U46619 (a thromboxane A2 analogue) and ADP. Furthermore, voltage-dependent Ca2+ release was observed in the absence of exogenous agonist following sensitization of IP3 receptors with thimerosal. Depolarization-induced Ca2+ release was not detected during depletion of intracellular Ca2+ stores by thapsigargin. Thus, depletion of stores alone is not sufficient to confer voltage dependence upon the Ca2+ release mechanism. Block of IP3 receptors by carbacyclin-stimulated elevations in cAMP, uncaging of cAMP or exposure to a high concentration of caffeine reversibly abolished Ca2+ increases stimulated by both ADP and depolarization. The cAMP-dependent block was prevented by a peptide inhibitor of protein kinase A, indicating that an alteration of adenylate cyclase activity leading to modulation of protein kinase A activity does not underlie the control of Ca2+ release by voltage. These results are consistent with the requirement for functional IP3 receptors for voltage control of Ca2+ release from intracellular stores during inositol lipid signalling. The data also indicate the involvement of a voltage sensor downstream of surface membrane receptors in the depolarization-evoked Ca2+ response.

Platelet shape change evoked by selective activation of P2X1 purinoceptors with alpha,beta-methylene ATP

Simultaneous measurements of [Ca2+]i and light transmission were used to examine the relationship between P2X1 receptor activation and functional platelet responses. The P2X1 agonist alpha,beta-MeATP evoked a transient [Ca2+]i increase and a reversible decrease in light transmission; both responses required external Ca2+ and the nucleotidase apyrase. The transmission response was due to shape change only, verified by scanning electron microscopy and insensitivity to Reopro, a GPIIbIIIa antagonist. Alpha,beta-MeATP stimulated smaller shape changes than ADP, however P2X1 responses had a lifespan of <2 h following resuspension in saline and may be considerably larger in vivo. A peak [Ca2+]i increase of >50 nM was required for detectable shape change. Overlap of concentration-response relationships for alpha,beta-MeATP-evoked [Ca2+]i and shape change suggests that other second messengers are not involved. Therefore, the physiological P2X1 agonist ATP can contribute to platelet activation, in contrast to its previously described inhibitory action at metabotropic platelet purinoceptors.

A novel role for membrane potential in the modulation of intracellular Ca2+ oscillations in rat megakaryocytes

1. The effect of membrane potential (Vm) on ADP-evoked [Ca2+]i oscillations was investigated in rat megakaryocytes, a non-excitable cell type recently shown to exhibit depolarisation-evoked Ca2+ release from intracellular stores during metabotropic purinoceptor stimulation. 2. Hyperpolarising voltage steps caused a transient fall in [Ca2+]i and either abolished Ca2+ oscillations or reduced the oscillation amplitude. These effects were observed in both the presence and absence of extracellular Ca2+ and also in Na+-free saline solutions, suggesting that hyperpolarisation leads to a reduction in the level of ADP-dependent Ca2+ release without a requirement for altered transmembrane Ca2+ fluxes. 3. In the presence of Ca2+ oscillations, depolarising voltage steps transiently enhanced the amplitude of Ca2+ oscillations. Following run-down of Ca2+ oscillations, depolarisation briefly restimulated oscillations. 4. Simultaneous [Ca2+]i and current-clamp recordings showed that Ca2+ and Vm oscillate in synchrony, with an average fluctuation of approximately 30-40 mV, due to activation and inactivation of Ca2+-dependent K+ channels. Application of a physiological oscillating Vm waveform to non-oscillating cells under voltage clamp stimulated [Ca2+]i oscillations. 5. Analysis of the relationship between [Ca2+]i and Vm showed a threshold for activation of hyperpolarisation at about 250-300 nM. The implications of this threshold in the interaction between Vm and Ca2+ release during oscillations are discussed. 6. We conclude that the ability of voltage to control release of endosomal Ca2+ in ADP-stimulated megakaryocytes is bipolar in nature. Our data suggest that Vm changes are active components of the feedback/feedforward mechanisms contributing to the generation of Ca2+ oscillations.

Depolarization-evoked Ca2+ release in a non-excitable cell, the rat megakaryocyte

1. The effect of membrane potential on [Ca2+]i in rat megakaryocytes was studied using simultaneous whole-cell patch clamp and fura-2 fluorescence recordings. 2. Depolarization from -75 to 0 mV had no effect on [Ca2+]i in unstimulated cells, but evoked one or more spikes of Ca2+ increase (peak increase: 714 +/- 95 nM) during activation of metabotropic purinoceptors by 1 microM ADP. 3. The depolarization-evoked Ca2+ increase was present in Ca2+-free medium and also following removal of Na+. Thus depolarization mobilizes Ca2+ from an intracellular store without a requirement for altered Na+-Ca2+ exchange activity. 4. Intracellular dialysis with heparin blocked the depolarization-evoked Ca2+ increase, indicating a role for functional IP3 receptors. 5. Under current clamp, ADP caused the membrane potential to fluctuate between -43 +/- 1 and -76 +/- 1 mV. Under voltage clamp, depolarization from -75 to -45 mV evoked a transient [Ca2+]i increase (398 +/- 91 nM) during exposure to ADP. 6. We conclude that during stimulation of metabotropic purinoceptors, membrane depolarization over the physiological range can stimulate Ca2+ release from intracellular stores in the rat megakaryocyte, a non-excitable cell type. This may represent an important mechanism by which electrogenic influences can control patterns of [Ca2+]i increase.

