Intracellular mechanisms of cytoplasmic Ca2+ oscillation in rat megakaryocyte

Attenuation of calmodulin regulation evokes Ca2+ oscillations: evidence for the involvement of intracellular arachidonate-activated channels and connexons

Intracellular Са²⁺ controls its own level by regulation of Ca²⁺ transport across the plasma and organellar membranes, often acting via calmodulin (CaM). Drugs antagonizing CaM action induce an increase in cytosolic Ca²⁺ concentration in different cells. We have found persistent Са²⁺ oscillations in cultured white adipocytes in response to calmidazolium (CMZ). They appeared at [CMZ] > 1 μM as repetitive sharp spikes mainly superimposed on a transient or elevated baseline. Similar oscillations were observed when we used trifluoperazine. Oscillations evoked by 5 μM CMZ resulted from the release of stored Ca²⁺ and were supported by Са²⁺ entry. Inhibition of store-operated channels by YM-58483 or 2-APB did not change the responses. Phospholipase A₂ inhibited by AACOCF₃ was responsible for initial Ca²⁺ mobilization, but not for subsequent oscillations, whereas inhibition of iPLA₂ by BEL had no effect. Phospholipase C was partially involved in both stages as revealed with U73122. Intracellular Са²⁺ stores engaged by CMZ were entirely dependent on thapsigargin. The oscillations existed in the presence of inhibitors of ryanodine or inositol 1,4,5-trisphosphate receptors, or antagonists of Ca²⁺ transport by lysosome-like acidic stores. Carbenoxolone or octanol, blockers of hemichannels (connexons), when applied for two hours, prevented oscillations but did not affect the initial Са²⁺ release. Incubation with La³⁺ for 2 or 24 h inhibited all responses to CMZ, retaining the thapsigargin-induced Ca²⁺ rise. These results suggest that Ca²⁺-CaM regulation suppresses La³⁺-sensitive channels in non-acidic organelles, of which arachidonate-activated channels initiate Ca²⁺ oscillations, and connexons are intimately implicated in their generation mechanism.

The Plant Hormone Abscisic Acid Is a Prosurvival Factor in Human and Murine Megakaryocytes

Abscisic acid (ABA) is a phytohormone involved in pivotal physiological functions in higher plants. Recently, ABA has been proven to be also secreted and active in mammals, where it stimulates the activity of innate immune cells, mesenchymal and hematopoietic stem cells, and insulin-releasing pancreatic β cells through a signaling pathway involving the second messenger cyclic ADP-ribose (cADPR). In addition to behaving like an animal hormone, ABA also holds promise as a nutraceutical plant-derived compound in humans. Many biological functions of ABA in mammals are mediated by its binding to the LANCL-2 receptor protein. A putative binding of ABA to GRP78, a key regulator of endoplasmic reticulum stress, has also been proposed. Here we investigated the role of exogenous ABA in modulating thrombopoiesis, the process of platelet generation. Our results demonstrate that expression of both LANCL-2 and GRP78 is up-regulated during hematopoietic stem cell differentiation into mature megakaryocytes (Mks). Functional ABA receptors exist in mature Mks because ABA induces an intracellular Ca²⁺ increase ([Ca²⁺] _i ) through PKA activation and subsequent cADPR generation. In vitro exposure of human or murine hematopoietic progenitor cells to 10 μm ABA does not increase recombinant thrombopoietin (rTpo)-dependent Mk differentiation or platelet release. However, under conditions of cell stress induced by rTpo and serum deprivation, ABA stimulates, in a PKA- and cADPR-dependent fashion, the mitogen-activated kinase ERK 1/2, resulting in the modulation of lymphoma 2 (Bcl-2) family members, increased Mk survival, and higher rates of platelet production. In conclusion, we demonstrate that ABA is a prosurvival factor for Mks in a Tpo-independent manner.

Calcium signalling in platelets and other cells

Reviewed are new concepts and models of Ca(2+) signalling originating from work with various animal cells, as well as the applicability of these models to the signalling systems used by blood platelets. The following processes and mechanisms are discussed: Ca(2+) oscillations and waves; Ca(2+) -induced Ca(2+) release; involvement of InsP(3)-receptors and quanta1 release of Ca(2+); different pathways of phospholipase C activation; heterogeneity in the intracellular Ca(2+) stores; store-and receptor-regulated Ca(2+) entry. Additionally, some typical aspects of Ca(2+) signalling in platelets are reviewed: involvement of protein serine/threonine and tyrosine kinases in the regulation of signal transduction; possible functions of platelet glycoproteins; and the importance of Ca(2+) for the exocytotic and procoagulant responses.

Acidic NAADP-releasable Ca(2+) compartments in the megakaryoblastic cell line MEG01

BACKGROUND

A novel family of intracellular Ca(2+)-release channels termed two-pore channels (TPCs) has been presented as the receptors of NAADP (nicotinic acid adenine dinucleotide phosphate), the most potent Ca(2+) mobilizing intracellular messenger. TPCs have been shown to be exclusively localized to the endolysosomal system mediating NAADP-evoked Ca(2+) release from the acidic compartments.

OBJECTIVES

The present study is aimed to investigate NAADP-mediated Ca(2+) release from intracellular stores in the megakaryoblastic cell line MEG01.

METHODS

Changes in cytosolic and intraluminal free Ca(2+) concentrations were registered by fluorimetry using fura-2 and fura-ff, respectively; TPC expression was detected by PCR.

