Inositol 1,4,5-trisphosphate releases Ca2+ from a Ca2+-transporting membrane vesicle fraction derived from human platelets

Calcium sequestration by human platelet acidic organelles is regulated by the actin cytoskeleton and autocrine 5-hydroxytryptamine

Human platelets regulate agonist-evoked Ca²⁺ signalling through Ca²⁺ release from and sequestration into acidic organelles. Previous studies have pharmacologically characterised the presence of a Ca²⁺-H⁺ exchanger in these organelles. This exchanger appears to regulate a secondary plateau phase in agonist-evoked cytosolic Ca²⁺ signals in fura-2-loaded human platelets. Here we demonstrate that cytochalasin D treatment removes the secondary plateau in ADP-evoked Ca²⁺ signals elicited in the absence of external Ca²⁺. This effect was reversed by pretreatment with nigericin, a K⁺/H⁺ exchanger that short-circuits the Ca²⁺-H⁺ exchanger. Using Fluo-5N- or Lysosensor Green-loaded cells, cytochalasin D was found to enhance Ca²⁺ sequestration into acidic organelles by preventing their alkalinisation. Additional experiments demonstrated that ADP-evoked alkalinisation of acidic organelles and subsequent slowing of acidic organellar Ca²⁺ sequestration was mediated by autocrine 5-HT signalling. Enhancing this 5-HT signalling using fluoxetine overcame the inhibitory effect of cytochalasin D on ADP-evoked Ca²⁺ signals, indicating that cytochalasin D interferes with 5-HT autocrine signalling. The ability of Cytochalasin D to interfere with autocrine 5-HT signalling was downstream of the 5-HT_2A receptor as secretion of [³H]-5-HT from ADP-stimulated human platelets was not reduced. These data provide the first evidence that the pH gradient across acidic organelles is dynamically regulated upon human platelet activation, and that this can play a significant role in controlling human platelet function by modulating Ca²⁺-H⁺ exchange and so [Ca²⁺]_i.

Attenuated platelet aggregation in patients with septic shock is independent from the activity state of myosin light chain phosphorylation or a reduction in Rho kinase-dependent inhibition of myosin light chain phosphatase

Background

Impaired coagulation contributes to the morbidity and mortality associated with septic shock. Whether abnormal platelet contraction adds to the bleeding tendency is unknown. Platelets contract when Ca²⁺-dependent myosin light chain kinase (MLCK) phosphorylates Ser19 of myosin light chain (MLC₂₀), promoting actin-myosin cross-bridge cycling. Contraction is opposed when myosin light chain phosphatase (MLCP) dephosphorylates MLC₂₀. It is thought that Rho kinase (ROK) inhibits MLCP by phosphorylating Thr855 of the regulatory subunit MYPT, favouring platelet contraction. This study tested the hypotheses that in septic shock, (i) platelet function is inversely correlated with illness severity and (ii) ROK-dependent MLCP inhibition and myosin light chain phosphorylation are reduced.

Methods

Blood was sampled from non-septic shock patients and patients in the first 24 h of septic shock. Platelet function was assessed using whole blood impedance aggregation induced by 1) ADP (1.6 and 6.5 μM), 2) thrombin receptor-activating protein (TRAP; 32 μM), 3) arachidonic acid (500 μM) and 4) collagen (3.2 μg/ml). Arachidonic acid-induced aggregation was measured in the presence of the ROK inhibitor Y27632. Illness severity was evaluated using sequential organ failure assessment (SOFA) and acute physiology and chronic health evaluation (APACHE) II scores. Western blot analysis of [Ser19]MLC₂₀ and [Thr855]MYPT phosphorylation quantified activation and inhibition of platelet MLC₂₀ and MLCP, respectively. Data were analysed using Spearman's rank correlation coefficient, Student's t-test and Mann-Whitney test; p < 0.05 was considered significant.

Results

Agonist-induced aggregation was attenuated in septic shock patients (n = 22 to 34; p < 0.05). Aggregation correlated inversely with SOFA and APACHE II scores (n = 34; p < 0.05). Thr855 phosphorylation of MYPT from unstimulated platelets was not decreased in patients with septic shock (n = 22 to 24). Both septic shock and ROK inhibition attenuated arachidonic acid-induced platelet aggregation independent of changes in [Ser19]MLC₂₀ and [Thr855]MYPT phosphorylation (n = 14).

Conclusions

Impairment of whole blood aggregation in patients within the first 24 h of septic shock was correlated with SOFA and APACHE II scores. Attenuated aggregation was independent of molecular evidence of diminished platelet contraction or reduced ROK inhibition of MLCP. Efforts to restore platelet function in septic shock should therefore focus on platelet adhesion and degranulation.

The role of phosphoinositide-regulated actin reorganization in chemotaxis and cell migration

Reorganization of the actin cytoskeleton is essential for cell motility and chemotaxis. Actin-binding proteins (ABPs) and membrane lipids, especially phosphoinositides PI(4,5)P2 and PI(3,4,5)P3 are involved in the regulation of this reorganization. At least 15 ABPs have been reported to interact with, or regulated by phosphoinositides (PIPs) whose synthesis is regulated by extracellular signals. Recent studies have uncovered several parallel intracellular signalling pathways that crosstalk in chemotaxing cells. Here, we review the roles of ABPs and phosphoinositides in chemotaxis and cell migration.