Reversible and irreversible intracellular Ca2+ spiking in single isolated human platelets

1. We have developed conditions that permit long duration recordings of [Ca2+]i in single, isolated human platelets and studied the reversibility of Ca2+i spiking following activation by physiological and artificial stimuli. 2. Fura-2-loaded platelets were immobilized at the tip of a saline-filled glass pipette using gentle suction. 'Contact' activation of Ca2+i spiking was observed in a proportion (11 %) of platelets, which continued for the duration of each recording (range 8-45 min). 3. Platelets that displayed constant, resting Ca2+i levels were used to test the effects of agonists. ADP (10 microM) increased [Ca2+]i in the form of either one to two spikes followed by an elevated plateau level (60 % of cells) or multiple Ca2+ spikes of irregular amplitude (40 % of cells). ADP-induced Ca2+i mobilization was completely reversible and repeatable. 4. Thrombin (1 u ml-1) evoked Ca2+i spiking in the majority (88 %) of platelets tested, which was not inhibited by perfusion of agonist-free saline throughout the recording period (range 8-67 min). 5. The clear difference in the reversibility of activation by different stimuli may reflect the distinct roles of individual agonists in haemostasis and have important consequences in the design of treatments for thrombosis.

ADP and inositol trisphosphate evoke oscillations of a monovalent cation conductance in rat megakaryocytes

1. A combination of conventional whole-cell patch clamp recordings and fura-2 fluorescence photometry was used to study the membrane currents during oscillations of intracellular Ca2+ concentration ([Ca2+]i) in single rat megakaryocytes. 2. At a holding potential of -60 mV, in NaCl external saline and KCl internal saline with low levels of Ca2+ buffering, 10 microM ADP evoked [Ca2+]i oscillations and simultaneous Ca2+-gated K+ currents at a frequency of 3-10 spikes min-1. A smaller inward current was also activated, with a time course that identified this component as the inositol 1,4, 5-trisphosphate (IP3)-activated monovalent cation current previously demonstrated in rat megakaryocytes. 3. Cs+ replacement of internal K+ combined with 100 nM external charybdotoxin (CTX) abolished the outward currents and revealed that an inward current was also transiently activated during each [Ca2+]i spike. This underlying conductance was permeable to Na+ and Cs+, but possessed little or no permeability to Cl- or divalent cations. 4. Intracellular dialysis with IP3 (5-50 microM) activated the monovalent cationic conductance prior to release of Ca2+ from intracellular stores. The [Ca2+]i increase was associated with a second phase of cationic current, implying that both IP3 and Ca2+ can activate this conductance. Buffering of [Ca2+]i with BAPTA abolished the second phase of current, leaving monophasic spikes of inward current, often occurring at regular intervals. 5. These data demonstrate that a monovalent cation current, which results in Na+ influx under normal ionic conditions, oscillates in response to ADP receptor stimulation due to activation by both IP3 and [Ca2+]i. This provides a route for long-term Na+ entry in the megakaryocyte following stimulation of receptors coupled to phospholipase C activation and may play a role in cell shape change.

An infra-red light-transmitting aperture controller for use in single-cell fluorescence photometry

Photometric techniques are commonly used to monitor the output from fluorescent indicators during the study of cellular signalling. At the single-cell level, the region of interest is normally set by a variable aperture placed within the microscope emission pathway. The present study reports an improved aperture controller which adjusts the area for fluorescence measurement, whilst allowing objects throughout the field of view to be continuously monitored using infra-red illumination. A rectangular aperture is selected by four 715-nm long-pass glass filters which block > 99.9% of the fluorescence emission at 480-600 nm. A 780-nm long-pass glass filter is used to provide infra-red illumination which does not interfere with the fluorescence signal, yet is detectable by a standard CCD camera. This allows detection of morphological events throughout the field of view and facilitates manipulation of extracellular pipettes, without interruption to a single-cell fluorescence recording. The infra-red light-transmitting controller is suitable for use with a range of other fluorescent indicators, including those routinely used to detect Ca2+, Cl-, Na+ and pH. Data are presented which demonstrate the use of this controller to measure ADP-evoked [Ca2+]i increases in single human erythroleukaemia cells loaded with the Ca2+ indicator fura-2.

Purinoceptor-evoked calcium signalling in human platelets

ADP evokes a rise in platelet cytosolic Ca2+ concentration by stimulating Ca2+ entry and releasing Ca2+ from intracellular stores. Single cell studies indicate that the response consists of a series of spikes in cytosolic Ca2+. The release of stored Ca2+ is mediated by the generation of inositol 1,4,5-trisphosphate. Store depletion in turn leads to activation of a store-regulated Ca2+ entry pathway via a mechanism which appears to involve a protein tyrosine phosphorylation step. Preceding these events, ADP activates a receptor-operated non-selective cation channel, which mediates the entry of Ca2+ and Na+ with a latency of just a few milliseconds. Recent studies indicate that this channel is activated via a P2X1 purinoceptor at which ATP and diadenosine tetraphosphate are agonists. This receptor is distinct from that leading to the release of stored Ca2+ and to store-regulated Ca2+ entry.