RESULTS

Treatment of MEG01 cells with the H(+)/K(+) ionophore nigericin or the V-type H(+)-ATPase selective inhibitor bafilomycin A1 revealed the presence of acidic Ca(2+) stores in these cells, sensitive to the SERCA inhibitor 2,5-di-(tert-butyl)-1,4-hydroquinone (TBHQ). NAADP releases Ca(2+) from acidic lysosomal-like Ca(2+) stores in MEG01 cells probably mediated by the activation of TPC1 and TPC2 as demonstrated by TPC1 and TPC2 expression silencing and overexpression. Ca(2+) efflux from the acidic lysosomal-like Ca(2+) stores or the endoplasmic reticulum (ER) results in ryanodine-sensitive activation of Ca(2+)-induced Ca(2+) release (CICR) from the complementary Ca(2+) compartment.

CONCLUSION

Our results show for the first time NAADP-evoked Ca(2+) release from acidic compartments through the activation of TPC1 and TPC2, and CICR, in a megakaryoblastic cell line.

Direct voltage control of signaling via P2Y1 and other Galphaq-coupled receptors

Emerging evidence suggests that Ca2+ release evoked by certain G-protein-coupled receptors can be voltage-dependent; however, the relative contribution of different components of the signaling cascade to this response remains unclear. Using the electrically inexcitable megakaryocyte as a model system, we demonstrate that inositol 1,4,5-trisphosphate-dependent Ca2+ mobilization stimulated by several agonists acting via Galphaq-coupled receptors is potentiated by depolarization and that this effect is most pronounced for ADP. Voltage-dependent Ca2+ release was not induced by direct elevation of inositol 1,4,5-trisphosphate, by agents mimicking diacylglycerol actions, or by activation of phospholipase Cgamma-coupled receptors. The response to voltage did not require voltage-gated Ca2+ channels as it persisted in the presence of nifedipine and was only weakly affected by the holding potential. Strong predepolarizations failed to affect the voltage-dependent Ca2+ increase; thus, an alteration of G-protein betagamma subunit binding is also not involved. Megakaryocytes from P2Y1(-/-) mice lacked voltage-dependent Ca2+ release during the application of ADP but retained this response after stimulation of other Galphaq-coupled receptors. Although depolarization enhanced Ca2+ mobilization resulting from GTPgammaS dialysis and to a lesser extent during AlF4- or thimerosal, these effects all required the presence of P2Y1 receptors. Taken together, the voltage dependence to Ca2+ release via Galphaq-coupled receptors is not due to control of G-proteins or down-stream signals but, rather, can be explained by a voltage sensitivity at the level of the receptor itself. This effect, which is particularly robust for P2Y1 receptors, has wide-spread implications for cell signaling.

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.

Sensitivity limits for voltage control of P2Y receptor-evoked Ca2+ mobilization in the rat megakaryocyte

G-protein-coupled receptor signalling has been suggested to be voltage dependent in a number of cell types; however, the limits of sensitivity of this potentially important phenomenon are unknown. Using the non-excitable rat megakaryocyte as a model system, we now show that P2Y receptor-evoked Ca2+ mobilization is controlled by membrane voltage in a graded and bipolar manner without evidence for a discrete threshold potential. Throughout the range of potentials studied, the peak increase in intracellular Ca2+ concentration ([Ca2+]i) in response to depolarization was always larger than the maximal reduction in [Ca2+]i following an equivalent amplitude hyperpolarization. Significant [Ca2+]i increases were observed in response to small amplitude (< 5 mV, 5 s duration) or short duration (25 ms, 135 mV) depolarizations. Individual cardiac action potential waveforms were also able to repeatedly potentiate P2Y receptor-evoked Ca2+ release and the response to trains of normally paced stimuli fused to generate prolonged [Ca2+]i increases. Furthermore, elevation of the temperature to physiological levels (36 degrees C) resulted in a more sustained depolarization-evoked Ca2+ increase compared with more transient or oscillatory responses at 20-24 degrees C. The ability of signalling via a G-protein-coupled receptor to be potentiated by action potential waveforms and small amplitude depolarizations has broad implications in excitable and non-excitable tissues.

Estrogen receptor, cyclic adenosine monophosphate, and protein kinase A are involved in the nongenomic pathway by which estradiol accelerates oviductal oocyte transport in cyclic rats

This investigation examined the role of estrogen receptor (ER) on the stimulatory effect of estradiol (E2) on protein phosphorylation in the oviduct as well as on E2-induced acceleration of oviductal oocyte transport in cyclic rats. Estrous rats were injected with E2 s.c. and with the ER antagonist ICI 182 780 intrabursally (i.b.), and 6 h later, oviducts were excised and protein phosphorylation was determined by Western blot analysis. ICI 182 780 inhibited the E2-induced phosphorylation of some oviductal proteins. Other estrous rats were treated with E2 s.c. and ICI 182 780 i.b. The number of eggs in the oviduct, assessed 24 h later, showed that ICI 182 780 blocked the E2-induced egg transport acceleration. The possible involvement of adenylyl cyclase, protein kinase A (PK-A), protein kinase C (PK-C), or tyrosine kinases on egg transport acceleration induced by E2 was then examined. Selective inhibitors of adenylyl cyclase or PK-A inhibited the E2-induced egg transport acceleration, whereas PK-C or tyrosine kinase inhibitors had no effect. Furthermore, forskolin, an adenylyl cyclase activator, mimicked the effect of E2 on ovum transport and E2 increased the level of cAMP in the oviduct of cycling rats. Finally, we measured PK-A activity in vitro in the presence of E2 or E2-ER complex. Activity of PK-A in the presence of E2 or E2-ER was similar to PK-A alone, showing that E2 or E2-ER did not directly activate PK-A. We conclude that the nongenomic pathway by which E2 accelerates oviductal egg transport in the rat requires absolute participation of ER and cAMP and partial participation of PK-A signaling pathways in the oviduct.