LINKED ARTICLES

This article is part of a themed section on Cytoskeleton, Extracellular Matrix, Cell Migration, Wound Healing and Related Topics. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2014.171.issue-24.

Inositol phosphate metabolism and platelet activation

Platelet activation by thrombin and most other agonists appears to require two second messenger systems that are both initiated by phospholipase C-catalysed cleavage of phosphatidylinositol phosphates leading to: 1. formation of inositol phosphates with a subsequent rise in intracellular calcium from intracellular stores and from outside the cell; 2. formation of diacylglycerol with subsequent activation of protein kinase C. This review examines inositol phosphate metabolism in platelets and its involvement in calcium metabolism.

Inositol 1,4,5-trisphosphate and its receptors

Activation of cells by many extracellular agonists leads to the production of inositol 1,4,5-trisphosphate (IP₃). IP₃ is a global messenger that easily diffuses in the cytosol. Its receptor (IP₃R) is a Ca(2+)-release channel located on intracellular membranes, especially the endoplasmic reticulum (ER). The IP₃R has an affinity for IP(3) in the low nanomolar range. A prime regulator of the IP₃R is the Ca(2+) ion itself. Cytosolic Ca(2+) is considered as a co-agonist of the IP₃R, as it strongly increases IP(3)R activity at concentrations up to about 300 nM. In contrast, at higher concentrations, cytosolic Ca(2+) inhibits the IP₃R. Also the luminal Ca(2+) sensitizes the IP₃R. In higher organisms three genes encode for an IP₃R and additional diversity exists as a result of alternative splicing mechanisms and the formation of homo- and heterotetramers. The various IP₃R isoforms have a similar structure and a similar function, but due to differences in their affinity for IP₃, their variable sensitivity to regulatory parameters, their differential interaction with associated proteins, and the variation in their subcellular localization, they participate differently in the formation of intracellular Ca(2+) signals and this affects therefore the physiological consequences of these signals.

Anti-thrombotic effect of milrinone is caused by inhibition of calcium release from the dense tubular system in human platelets

AIM

Milrinone, a phosphodiesterase III inhibitor, exerts positive inotropic effects which induce an increase in the intracellular calcium concentration by raising the cyclic adenosine monophosphate level in cardiac muscle. Milrinone was also reported to inhibit platelet aggregation, however, its mechanism remains unknown. Therefore, we investigated the effects of milrinone on intracellular calcium mobilization when platelets were activated.

METHODS

Washed platelets, obtained from six healthy volunteers, were preincubated with milrinone (0.9 micro M) for 1 min and then exposed to 0.015 i micro ml-1 thrombin for 5 min. The effect of milrinone on changes in the intracellular calcium level using a fluorescent dye, fura-2, was also observed. Calcium mobilizations via plasma membrane calcium channels and the dense tubular system were assessed differentially.

RESULTS

Milrinone (0.9 micro M) significantly suppressed the aggregation ratios at 5 min compared with those in controls (86+/-5%) to 75+/-8%. The increase in the intracellular calcium concentration was also significantly suppressed (controls, 915+/-293 nM vs. 405+/-240 nM) when stimulated by thrombin. Milrinone also significantly inhibited the release of calcium from the dense tubular system (controls, 284+/-111 nM vs. 158+/-51 nM). Calcium influx through the plasma membrane was suppressed by milrinone 2.4 micro M.

CONCLUSION

Milrinone (0.9 micro M) inhibited thrombin-induced platelet aggregation. This inhibitory effect was mainly mediated by suppressing calcium release from the dense tubular system.

Antiplatelet effect of marchantinquinone, isolated from Reboulia hemisphaerica, in rabbit washed platelets

Platelet activation is involved in serious pathological situations, including atherosclerosis and restenosis. It is important to find efficient antiplatelet medicines to prevent fatal thrombous formation during the course of these diseases. Marchantinquinone, a natural compound isolated from Reboulia hemisphaerica, inhibited platelet aggregation and ATP release stimulated by thrombin (0.1 units mL(-1)), platelet-activating factor (PAF; 2 ng mL(-1)), collagen (10 microg mL(-1)), arachidonic acid (100 microM), or U46619 (1 microM) in rabbit washed platelets. The IC50 values of marchantinquinone on the inhibition of platelet aggregation induced by these five agonists were 62.0 +/- 9.0, 86.0 +/- 7.8, 13.6 +/- 4.7, 20.9 +/- 3.1 and 13.4 +/- 5.3 microM, respectively. Marchantinquinone inhibited thromboxane B2 (TxB2) formation induced by thrombin, PAF or collagen. However, marchantinquinone did not inhibit TxB2 formation induced by arachidonic acid, indicating that marchantinquinone did not affect the activity of cyclooxygenase and thromboxane synthase. Marchantinquinone did inhibit the rising intracellular Ca2+ concentration stimulated by the five platelet-aggregation inducers. The formation of inositol monophosphate induced by thrombin was inhibited by marchantinquinone. Platelet cAMP and cGMP levels were unchanged by marchantinquinone. The results indicate that marchantinquinone exerts antiplatelet effects by inhibiting phosphoinositide turnover.