Thrombin-dependent calcium signalling in single human erythroleukaemia cells

1. A combination of single cell fluorescence and patch clamp techniques were used to study the mechanisms underlying thrombin-evoked Ca2+ signals in human erythroleukaemia (HEL) cells, a leukaemic cell line of platelet-megakaryocyte lineage. 2. Thrombin caused a transient increase in intracellular Ca2+ ([Ca2+]i), consisting of both release of Ca2+ from intracellular stores and influx of extracellular Ca2+. Mn2+ quench studies indicated that the thrombin-evoked divalent cation-permeable pathway was activated during, but not prior to, release from internal stores. 3. Thapsigargin (1 microM) irreversibly released internal Ca2+ from the same store as that released by thrombin and continuously activated a Ca(2+)-influx mechanism. The amplitude of the thrombin- and thapsigargin-induced Ca2+ influx displayed a marked single cell heterogeneity which showed no correlation with the size of the store Ca2+ transient. 4. In whole-cell patch clamp recordings, both thrombin and thapsigargin evoked an inwardly rectifying Ca2+ current which developed with little or no increase in current noise, showed no reversal in the voltage range -110 to +60 mV and was blocked by 1 mM Zn2+. The apparent divalent cation permeability sequence of this pathway was Ca2+ > > Ba2+ > Mn2+, Mg2+. The thapsigargin-evoked current density at -100 mV varied between 0.42 and 2.1 pA pF-1 in different cells. Thrombin failed to activate additional Ca2+ current if it was added after the thapsigargin-induced inward current had fully developed. 5. These studies indicate that thrombin activates Ca2+ influx in HEL cells entirely via a Ca(2+)-store-release-activated Ca2+ current (Icrac) rather than via receptor-operated or second messenger-dependent Ca2+ channels. The level of expression of Icrac appears to be a major factor in determining the duration of the thrombin-evoked [Ca2+]i response and therefore represents a means by which cells can exert control over [Ca2+]i-dependent events.

Fcgamma receptor I activation triggers a novel Ca2+-activated current selective for monovalent cations in the human monocytic cell line, U937

Previous reports have suggested that receptors for immunoglobulin G (IgG), FcgammaRs, directly activate a nonselective cation channel (Young, J. D.-E., Unkeless, J. C., Young, T. M., Mauro, A., and Cohn, Z. A. (1983) Nature 306, 186-189; Nelson, D. J., Jacobs, E. R., Tang, J. M., Zeller, J. M., and Bone, R. C. (1985) J. Clin. Invest. 76, 500-507). To investigate the mechanisms underlying membrane conductance changes following human high affinity (FcgammaRI) receptor activation, we have used the human monocytic cell line U937 and combined conventional whole cell patch-clamp recordings with single cell fura-2 Ca2+ measurements. Using a K+-free internal solution, antibody cross-linking of IgG-occupied FcgammaRI activated an inward current at negative potentials, whose amplitude and time course mirrored the concomitant rise in intracellular Ca2+. Current-voltage relationships, obtained under different ionic conditions, revealed a monovalent cation-selective conductance that, under physiological conditions, would result in Na+ influx. Noise analysis of current recordings indicated a single channel conductance of 18 picosiemens and a mean opening time of 4.5 ms. This current was also activated by rises in intracellular Ca2+ induced by ionomycin (3 microM) or thapsigargin (1 microM). Addition of the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid to the intracellular medium abolished any channel activation by ionomycin, FcgammaRI, or the low affinity receptor, FcgammaRII. These results demonstrate that FcgammaRI activation triggers a novel Ca2+-activated channel selective for monovalent cations and that neither FcgammaRI nor FcgammaRII can directly activate a channel.

Temperature-dependent block of capacitative Ca2+ influx in the human leukemic cell line KU-812

The mechanism by which depletion of intracellular Ca2+ stores activates Ca2+ influx is not understood. We recently showed that primaquine, an inhibitor of vesicular transport, blocks the activation of the calcium release-activated calcium current (ICRAC) in rat megakaryocytes (Somasundaram, B., Norman, J. C., and Mahaut-Smith, M. P. (1995) Biochem. J. 309, 725-729). Since it is well established that vesicular transport is temperature-sensitive, we have investigated the effect of temperature on both the activation and maintenance of store-mediated Ca2+ and Mn2+ influx in the human leukemic cell line KU-812 using a combination of whole cell ICRAC recordings and measurements of Mn2+ photoquench of fura-2. Activation of ICRAC was temperature-sensitive, showing a nonlinear reduction when the temperature was lowered from 27 to 17 degrees C with an abrupt change at 21-22 degrees C and complete inhibition at 17 degrees C. Once activated, ICRAC also displayed an abrupt reduction at 21-22 degrees C but was not completely blocked even when the temperature was reduced to 14 degrees C, suggesting that at least one of the temperature-sensitive components is exclusively involved in ICRAC activation. Activation of store-mediated Mn2+ influx also showed similar nonlinear temperature sensitivity and complete inhibition at 19 degrees C. However, in contrast to ICRAC measurements, lowering the temperature following maximal activation of the influx pathway at 37 degrees C did not result in any detectable residual Mn2+ entry below 19 degrees C. We conclude that the mechanism of store-mediated Ca2+ influx involves temperature-dependent steps in both its maintenance and activation, suggesting dependence on a lipid membrane environment.

Differentiation of the human monocytic cell line U937 results in an upregulation of the calcium release-activated current, ICRAC

1. Single cell fura-2 fluorescence measurements and whole-cell patch clamp recordings were used to investigate the effects of macrophage-like differentiation, induced by dibutyryl cAMP (dbcAMP), on Ca2+ influx triggered by Ca2+ store depletion in the human monocytic cell line, U937. 2. In differentiated cells, the rise in intracellular [Ca2+] following store depletion by thapsigargin (TG) in nominally Ca(2+)-free solution was 94% greater and the [Ca2+]i rise on subsequent re-addition of external Ca2+ (2 mM) was 292% greater than in undifferentiated cells. 3. Under conditions where [Ca2+]i was buffered by BAPTA, TG-induced store depletion failed to activate a detectable inward Ca2+ current in undifferentiated U937 cells. Under identical conditions, store depletion of differentiated U937 cells generated an inwardly rectifying Ca(2+)-selective current which showed no reversal from -140 to +30 mV and was blocked by 1 microM external La3+; characteristics of the calcium release-activated Ca2+ current (ICRAC) identified in other cells. 4. We conclude that U937 cells show a differentiation-dependent upregulation of a store-mediated Ca2+ entry pathway, identified as ICRAC, which is not correlated with the small associated increase in the size of TG-sensitive Ca2+ pools.