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.

Inhibitory effects of ruthenium red on inositol 1,4, 5-trisphosphate-induced responses in rat megakaryocytes

The effects of ruthenium red (RR) on inositol 1,4,5-trisphosphate (InsP(3))-induced responses were studied in rat bone marrow megakaryocytes with the patch-clamp whole-cell recording technique in combination with fura-2 microfluorometry. Internal application of InsP(3) (100 microM) increased intracellular Ca(2+) concentration ([Ca(2+)](i)) and activated the Ca(2+)-dependent K(+) current. Administering InsP(3) together with RR (100-500 microM) inhibited InsP(3)-induced responses (both Ca(2+) and current responses) in a dose-dependent fashion. Pretreatment of megakaryocytes with extracellular RR (50 microM) also inhibited InsP(3)-induced responses. Intracellular and extracellular application of RR reduced ADP-induced increases in [Ca(2+)](i). In contrast, in isolated single pancreatic acinar cells, RR had no effect on InsP(3)-induced responses. Taken together, these results suggest that the site of the inhibitory action of RR is at the InsP(3) receptor, or its closely associated proteins. In addition, we have shown that RR is a useful pharmacological tool with which to examine the InsP(3)-mediated responses of megakaryocytes.

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.

Dual regulation of calcium mobilization by inositol 1,4, 5-trisphosphate in a living cell

Changes in cytosolic free calcium ([Ca(2+)](i)) often take the form of a sustained response or repetitive oscillations. The frequency and amplitude of [Ca(2+)](i) oscillations are essential for the selective stimulation of gene expression and for enzyme activation. However, the mechanism that determines whether [Ca(2+)](i) oscillates at a particular frequency or becomes a sustained response is poorly understood. We find that [Ca(2+)](i) oscillations in rat megakaryocytes, as in other cells, results from a Ca(2+)-dependent inhibition of inositol 1,4,5-trisphosphate (IP(3))-induced Ca(2+) release. Moreover, we find that this inhibition becomes progressively less effective with higher IP(3) concentrations. We suggest that disinhibition, by increasing IP(3) concentration, of Ca(2+)-dependent inhibition is a common mechanism for the regulation of [Ca(2+)](i) oscillations in cells containing IP(3)-sensitive Ca(2+) stores.

Spontaneous and stimulated transients in cytoplasmic free Ca(2+) in normal human osteoblast-like cells: aspects of their regulation

We characterize two patterns of transients in cytoplasmic free calcium ([Ca2+]i) in normal human osteoblast-like cells (hOB cells). Firstly, spontaneous oscillations in [Ca2+]i were found to be common. The [Ca2+]i oscillations were completely inhibited by thapsigargin, indicating that Ca2+ fluxes between intracellular Ca2+ pools and the cytosol contributed to the generation of the [Ca2+]i oscillations. Removing extracellular Ca2+ either attenuated or completely inhibited spontaneous [Ca2+]i oscillations. Gadolinium, an inhibitor of stretch activated cation channels (SA-cat channels), reduced the frequency of [Ca2+]i oscillations. Hence, entry of calcium from the extracellular space, possibly through SA-cat channels also seemed to be of importance in the regulation of these [Ca2+]i oscillations. The role of the observed spontaneous [Ca2+]i oscillations in hOB cell function is not clear. Secondly, a decrease in pericellular osmolality, which causes the plasma membrane to stretch, transiently increased [Ca2+]i in hOB cells. This effect was also observed in a Ca2+ free extracellular environment, suggesting that osmotic stimuli release Ca2+ from intracellular pools. This finding indicates a possible signaling pathway by which mechanical strain can promote anabolic effects on the human skeleton.

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.

Alkalinization-induced K+ current of the mouse megakaryocyte

We have recently found that mouse megakaryocytes responded to extracellular alkalinization to pH > 8.0, generating a K+ current under voltage-clamped conditions with the whole cell recording mode of the patch-clamp technique. The purpose of this study was to physiologically and pharmacologically characterize the alkaline-dependent K+ conductance of the megakaryocyte membrane. The alkalinization-induced K+ current (I(ALK)) did not seem to be Ca2+-dependent since I(ALK) was allowed to be generated under intracellularly Ca2+-buffered conditions with 10 mM EGTA, which completely prevented the generation of caffeine-induced Ca2+-activated currents of mouse megakaryocytes; and no [Ca2+]i elevation was evoked by the alkalinization protocol in contrast to a significant increase in [Ca2+]i in response to caffeine when [Ca2+]i was measured with a fura 2 ratiometry. I(ALK) was strongly suppressed with tetraethylammonium (TEA), 4-aminopyridine (4-AP) and streptomycin (SM), but was completely resistant to quinidine (QND). The values of IC50 for the suppression of I(ALK) with TEA, 4-AP and SM were 5.6, 0.47 and 1.5 mM, respectively. Voltage-gated K+ currents (I(K)) of the same megakaryocyte preparation were weakly suppressed with TEA and 4-AP, while they were significantly suppressed with either SM or QND. These results suggest that mouse megakaryocytes possess K+ conductance that was activated by extracellular alkalinization and that probably differs from conventional K+ conductance in its pharmacological properties.