Activation of receptor-operated cation channels via P2X1 not P2T purinoceptors in human platelets

We have investigated the purinoceptor subtypes responsible for calcium signaling in human platelets, which previous studies have shown to involve both Ca2+ influx via receptor-operated cation channels and release of Ca2+ from intracellular stores. Fura-2 measurements of [Ca2+]i in stirred platelet suspensions showed that both ADP (40 microM) and the non-hydrolyzable ATP analogue alphabeta-meATP (alpha, beta-methyleneadenosine 5-triphosphate, 10 microM) activated a rapid Ca2+ influx whereas only ADP mobilized Ca2+ from internal stores. In "nystatin" whole-cell patch clamp recordings, ATP, ADP, and the non-hydrolyzable ATP analogues, alpha, beta-meATP and ATPgammaS (adenosine 5 -O-(3-thiotriphosphate), all activated a cation channel permeable to both monovalent and divalent cations with a single-channel conductance of 11 picosiemens in NaCl saline. The current response to ATP (40 microM) was activated within 20 ms and desensitized with a time constant of 47-107 ms in the continued presence of agonist, which are characteristics of P2X1 receptors in other tissues. We conclude that human platelets possess a P2X1 purinoceptor, which mediates a rapid phase of ADP- or ATP-evoked Ca2+ entry via a cation channel, whereas one or more separate ADP-selective P2 purinoceptors evoke release of calcium from intracellular stores.

Chloride channels in excised membrane patches from human platelets: effect of intracellular calcium

Human platelets were studied by patch clamp recordings from inside-out membranes; there were formed by briefly dipping the platelet, in cell-attached mode, into silicone grease. At 20 degrees C in symmetrical 150 mM NaCl, spontaneous channel openings were rarely observed at negative potentials, whereas depolarised potentials (+ 60 to + 100 mV) elicited sustained channel activity in 38% of patches. The single channel conductance was 53 +A- 1 pS at + 80 mV (outward current), decreasing to 20 +/- 2 pS at -80 mV (inward current). Ion substitution experiments indicated that this channel conducts Cl- and not Na+. Furthermore, 5-nitro-2-(3-phenylpropylamino)benzoate (100 microM), a recognized inhibitor of anion channels, induced a reversible 'flickery' channel block. We estimate that each platelet possesses < or = 30 such channels. Kinetic analysis suggested at least two open channel states (tau = 0.8 +/- 0.2 ms, tau = 22 +/- 14 ms, n = 4) and two closed states (tau = 0.8 +/- 0.2 ms, tau = 12 +/- 0.6 ms, n = 4). Increasing [Ca2+]i to 10 microM, following channel activation by depolarisation, had no significant effect on channel kinetics or open probability, however, elevated [Ca2+]i (300 nM-10 microM) increased the number of anion channels activated by subsequent depolarisation. This study represents the first recordings of ionic currents in excised, inside-out membrane patches from human platelets, and provides further evidence for the existence of chloride channels in these cells.

IgG-induced Ca2+ oscillations in differentiated U937 cells; a study using laser scanning confocal microscopy and co-loaded fluo-3 and fura-red fluorescent probes

We have investigated, at the single cell level, intracellular Ca2+ ([Ca2+]i) modulations triggered by the high affinity receptor for IgG, Fc gamma RI, in the monocytic cell line, U937. Cells were co-loaded with the Ca(2+)-sensitive dyes, Fluo-3 and Fura-Red, by incubation with their acetoxymethyl (AM) esters and confocal ratio imaging was used to monitor the [Ca2+]i changes induced by antibody cross-linking of IgG-loaded Fc gamma RI. A single Ca2+ spike was observed in 81% of untreated cells whereas dibutyryl cAMP-induced differentiation into a more macrophage cell type resulted in a sub-population of cells (44%) responding to receptor cross-linking with calcium oscillations. This change in calcium signalling may explain the difference in functional responses triggered by Fc gamma RI in monocytes and macrophages. Analysis of the Fluo-3 and Fura-Red fluorescence, after AM-ester loading, showed that both dyes have similar photobleach rates and intracellular localization allowing compensation for shifts in focal plane, dye photobleaching and non-uniformity of dye loading. In addition, because the binding kinetics of both dyes are equivalent, accurate temporal information can be gained about [Ca2+] changes. There are, however, two major problems with this dual indicator technique. Firstly, loading from AM esters results in considerable variation between cells in the intracellular concentration ratio of the two dyes, making calibration difficult. Secondly, the fluorescence ratio, Fluo-3/Fura-Red, behaves non-linearly at Ca2+ concentrations less than approximately 500 nM and comparison with Fura-2-loaded single cell photometry studies suggests there is considerable amplitude distortion of the signal when the ratios are displayed on a linear scale. These problems may considerably limit the application of Fluo-3/Fura-Red ratiometric measurements.

Primaquine, an inhibitor of vesicular transport, blocks the calcium-release-activated current in rat megakaryocytes

The whole-cell patch-clamp technique was used to study the effect of primaquine, an inhibitor of vesicular transport, on the calcium-release-activated current (Icrac) in rat megakaryocytes. Addition of primaquine, before emptying of internal Ca2+ stores by ionomycin, prevented the development of Icrac, with a half-maximal concentration of near 100 microM. Maximal inhibition (> or = 83%) was observed at 0.6-1 mM primaquine. At 1 mM, chloroquine, a related compound which is less effective at blocking vesicular secretion, had no effect on Icrac. Primaquine (0.8 mM) added after sustained activation of Icrac caused a gradual block of current, with maximal inhibition of 50% observed after 2-3 min. At 1 mM, internal guanosine 5'-[gamma-thio]triphosphate reduced Icrac by 65 +/- 13%. Neither 1 mM GTP nor 2 mM guanosine 5'-[beta-thio]diphosphate had any significant effect on Icrac. The recognized role of GTPases in the regulation of vesicular trafficking, together with block of Icrac activation by primaquine, provide evidence that the channels carrying Icrac may be stored in a vesicular membrane compartment and transferred to the plasma membrane following store depletion.

A novel monovalent cation channel activated by inositol trisphosphate in the plasma membrane of rat megakaryocytes

The activation of a monovalent cation current was studied in rat megakaryocytes using patch clamp techniques combined with photometric measurements of intracellular concentrations of Ca2+ ([Ca2+]i) and Na+. ADP evoked a release of [Ca2+]i and transiently activated a monovalent cation-selective channel, which, at negative potentials and under physiological conditions, would be expected to carry an inward Na+ current. The single channel conductance, estimated by noise analysis from whole cell currents at -50 to -60 mV was 9 picosiemens. Thapsigargin-induced [Ca2+]i increases failed to stimulate the monovalent cation current, suggesting that neither [Ca2+]i nor the depletion of internal Ca2+ stores were activators of this conductance. However, buffering of [Ca2+]i changes with 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid showed that both activation and inactivation of the current were accelerated by a rise in [Ca2+]i. The monovalent cation conductance was activated by internal perfusion with inositol 1,4,5-trisphosphate, both in the presence and in the absence of a rise in [Ca2+]i. Internal perfusion with inositol 2,4,5-trisphosphate, the poorly metabolizable isomer of inositol trisphosphate, similarly activated the monovalent cation current, whereas 1,3,4,5-tetrakisphosphate neither activated a current nor modified the ADP-induced monovalent current. Heparin, added to the pipette, blocked activation of the channel by ADP. The intracellular concentration of Na+, monitored by sodium-binding benzofuran isopthalate, increased by 10-20 mM in response to ADP under pseudophysiological conditions. We conclude the existence of a novel nonselective cation channel in the plasma membrane of rat megakaryocytes, which is activated by IP3 and can lead to increases in cytosolic Na+ after stimulation by ADP.

Calcium-activated potassium channels in human platelets

1. The effect of intracellular [Ca2+] ([Ca2+]i) on human platelet ion channels was studied using the nystatin whole-cell patch clamp recording technique. 2. Ionomycin-induced increases in [Ca2+]i rapidly activated a voltage-independent K(+)-selective channel with a slope conductance of 30 pS in 154 mM K+ saline. The single-channel conductance decreased in proportion to the square root of the external K+ concentration such that the estimated conductance in 5 mM K+ was approximately 5 pS. 3. The peak current under conditions expected to increase [Ca2+]i to micromolar levels indicated that each platelet possesses a small number (5-7) of 30 pS Ca(2+)-dependent K+ channels (KCa channels). 4. Spontaneous [Ca2+]i spiking was observed in many patch-clamped platelets using fura-2 fluorescence measurements. Each Ca2+ spike triggered up to five KCa channels at any one time. KCa channels were not active at resting levels of [Ca2+]i. 5. The results suggest that platelet KCa channels are not active under resting conditions but may have an important role in determining the membrane potential during Ca2+ signalling.

Three cation influx currents activated by purinergic receptor stimulation in rat megakaryocytes

1. Simultaneous patch clamp and fura-2 fluorescence measurements were used to study ATP-evoked membrane currents and intracellular [Ca2+] ([Ca2+]i) changes in rat megakaryocytes. 2. At negative potentials, under conditions that blocked K+ currents, 20 microM ATP activated a biphasic inward current and a concurrent biphasic increase in [Ca2+]i. The initial [Ca2+]i increase was due to Ca2+ influx whereas the delayed (1.70 +/- 0.13 s, mean +/- S.D.) increase was at least partly due to the release of internal Ca2+ stores. 3. The initial current was activated within 100 ms, inactivated within 1-4 s and was carried by both Na+ and Ca2+. 4. The delayed current was also transient and carried mainly by Na+ when Ca2+ buffering in the pipette was low. This Na+ conductance did not require an increase in [Ca2+]i for activation, but was triggered by inositol 1,4,5-trisphosphate (IP3), or a metabolite of IP3. 5. Buffering of [Ca2+]i changes with BAPTA revealed a third current activated by Ca2+ release from internal stores. This channel was selective for divalent cations with the permeability sequence Ca2+ >> Ba2+ > Mn2+, Mg2+. 6. Adenosine-5'-O-3-thiotriphosphate (ATP gamma S), like ATP, evoked all three influx currents, whereas ADP only stimulated Ca2+ release and the two currents associated with it. Increasing the external divalent cation concentration abolished the ATP-evoked Ca2+ release and delayed currents but not the initial transient current. 7. We conclude that rat megakaryocytes express two types of purinergic receptor. One type, activated by ATP, is closely coupled to a non-selective cation channel.(ABSTRACT TRUNCATED AT 250 WORDS)

Single-cell analysis of cyclic AMP response to parathyroid hormone in osteoblastic cells