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.

Cyclic GMP inhibits cytoplasmic Ca2+ oscillation by increasing Ca2+-ATPase activity in rat megakaryocytes

The regulatory effects of cyclic GMP on purinoceptor-operated cytoplasmic Ca2+ oscillation of rat megakaryocytes were investigated by using whole-cell patch-clamp technique. ATP-induced oscillatory K+ currents though Ca2+-activated K+ channels (I(KCa)S) were depressed by pretreatment with the guanylate cyclase activator, sodium nitroprusside, and a stable membrane-permeable cGMP analogue, 8-bromo-cGMP. The inhibition by sodium nitroprusside was blocked by treatment with a cyclic nucleotide-dependent protein kinase inhibitor, N-[2-(methylamino)]-5-isoquinolinesulfonamide x HCl (H-8) (10 microM), but not by a selective cAMP-dependent-protein kinase inhibitor, Rp-cAMPS (100 microM). The oscillatory I(KCa) directly evoked by intracellular D-myo-inositol-trisphosphate (IP3) perfusion was also inhibited by the application of sodium nitroprusside. The inhibitory effect of sodium nitroprusside disappeared when the ATP-induced oscillatory I(KCa) was changed to a monophasic sustained I(KCa) current by inhibition of Ca2+-ATPase. These results suggested that cGMP depressed Ca2+ mobilization by improving Ca2+-ATPase activity by phosphorylation.

Inhibition of inositol 1,4,5-trisphosphate-induced Ca2+ release by cAMP-dependent protein kinase in a living cell

Interaction of intracellular free calcium ([Ca2+]i) and cAMP signaling mechanisms was examined in intact single megakaryocytes by using a combination of single-cell fluorescence microscopy to measure [Ca2+]i and flash photolysis of caged Ca2+, inositol 1,4, 5-trisphosphate (IP3), or cAMP to elevate rapidly the concentration of these compounds inside the cell. Photolysis of caged IP3 stimulated Ca2+ release from an IP3-sensitive store. The cAMP-elevating agent carbacyclin inhibited this IP3-induced rise in [Ca2+]i but did not affect the rate of Ca2+ removal from the cytoplasm after photolysis of caged Ca2+. Photolysis of caged cAMP during ADP-induced [Ca2+]i oscillations caused the [Ca2+]i oscillation to transiently cease without affecting the rate of Ca2+ uptake and/or extrusion. We conclude that the principal mechanism of cAMP-dependent inhibition of Ca2+ mobilization in megakaryocytes appears to be by inhibition of IP3-induced Ca2+ release and not by stimulation of Ca2+ removal from the cytoplasm. Two inhibitors of cAMP-dependent protein kinase, a specific peptide inhibitor of the catalytic subunit of cAMP protein kinase and KT5720, blocked the inhibitory effect of carbacyclin, indicating that the inhibition of IP3-induced Ca2+-release by carbacyclin is mediated by cAMP-dependent protein kinase.

Blockade of N-type Ca2+ current by cilnidipine (FRC-8653) in acutely dissociated rat sympathetic neurones

1 The inhibitory effects of cilnidipine (FRC-8653) and various organic Ca2+ channel blockers on high voltage-activated Ba2+ currents (HVA IBa) in rat sympathetic neurones were examined by means of the conventional whole-cell patch-clamp recording mode under voltage-clamped conditions. 2 HVA IBa was classified into three different current components with subtype selective peptide Ca2+ channel blockers. No omega-Agatoxin IVA-sensitive (P-type) or omega-conotoxin MVIIC-sensitive (Q-type) current components were observed. Most (> 85%) IBa was found to consist of omega-conotoxin GVIA-sensitive N-type components. 3 The application of cilnidipine inhibited HVA 1Ba in a concentration-dependent manner. The Kd value for cilnidipine was 0.8 microM. Cilnidipine did not shift the current-voltage (I-V) relationship for HVA IBa, as regards the threshold potential and peak potential where the amplitude reached a maximum. 4 High concentration of three hypotensive Ca2+ channel blockers, nifedipine, diltiazem and verapamil, all inhibited HVA IBa in a concentration-dependent manner. The Kd values for nifedipine, diltiazem and verapamil were 131, 151 and 47 microM, respectively. A piperazine-type Ca2+ channel blocker, flunarizine, showed a relatively potent blocking action on IBa. The Kd value was about 3 microM. 5 These results thus show that cilnidipine potently inhibits the sympathetic Ca2+ channels which predominantly consist of an omega-Cg-GVIA-sensitive component. This blockade of the N-type Ca2+ channel, as well as the L-type Ca2+ channel by cilnidipine suggests that it could be used therapeutically for treatment of hypersensitive sympathetic disorders associated with hypertension.