We previously demonstrated that the [Ca2+]i response to PTH is heterogeneous in single UMR-106-01 osteogenic sarcoma cells. To verify whether response heterogeneity is a universal feature of PTH signal transduction, cAMP production was monitored in monolayer cultures of UMR-106-01 cells and human trabecular bone osteoblasts (HOB) using the cAMP-sensitive fluorescent indicator FlCRhR. FlCRhR was microinjected into single cells, and the 500-530/> 560 nm fluorescence ratio was monitored by confocal laserscanning video imaging as a measure of cAMP concentration ([cAMP]). Virtually all UMR-106-01 cells exposed to bovine PTH(1-34) (10(-7) M) exhibited an increase in intracellular [cAMP], with an average fluorescence ratio change of 145 +/- 17% of baseline (n = 15), corresponding to nearly maximal dissociation of protein kinase A. In the continued presence of the hormone (10(-7) M), [cAMP] remained elevated for at least 30 minutes. This effect was accompanied by a slow translocation of the fluorescein-labeled catalytic subunit of protein kinase A from the cytoplasm to the nucleus. In contrast, PTH(1-34) caused no detectable increase in [cAMP] in HOB cells, although PGE2 (3 x 10(-6) M) stimulation was able to increase the FlCRhR ratio (154 +/- 27%, n = 10). The truncated fragment PTH(2-34) was only 67% as potent at PTH(1-34), but deletion of the first two amino acids at the N terminus abolished the hormone's ability to stimulate cAMP production in UMR-106-01 cells. Brief exposure to 10(-7) M of either PTH(3-34) or PTH(7-34) did not affect the amplitude of the fluorescence ratio change induced by equimolar doses of PTH(1-34). Thus, in osteoblast-like cells stimulated with PTH, the [cAMP] response is much more homogeneous from cell to cell than the [Ca2+]i response.

Isolation and characterization of membrane potential changes associated with release of calcium from intracellular stores in rat thymic lymphocytes

Membrane potential changes accompanying Ca2+ influx stimulated by release of Ca2+ from intracellular stores (store-regulated Ca2+ uptake) were monitored in BAPTA-loaded rat thymic lymphocytes using the fluorescent indicator bis(1,3-diethylthiobarbituric acid)trimethine oxonol. Depletion of [Ca2+]i stores by the application of thapsigargin, ionomycin or cyclopiazonic acid induced a depolarization which was (i) dependent upon BAPTA-loading, (ii) dependent upon extracellular Ca2+, (iii) independent of extracellular Na+ and (iv) abolished by 5 mM extracellular Ni2+. This depolarization was followed by a charybdotoxin-sensitive repolarization and subsequent hyperpolarization to values approximating the K+ equilibrium potential, consistent with secondary activation of a K+ conductance. These membrane potential changes temporally correlated with Ca2+ influx from the extracellular medium as measured fluorimetrically with indo-1. The divalent cation permeability sequence was investigated by monitoring the magnitude of the depolarization observed following the addition of 4 mM Ca2+, Mn2+, Ba2+ or Sr2+ to cells pretreated with doses of thapsigargin or ionomycin known to activate the store-regulated calcium uptake pathway. On the basis of these experiments, we conclude that the store-regulated Ca2+ uptake pathway has the following permeability sequence: Ca2+ > Mn2+ >> Ba2+, Sr2+ with Mn2+ displaying significant permeability relative to Ca2+. This pathway is distinguishable from other divalent cation uptake pathways reported in other cells types on the basis of its activation by thapsigargin and its high Mn2+ permeability.

Calcium Entry in Nonexcitable Cells: Lessons from Human Platelets

A rise in cytosolic calcium concentration plays an important role in platelet activation. As well as helping define stimulus-response coupling in these intriguing and clinically important cells, the study of calcium mobilisation in platelets serves as an important model for elucidating calcium signaling mechanisms in general.

Rapid ADP-evoked currents in human platelets recorded with the nystatin permeabilized patch technique

"Whole-cell" patch recordings using nystatin permeabilization were made from single human platelets during application of agonists from a "puffer" pipette. In platelets clamped near the resting potential and bathed in Na+ saline, 40 microM ADP activated a transient inward current within tens of milliseconds. At -73 mV the current lasted between 0.1 and 1 s and had a peak of between 13 and 31 pA in different cells. Ion substitution experiments indicated that the channel is permeable to Na+,K+, and Ba2+ and presumably also to Ca2+, but is not permeable to Cl-. The single channel conductance was 15 pS (near the resting potential) in nominally Ca(2+)-free saline and 11 picosiemens in BaCl2 saline. Thrombin, at 1 unit/ml, did not elicit detectable currents during a 3-s application in platelets bathed in 1 mM Ca2+, Na+ saline. Under the same conditions, in fura-2-loaded cells, thrombin-evoked Ca2+ entry (monitored by Mn2+ quench) was detectable after a delay of 1.4 s. This suggests that early thrombin-evoked Ca2+ entry occurs via small conductance channels, below the resolution of the patch clamp technique, or by an electroneutral pathway. The ADP-evoked channel has the requisite speed of activation to account for the rapid Ca2+ influx observed during stopped-flow studies of agonist-evoked changes in [Ca2+]i.

Resting and ADP-evoked changes in cytosolic free sodium concentration in human platelets loaded with the indicator SBFI

1. Cytosolic free Na+ concentration, [Na+]i, was investigated in human platelets loaded with the fluorescent indicator SBFI (sodium-binding benzofuran isophthalate). 2. SBFI fluorescence from platelet suspensions was measured at excitation wavelengths of 340 and 385 nm and the 340/385 nm fluorescence ratio was calibrated in terms of [Na+]i in situ. [Na+]i was set to known values by resuspending cells in media with various [Na+], in the presence of the Na(+)-K+ ionophore, gramicidin. 3. Basal free [Na+]i was 5.5 +/- 0.3 mM (n = 50). This is considerably lower than estimates of total platelet Na+, suggesting that much intracellular Na+ is sequestered or bound. 4. ADP (40 microM) evoked a rise in [Na+]i from 6.4 +/- 0.7 to 18.3 +/- 1.1 mM (n = 8). The ADP-evoked rise in [Na+]i was abolished when external Na+ was replaced with N-methyl-D-glucamine. This indicates that the rise in [Na+]i was due to Na+ entry. 5. In platelets loaded with the fluorescent pH indicator, BCECF, 40 microM-ADP was shown to evoke a fall in cytosolic pH (pHi) from 7.21 +/- 0.03 to 7.12 +/- 0.03 (n = 10). Three minutes after ADP addition pHi had only recovered to 7.15 +/- 0.03. The recovery was dependent on external Na+, suggesting it was mediated by Na(+)-H+ exchange. However, this would only account for an increase in [Na+]i of approximately 0.5 mM, indicating most of the ADP-evoked Na+ entry occurred by other mechanisms. 6. Stopped-flow fluorimetry showed that the ADP-evoked rise in [Na+]i commenced without measurable delay and peaked within 1 s. The initial kinetics were thus similar to those reported for ADP-evoked rises in [Ca2+]i. 7. Cell-attached patch-clamp recordings showed that ADP evoked single-channel inward currents when included in the pipette-filling solution. The currents were similar whether Ca2+ was present or absent from the pipette. The slope conductance was 11 pS in the presence of external Ca2+ and 10 pS in its absence. Current-voltage relationships were similar and the reversal potentials were close to 0 mV under both conditions. 8. SK & F 96,365 (20 microM), a blocker of receptor-mediated Ca2+ entry in several non-excitable cells, blocked the ADP-evoked rise in [Na+]i. This compound has been shown to only partly block the biphasic ADP-evoked rise in [Ca2+]i, being selective for the fast, receptor-operated phase of entry. 9. These data suggest that ADP rapidly activates a channel in that platelet plasma membrane which is permeable to Na+ and divalent cations.

Ca(2+)-activated K+ channels in rat thymic lymphocytes: activation by concanavalin A

1. The role of ion channels in the mitogenic response of rat thymic lymphocytes to concanavalin A (ConA) was studied using single-channel patch-clamp recordings and measurements of membrane potential with the fluorescent probe bis-oxonol. 2. ConA (20 micrograms ml-1) evoked a rapid membrane hyperpolarization; Indo-1 measurements indicated a concurrent increase in [Ca2+]i. The hyperpolarization was blocked by cytoplasmic loading with the Ca2+ buffer BAPTA (bis(O-amino-phenoxy)ethane-N,N,N',N'-tetraacetic acid), or charybdotoxin, a component of scorpion venom known to block K+ channels in lymphocytes. 3. Cell-attached patch-clamp recordings showed that both ConA and the Ca2+ ionophore ionomycin activated channels with high selectivity for K+. Two conductance levels were observed -6-7 pS and 17-18 pS-measured as inward chord conductance at 60 mV from reversal potential (Erev) with 140 mM-KCl in the pipette. The current-voltage relationship for the larger channel displayed inward rectification and channel open probability was weakly dependent upon membrane potential. 4. These experiments provide the first direct evidence for mitogen-activated Ca(2+)-gated K+ channels (IK(Ca)) in lymphocytes. This conductance is relatively inactive in unstimulated rat thymocytes but following the intracellular Ca2+ rises induced by ConA, IK(Ca) channels are activated and produce a significant hyperpolarization of the cell potential.

The role of intracellular Ca2+ in the regulation of the plasma membrane Ca2+ permeability of unstimulated rat lymphocytes

The mechanism responsible for the increase in cytosolic free Ca2+ concentration ([Ca2+]i) during mitogenic stimulation of lymphocytes has been widely investigated. By contrast, little is known about the processes underlying Ca2+i homeostasis in resting (unstimulated) cells. It has been suggested that [Ca2+]i is an important determinant of the rate of Ca2+ influx following mitogenic activation. Using rat thymic lymphocytes, we investigated whether the resting influx pathway is similarly controlled by [Ca2+]i. Otherwise untreated cells were Ca(2+)-depleted by loading with Ca2+ chelators while suspended in Ca(2+)-free solution. Ca2+ depletion induced an 8-fold increase in the rate of unidirectional Ca2+ uptake. The depletion-activated flux was voltage-sensitive and was blocked by La3+ and by compound SK&F 96365, a receptor-operated Ca2+ channel blocker. Upon reintroduction to Ca(2+)-containing solution, the increased influx brought about a rapid recovery of [Ca2+]i. Detailed analysis of the magnitude of the 45Ca2+ flux during this recovery indicated that [Ca2+]i is not the primary determinant of the plasmalemmal Ca2+ permeability. Instead, depletion of an internal thapsigargin-sensitive store correlates with and appears to be responsible for the increased permeability of the plasma membrane. Accordingly, the Ca2+ fluxes induced by intracellular Ca2+ depletion and by thapsigargin were pharmacologically indistinguishable. Mitogenic lectins also released Ca2+ from a thapsigargin-sensitive store and activated a plasmalemmal Ca2+ permeability displaying identical pharmacology. The data support the existence of a coupling process whereby the degree of filling of an internal Ca2+ store dictates the Ca2+ permeability of the plasma membrane. This coupling mechanism is important not only in mediating the effects of mitogens and other agonists, as suggested before, but seemingly also in the control of resting Ca2+i homeostasis in unstimulated cells.

Chloride channels in human platelets: evidence for activation by internal calcium

Whole-cell patch-clamp recordings were made from freshly isolated human platelets. The pipette contained a high concentration of divalent cations, which permitted easy disruption of cell-attached membrane patches by suction. Single-channel currents were measured when the pipette contained isotonic BaCl2 or MgCl2 saline; over 30 sec -5 min an increasing number of channels appeared until conductance steps through individual channels could no longer be distinguished. The current-voltage relationship was curvilinear; chord conductance at -35 mV was 25 pS increasing to 45 to 52 pS at +45 mV. Ion substitution experiments showed the current to be primarily carried by Cl-. Erev was shifted 30 mV/10-fold change in external Cl- (replaced by gluconate), was similar with BaCl2 or MgCl2 in the pipette and was not significantly shifted by replacing external Na+ with K+. Addition of 1 mM BAPTA to the MgCl2 pipette saline prevented activation of Cl- currents; with isotonic CaCl2 internal saline, current appeared immediately upon patch rupture, suggesting that the Cl- channels are dependent on internal Ca2+. 5-nitro-2-(3-phenylpropylamino)-benzoate, reported to block a Cl- conductance in studies of rat epithelial cells, caused a potent flickery block and may be a useful tool with which to investigate the physiological role of Cl- currents in human platelets.