Rapid and long-term effects of PTH(1-34) on growth plate chondrocytes are mediated through two different pathways in a cell-maturation-dependent manner

The aims of this study were to clarify the role of cell maturation stage on chondrocyte response to parathyroid hormone (PTH) by examining the effect of PTH(1-34) on alkaline-phosphatase-specific activity (ALPase) of chondrocyte cultures at two distinct stages of maturation, and to determine the signaling pathways used by the cells to mediate this effect. Confluent, fourth passage rat costochondral resting zone (RC) and growth zone (GC) chondrocytes were used. ALPase was measured in the cell layer, as well as in matrix vesicles (MV) and plasma membranes (PM), after the addition of 10(-7) 10(-11) mol/L bovine PTH(1-34), the active peptide, or bovine PTH(3-34), the inactive peptide, to the cultures. PTH(1-34) increased ALPase in the GC cultures at two separate times: between 5 and 180 min, with maximal stimulation at 10 min, and 36 to 48 h. In contrast, PTH(3-34) had no effect. At 10 min and 48 h, PTH(1-34) produced a dose-dependent increase in ALPase of both MV and PM isolated from GC cultures. Addition of forskolin and IBMX to increase cAMP increased ALPase in GC cultures to a level similar to that seen after addition of PTH(1-34). In contrast, the addition of PTH(1-34) to RC cells only increased ALPase between 5 and 60 min, with peak activity at 10 min. As with GC, PTH increased ALPase in both MV and PM. Moreover, the addition of PTH(3-34) or forskolin and IBMX had no effect on ALPase in RC. PTH(1-34) had no effect on GC protein kinase C (PKC) activity; however, the addition of PTH(1-34) to RC caused a dose-dependent increase in PKC activity. H8, an inhibitor of PKA, had no effect on PTH-stimulated ALPase in RC cells, but inhibited the PTH-dependent response in GC cells. In contrast, chelerythrine, an inhibitor of PKC activity, inhibited PTH-stimulated ALPase in RC cells, but had no effect on PTH-stimulated ALPase in GC cells. This study shows that the effect of PTH(1-34) on RC and GC cells is maturation dependent in terms of time course and mechanism. Whereas both cell types exhibit a rapid response to PTH, only GC cells show a long-term response. In GC, the effects of PTH are associated with changes in cAMP and may also involve at least one other pathway, whereas, in RC, the PTH effects appear to be associated with changes in PKC.

Complexity of the outward K+ current of the rat megakaryocyte

Megakaryocytes isolated from rat bone marrow express a voltage-dependent, outward K+ current with complex kinetics of activation and inactivation. We found that this current could be separated into at least two components based on differential responses to K+ channel blockers. One component, which exhibited features of the "transient" or "A-type" K+ current of excitable cells, was more strongly blocked by 4-aminopyridine (4-AP) than by tetrabutylammonium (TBA). This current, which we designated as "4-AP-sensitive" current, activated rapidly at potentials more positive than -40 mV and subsequently underwent rapid voltage-dependent inactivation. A separate current that activated slowly was blocked much more effectively by TBA than by 4-AP. This "TBA-sensitive" component, which resembled a typical delayed rectifier current, was much more resistant to voltage-dependent inactivation. The relative contribution of each of these components varied from cell to cell. The effect of charybdotoxin was similar to that of 4-AP. Our data indicate that the voltage-dependent K+ current of resting megakaryocytes is more complex than heretofore believed and support the emerging concept that megakaryocytes possess intricate electrophysiological properties.

[Ca2+]i oscillations and [Ca2+]i waves in rat megakaryocytes

ATP activated [Ca2+]i oscillations were measured in single rat megakaryocytes using fluorescence ratio microscopy. With increasing ATP concentration the duration of the [Ca2+]i oscillations increased, however, there was considerable variation from cell to cell in the absolute value of the peak [Ca2+]i and the frequency and duration of the oscillations. This variation depended, in part, on the level of Fura-2 loading suggesting that megakaryocytes are sensitive to buffering of [Ca2+]i by Fura-2. Agents, that increase the level of intracellular cGMP (sodium nitroprusside and 8-pCPT-cGMP) or cAMP (prostacyclin, IBMX, forskolin and 8-bromo-cAMP) inhibited [Ca2+]i oscillations. Despite the large cell to cell variation in the patterns of [Ca2+]i oscillations, reapplication of the agents that elevated cAMP or cGMP inhibited the oscillations similarly. Using video rate fluorescence ratio imaging we found that the agonist-induced [Ca2+]i oscillations were the result of a well-defined [Ca2+]i wave, which spread across the cell with an average speed of about 35 microns/s, during the rising phase of each oscillatory spike. After reaching a peak, [Ca2+]i decreased uniformly across the whole cell during the falling phase of the spike. Analysis of the temperature dependence of [Ca2+]i waves showed that the rate of [Ca2+]i decay exhibited a strong temperature dependence (Q10 approximately 4), whereas, the rate of rise exhibited a weak temperature dependence (Q10 approximately 1.3), suggesting, that the rate limiting process for [Ca2+]i wave propagation in rat megakaryocytes is the rate of [Ca2+]i diffusion.