Voltage-gated potassium channels and the control of membrane potential in human platelets

1. Human platelets were studied using a combination of patch-clamp and fluorescent indicators of membrane potential and [Ca2+]i. 2. Whole-cell and cell-attached patch recordings showed voltage-gated channels selective for K+ (IK(V]. These channels were activated by depolarization at a threshold close to the platelet resting potential and were blocked by the venom charybdotoxin (CTX; 10-20 nM). Several different conductance states were observed, ranging from 5 to 34 pS, with isotonic KCl in the patch pipette and bath. 3. Measurements with the potential-sensitive dye 3,3'-dipropylthia-dicarbocyanine, diS-C3-(5), in platelet suspensions showed that CTX depolarized the resting potential by approximately 25 mV. Thus, CTX-sensitive, voltage-gated K+ channels appear to play a major part in setting the resting potential. 4. ADP-evoked Ca2+ influx, monitored with Fura-2, was reduced by 10 nM-CTX. Restoration of a large negative membrane potential with valinomycin reversed this effect of CTX. These results suggest that the Ca2+ influx depends on the negative membrane potential and that K+ channels may be important in maintaining this potential during activation.

Receptor-activated single channels in intact human platelets

We report "cell-attached" patch clamp studies of intact human platelets which show receptor-activated single channels. Inclusion of ADP in the patch pipette, but not in the bath, resulted in the appearance of inward currents indicative of single channels tightly coupled to the ADP receptors. The channels had a slope conductance of 11 picosiemens at the resting potential. Removal of 1 mM Ca2+ or replacement of chloride by gluconate in the pipette filling solution had little effect on the slope conductance at the resting potential or on the estimated reversed potential. With isotonic BaCl2 in the pipette, ADP evoked single channel currents with a slope conductance of 10 picosiemens. Thus these channels appear to be permeable to monovalent and divalent cations and selective for cations over anions. Addition of 5 mM Ni2+ (which blocks ADP-evoked rapid calcium entry in fura-2-loaded platelets) to the pipette solution blocked ADP-evoked channel activity. These channels may therefore provide an important mechanism for ADP to activate human platelets within a small fraction of a second.

Rapid kinetics of agonist-evoked changes in cytosolic free Ca2+ concentration in fura-2-loaded human neutrophils

The initial kinetics of agonist-evoked rises in the cytosolic Ca2+ concentration [Ca2+]i were investigated in fura-2-loaded human neutrophils by stopped-flow fluorimetry. The rises in [Ca2+]i evoked by chemotactic peptide (fMet-Leu-Phe), platelet-activating factor and ADP all lagged behind agonist addition by 1-1.3 s. Lag times were not significantly different in the presence and in the absence of external Ca2+. Stimulation of the cells in the presence of extracellular Mn2+ resulted in a quench of fluorescence with a similar lag time to [Ca2+]i rise. The delay in onset of the rise in [Ca2+]i evoked by fMet-Leu-Phe was dependent on concentration, becoming longer at lower concentrations of agonist. These results indicate that both the agonist-evoked discharge of the intracellular Ca2+ stores and the generation of bivalent-cation influx lag behind agonist-receptor binding in neutrophils. Both pathways thus appear to be mediated by indirect mechanisms, rather than by a directly coupled process such as a receptor-operated channel. The temporal coincidence of the onset of store discharge with the commencement of bivalent-cation influx suggests that the two events may be causally linked.

Ca2(+)-activated K+ channels in human B lymphocytes and rat thymocytes

1. Previous evidence for the existence of Ca2(+)-activated K+ channels in lymphocytes comes from measurements using voltage-sensitive dyes and from tracer flux studies. We have now directly measured these channels in human tonsillar B lymphocytes and rat thymocytes in single-channel recordings from cell-attached and excised patches. 2. In cell-attached recordings, intracellular Ca2+ was raised by either ionomycin or replacement of external Ca2+ following incubation in Ca2(+)-free medium. Indo-1 measurements during the Ca2(+)-replacement technique showed that [Ca2+]i rose from approximately 90 to 260 nM. Both techniques activated two channels of approximately 25 and 8 pS (slope conductance at 0 mV applied, with 140 mM-K+ in the pipette). Over 90% of patches displayed this activity, indicating a high density of these channels in the membrane. 3. Both channels reversed near the K+ equilibrium potential with either KCl or potassium aspartate in the pipette, when the cells were bathed in normal or high-K+ saline. Therefore, these channels are selective for K+. 4. The larger channel was studied in more detail. It displayed inward rectification in symmetrical K+ solutions. The open-channel probability was weakly dependent on membrane potential. 5. Ca2(+)-dependent K+ channels were also recorded from excised, inside-out membrane patches. The threshold for activation was 200-300 nM [Ca2+i]. 6. Patch excision altered some characteristics of IK(Ca). Channels were activated in fewer than 50% of patches and the main conductance level was approximately 34 pS (at -80 mV). The duration of single-channel events was shorter than in cell-attached patches; kinetic analysis suggested that this was due to the loss of an open state in excised patches. 7. We conclude that B and T lymphocytes have K(+)-selective channels which are activated by internal [Ca2+] in the physiological range and which will influence the membrane potential during cell activation.