Reversible phosphorylation as a controlling factor for sustaining calcium oscillations in HeLa cells: Involvement of calmodulin-dependent kinase II and a calyculin A-inhibitable phosphatase

The role of reversible phosphorylation in histamine-induced Ca2+ oscillations in HeLa cells has been investigated by using various activators and inhibitors of protein kinases and phosphatases. Electroporation was employed to introduce impermeable materials into single cells, which proved to be a useful and convenient tool. Of the kinases examined, cAMP-dependent kinase, protein kinase C, and calmodulin-dependent kinase II (CaMK II), only CaMK II was essential. When added during oscillations, both W-7, a calmodulin antagonist, and KN-62, a specific CaMK II inhibitor, caused one large Ca2+ spike before halting the process. Introduction of the Ca2+/calmodulin-independent catalytic domain of CaMK II into the cells forestalled their response to histamine. These results show that intracellular Ca2+ cannot oscillate when CaMK II is locked in either the inactive or the stimulated state. External Ca2+ electroporated into cells preloaded with the catalytic domains was quickly removed (but not when the cells were pretreated with the endoplasmic reticulum Ca(2+)-ATPase inhibitor, tapsigargin), indicating that the ATP-driven Ca2+ pump was somehow activated by CaMK II. Protein phosphatase inhibitors calyculin A and okadaic acid abolished ongoing oscillations and, when added at low concentrations, prolonged the interspike interval. Immunoprecipitation experiments with 32P(i)-labeled cells provided the first evidence that inositol 1,4,5-trisphosphate receptor (IP3R) was phosphorylated by CaMK II in vivo. The extent of phosphorylation was increased in the presence of histamine, significantly enhanced by calyculin A, and greatly reduced by W-7. Our observations are consistent with the concept that repetitive phosphorylation-dephosphorylation cycles regulating IP3R and Ca2+ pumps are a controlling factor for sustained Ca2+ oscillations in HeLa, and possibly other, cells.

ATP-regulated K+ channel and cytosolic Ca2+ mobilization in cultured rat spinal neurons

ATP activated the K+ channel responsible for outwardly rectifying currents via a P2Y purinoceptor linked to a pertussis toxin-insensitive G-protein in cultured rat spinal neurons. The evoked currents were inhibited by a selective protein kinase C inhibitor, GF109203X, whereas a phospholipase C inhibitor, neomycin had no effect. These indicate that the currents are regulated by phospholipase C-independent protein kinase C activation. In addition, ATP enhanced intracellular free Ca2+ concentration. The increase in intracellular free Ca2+ concentration was inhibited by a broad G-protein inhibitor, GDP beta S, but not affected by neomycin or an inositol 1,4,5-triphosphate receptor antagonist, heparin, suggesting that the cytosolic Ca2+ mobilization is regulated by a mechanism independent of a phospholipase C-mediated phosphatidylinositol signaling. These results, thus, demonstrate that ATP has dual actions on the coupled K+ channel and cytosolic Ca2+ release.

Pharmacological studies on mechanisms involved in Ca2+ oscillations in rat megakaryocytes

Extracellular application of ATP evoked the oscillatory K+ currents (IKCa) reflecting oscillation in cytoplasmic Ca2+ concentration ([Ca2-]i) of megakaryocyte isolated from rat bone marrow. We have reported that the [Ca2+], oscillation was regulated by intracellular Ca(2+)-pumping activity (Uneyama H.C. Uneyama and N. Akaike, 1993, J. Biol. Chem. 268, 168). Here we found that the Ca2+ pump of the megakaryocyte could be divided into at least two classes according to the sensitivity to phosphorylation-modulating drugs. The effects of protein kinase C and cyclic AMP-dependent protein kinase are complementary, and the effect of Ca2+/calmodulin is independent of the above two kinases. In addition, this is the first report concerning the physiological regulation of Ca(2+)-ATPase in living cells.

Modulation of Ca2+ oscillation and apamin-sensitive, Ca2+-activated K+ current in rat gonadotropes

In rat pituitary gonadotropes, gonadotropin-releasing hormone (GnRH) stimulates rhythmic release of Ca2+ from stores sensitive to inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], which in turn induces an oscillatory activation of apamin-sensitive Ca2+-activated K+ current, IK(Ca). Since GnRH also activates protein kinase C (PKC), we investigate the action of PKC while simultaneously measuring intracellular Ca2+ concentration ([Ca2+]i) and IK(Ca). Stimulation of PKC by application of phorbol 12-myristate 13-acetate (PMA) did not affect basal [Ca2+]i. However, PMA or phorbol 12,13-dibutyrate (PdBu), but not the inactive 4alpha-phorbol 12,13-didecanoate (4alpha-PDD), reduced the frequency of GnRH-induced [Ca2+]i oscillation and augmented the IK(Ca) induced by any given level of [Ca2+]i. The slowing of oscillations and the enhancement of IK(Ca) were mimicked by synthetic diacylglycerol (1,2-dioctanoyl-sn-glycerol) and could be induced during ongoing oscillations that had been initiated irreversibly in cells loaded with guanosine 5'-O-(3-thiotriphosphate) (GTP-[gammaS]). In contrast, when oscillations were initiated by loading cells with Ins(1,4,5)P3, phorbol esters enhanced IK(Ca) without affecting the frequency of oscillation. The protein kinase inhibitor, staurosporine, reduced IK(Ca) without affecting [Ca2+]i and partially reversed the phorbol-ester-induced slowing of oscillation. Therefore, activation of PKC has two rapid effects on gonadotropes. It slows [Ca2+]i oscillations probably by actions on phospholipase C, and it enhances IK(Ca) probably by a direct action on the channels.

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.

Caffeine rapidly decreases potassium conductance of dissociated outer hair cells of guinea-pig cochlea

The effects of caffeine on the outer hair cells (OHCs) freshly dissociated from guinea-pig cochlea were investigated with the whole-cell patch-clamp technique, in both the conventional and the nystatin perforated patch-clamp configurations under voltage-clamp condition. Application of caffeine (> 1 mM for 10-30 s) induced an inward current (Icaffeine) with decrease of conductance in a dose-dependent manner at a holding potential (VH) of -60 mV. The reversal potential of Icaffeine (Ecaffeine) was close to the K+ equilibrium potential. The Icaffeine was not affected by Ca(2+)-free external solution. The internal perfusion of the Ca2+ chelator BAPTA had no effect on Icaffeine. The Icaffeine was not modulated by the external application of H-8 or staurosporine and by the internal perfusion of GDP-beta S. The amplitude of Icaffeine was the largest at the basal region of OHCs when caffeine was locally applied by the 'puffer' method. These results suggest that caffeine induces a decrease in membrane potassium conductance of the OHCs mainly at the basal region without mediating the intracellular signaling pathway.

Slow oscillations of free intracellular calcium ion concentration in human fibroblasts responding to mechanical stretch

Calcium transients in single, human gingival fibroblasts were studied after mechanical stretching of flexible culture substrates. A model system was developed to reproducibly stretch and rapidly (<1 sec) refocus cells in the same focal plane so that changes in the concentration of free intracellular calcium ions ([Ca2+]i) were monitored without delay. Attached cells were grown on flexible bottom Petriperm dishes, loaded with fura-2/AM, and stretched by 1% or 2.8% of substrate area. The stretch caused no significant cell detachment or membrane lesions. A 1% stretch induce no calcium response, but a 2.8% stretch stimulated an initial calcium transient and the subsequent generation of [Ca2+]i oscillations of up to 2,000 sec. At 1% stretch, there was no calcium response. Cell shape and plating time were important determinants in the calcium response to mechanical stimulation: the responder cells were small and round without long processes. Major calcium transients were inhibited completely by 5 mM EGTA or by 10 microM gadolinium ions, by 50 microM nifedipine, or 250 microM verapamil, suggesting an influx of calcium through stretch-activated (SA) channels and L-type calcium channels. Depolarization by high KCl (144 mM) in the extracellular medium enhanced the amplitude of calcium transients by 54%. Calcium oscillations were not inhibited by preincubation with thapsigargin, caffeine, cholera toxin, staurosporine or 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), indicating that IP3 sensitive pools, IP3 insensitive pools, GS alpha subunits, and protein kinase C, respectively, were not involved in the generation of calcium oscillations. Pretreatment with genistein, a specific tyrosine kinase inhibitor or cytochalasin D, an inhibitor of actin polymerization, or pertussis toxin, an inhibitor of Gi alpha and G(o) alpha subunits, completely abolished calcium transients and oscillations. These results indicate that Ca2+ flux due to mechanical stretching is likely mediated through SA ion channels and is dependent on tyrosine kinases, pertussis toxin-sensitive subunits of G-proteins, and actin filaments.

FCCP modulation of Ca2+ oscillation in rat megakaryocytes

The effects of a mitochondrial uncoupler, FCCP, (carbonyl cyanide-p-trifluromethoxyphenyl-hydrazone) on the regulation of cytoplasmic Ca2+ were investigated in ATP-induced Ca2+ oscillation system of rat megakaryocyte. Application of FCCP did not induce any detectable Ca(2+)-activated K+ current (IKCa) but pretreatment with FCCP modulated the ATP-induced repetitive IKCa. FCCP abolished the IKCa induced by low concentrations of ATP. However, when the concentration of ATP was high, the uncoupler also changed the periodic current to a sustained one. Similar biphasic regulation by the uncoupler was observed in the case of IP3-evoked repetitive IKCa. These results indicate that FCCP inhibits both Ca2+ mobilization and elimination processes after IP3 liberation induced by agonist stimulation.

Biological actions of purines on rat megakaryocytes: potentiation by adenine of the purinoceptor-operated cytoplasmic Ca2+ oscillation

We have found that adenine enhanced the purinoceptor-operated cytoplasmic Ca2+ oscillation in rat megakaryocytes at submillimolar concentrations. Guanine and other nucleic acid bases had no effect on this system. Adenine enhanced the reaction intensity but had no effect on the threshold concentration of ATP to evoke the oscillation.

Ca2+ spike initiation from sensitized inositol 1,4,5-trisphosphate-sensitive Ca2+ stores in megakaryocytes

Ca(2+)-mediated Ca2+ spikes were analysed in fura-2-loaded megakaryocytes. Direct Ca2+ loading using whole-cell dialysis induced an all-or-none Ca2+ spike on top of a tonic increase in cellular Ca2+ concentration ([Ca2+]i) with a latency of 3-7 s. The latency decreased with increasingly higher concentrations of Ca2+ in the dialysing solution. Spike size and its initiation did not correlate with the tonic level of [Ca2+]i. Thapsigargin completely abolished the Ca(2+)-induced spike initiation, suggesting that Ca2+ spikes originate from thapsigargin-sensitive Ca2+ pools. An inhibitor of phosphatidylinositide-specific phospholipase C (PLC), 2-nitro-4-carboxyphenyl-N,N-diphenyl-carbamate prolonged the latency without changes of spike size in most cases (6/9 cells), but abolished the spike initiation in the other cells (3/9). The results suggest that an increase in [Ca2+]i charges up the inositol-1,4,5-trisphosphate-(InsP3)- and thapsigargin-sensitive Ca2+ pools which progressively sensitize to low or slightly elevated levels of InsP3 by the action of Ca(2+)-dependent PLC until a critical Ca2+ content is reached, and then the Ca2+ spike is triggered. Thus, the limiting step of Ca2+ spike triggering is the initial filling process and the level of InsP3 in megakaryocytes.

Hormone-mediated Ca2+ transients in isolated renal cortical thick ascending limb cells

Peptide hormones control salt reabsorption in cortical thick ascending limb (cTAL) cells of the loop of Henle. These agonists act, in part, through alterations on intracellular Ca2+ ([Ca2+]i). Primary cell cultures were prepared from porcine kidneys using a double antibody technique (goat antihuman Tamm-Horsfall and rabbit antigoat IgG antibodies). [Ca2+]i was determined in single cells with fluorescent techniques using fura-2. Parathyroid hormone (PTH) and arginine vasopressin (AVP) transiently increased [Ca2+]i in a dose-dependent manner. [Ca2+]i maximally increased from 85 +/- 5 nmol/l to 608 +/- 99 nmol/l with PTH, 10(-6) M, and to 766 +/- 162 nmol/l with AVP, 10(-7) M. The increment in [Ca2+]i by both hormones was by intracellular Ca2+ release and entry through plasma membrane Ca2+ channels. 8-Bromo-adenosine-3',5'-cyclic monophosphate (8-BrcAMP), 10(-4) M, increased [Ca2+]i (basal 83 +/- 3 to 427 +/- 121 nmol/l) but only from internal sources as nifedipine (10 mumol), ([Ca2+]i changes: 86 +/- 4 to 390 +/- 29 nmol/l) and removal of bath Ca/+o, ([Ca2+]i changes: 84 +/- 6 to 517 +/- 142 nmol/l), were without effect on agonist-induced [Ca2+]i. Thapsigargin, 1.5 mumol, completely abolished the AVP- and cyclic adenosine monophosphate-(cAMP)-induced Ca2+ transients, and partially inhibited PTH-mediated Ca2+ transients by about 50%. Pretreatment with 8-BrcAMP inhibited the PTH and AVP responses likely through depletion of cAMP-sensitive Ca2+ stores. Activation of protein kinase C (PKC) with phorbol esters inhibited PTH and AVP responses and 8-BrcAMP-induced [Ca2+]i transients.(ABSTRACT TRUNCATED AT 250 WORDS)

Spatial and temporal signalling by calcium

Recent research has shown the importance of the spatial and temporal aspects of calcium signals, which depend upon regenerative properties of the inositol trisphosphate and ryanodine receptors that regulate the release of calcium from internal stores. Initiation sites have been found to spontaneously release calcium, recognized as 'hot spots' or 'sparks', and can trigger a wave that spreads through a process of calcium-induced calcium release.

Suramin and reactive blue 2 are antagonists for a newly identified purinoceptor on rat megakaryocyte

1. The effects of purinoceptor antagonists on ATP-induced oscillatory K(+)-currents in rat isolated megakaryocytes were investigated. 2. Both reactive blue-2 (RB-2), a selective antagonist of the P2Y purinoceptor, purinoceptor, at concentrations of 0.3-10 microM and suramin, a non-selective P2 purinoceptor antagonist, at 1-30 microM blocked the ATP-induced oscillation in a concentration-dependent manner. 3. RB-2 and suramin also blocked the ADP-induced K(+)-current oscillation at the same concentration range as in the case of ATP. However, both suramin and RB-2 had no effect on thrombin- and inositol 1,4,5-trisphosphate (IP3)-induced K+ current oscillation, indicating that they act as specific purinoceptor antagonists. 4. Thus, the purinoceptors on megakaryocytes show the properties of the P2 subtype according to their blockade by antagonists.

Existence of rolipram-sensitive phosphodiesterase in rat megakaryocyte

1. The effect of rolipram (ME3176) on ADP- and IP3-induced repetitive IK(Ca) in rat megakaryocyte was investigated by use of the nystatin perforated patch and conventional whole-cell patch-clamp techniques. 2. The ADP-induced IK(Ca) was depressed by treatment with rolipram in a concentration-dependent manner. The inhibition by rolipram disappeared after treatment with a cyclic nucleotide-dependent protein kinase inhibitor, H-8. The inhibition of IK(Ca) was also observed in the presence of cyclic AMP accumulating agents such as forskolin and isobutylmethylxanthine (IBMX). 3. Rolipram enhanced the inhibitory action of forskolin, suggesting that rolipram facilitates the accumulation of cyclic AMP by blocking its breakdown. Similar results was obtained with adenosine, an endogenous adenylate cyclase activator. 4. Intracellular application of inositol trisphosphate (IP3) induced repetitive IK(Ca) in megakaryocytes. The induced IK(Ca) was also inhibited by rolipram and by other cyclic AMP accumulating agents. 5. It was concluded that megakaryocytes possess rolipram-sensitive phosphodiesterase (PDE), which was not detected in platelets, but plays a distinct modulatory role in megakaryocytes for generating ADP-induced IK(Ca).