Mechanism of inhibition of platelet aggregation by HCL-31D

The antiplatelet effect of the pyridazinone analogue, 4, 5-dihydro-6-[4-[2-hydroxy-3-(3,4 dimethoxybenzylamino)propoxy]naphth-1-yl]-3(2H)-pyridazinone (HCL-31D), was investigated in vitro with rabbit platelets. HCL-31D dose-dependently inhibited the platelet aggregation and ATP release induced by collagen (10 microg/ml), arachidonic acid (100 microM) or thrombin (0.1 U/ml) with an IC(50) of about 0.95-5.41 microM. HCL-31D (0.5-5 microM) increased the platelet cyclic AMP level in a dose-dependent manner. Furthermore, HCL-31D potentiated cyclic AMP formation caused by prostaglandin E(1) but not that caused by 3-isobutyl-1-methylxanthine (IBMX). HCL-31D also attenuated phosphoinositide breakdown and intracellular Ca(2+) elevation induced by collagen, arachidonic acid or thrombin. HCL-31D inhibited the formation of thromboxane B(2) induced by collagen or thrombin but not by arachidonic acid. In addition, HCL-31D did not affect platelet cylooxygenase and thromboxane synthase activity. These data indicate that HCL-31D is an inhibitor of phosphodiesterase and that its antiplatelet effect is mainly mediated by elevation of cyclic AMP levels.

Human Platelets Contain SNARE Proteins and a Sec1p Homologue That Interacts With Syntaxin 4 and Is Phosphorylated After Thrombin Activation: Implications for Platelet Secretion

In response to thrombin and other extracellular activators, platelets secrete molecules from large intracellular vesicles (granules) to initiate thrombosis. Little is known about the molecular machinery responsible for vesicle docking and secretion in platelets and the linkage of that machinery to cell activation. We found that platelet membranes contain a full complement of interacting proteins—VAMP, SNAP-25, and syntaxin 4—that are necessary for vesicle docking and fusion with the plasma membrane. Platelets also contain an uncharacterized homologue of the Sec1p family that appears to regulate vesicle docking through its binding with a cognate syntaxin. This platelet Sec1 protein (PSP) bound to syntaxin 4 and thereby excluded the binding of SNAP-25 with syntaxin 4, an interaction critical to vesicle docking. As predicted by its sequence, PSP was detected predominantly in the platelet cytosol and was phosphorylated in vitro by protein kinase C (PKC), a secretion-linked kinase, incorporating 0.87 ± 0.11 mol of PO4 per mole of protein. PSP was also specifically phosphorylated in permeabilized platelets after cellular stimulation by phorbol esters or thrombin and this phosphorylation was blocked by the PKC inhibitor Ro-31-8220. Phosphorylation by PKC in vitro inhibited PSP from binding to syntaxin 4. Taken together, these studies indicate that platelets, like neurons and other cells capable of regulated secretion, contain a unique complement of interacting vesicle docking proteins and PSP, a putative regulator of vesicle docking. The PKC-dependent phosphorylation of PSP in activated platelets and its inhibitory effects on syntaxin 4 binding provide a novel functional link that may be important in coupling the processes of cell activation, intracellular signaling, and secretion.

Human Platelets Contain SNARE Proteins and a Sec1p Homologue That Interacts With Syntaxin 4 and Is Phosphorylated After Thrombin Activation: Implications for Platelet Secretion

In response to thrombin and other extracellular activators, platelets secrete molecules from large intracellular vesicles (granules) to initiate thrombosis. Little is known about the molecular machinery responsible for vesicle docking and secretion in platelets and the linkage of that machinery to cell activation. We found that platelet membranes contain a full complement of interacting proteins—VAMP, SNAP-25, and syntaxin 4—that are necessary for vesicle docking and fusion with the plasma membrane. Platelets also contain an uncharacterized homologue of the Sec1p family that appears to regulate vesicle docking through its binding with a cognate syntaxin. This platelet Sec1 protein (PSP) bound to syntaxin 4 and thereby excluded the binding of SNAP-25 with syntaxin 4, an interaction critical to vesicle docking. As predicted by its sequence, PSP was detected predominantly in the platelet cytosol and was phosphorylated in vitro by protein kinase C (PKC), a secretion-linked kinase, incorporating 0.87 ± 0.11 mol of PO4 per mole of protein. PSP was also specifically phosphorylated in permeabilized platelets after cellular stimulation by phorbol esters or thrombin and this phosphorylation was blocked by the PKC inhibitor Ro-31-8220. Phosphorylation by PKC in vitro inhibited PSP from binding to syntaxin 4. Taken together, these studies indicate that platelets, like neurons and other cells capable of regulated secretion, contain a unique complement of interacting vesicle docking proteins and PSP, a putative regulator of vesicle docking. The PKC-dependent phosphorylation of PSP in activated platelets and its inhibitory effects on syntaxin 4 binding provide a novel functional link that may be important in coupling the processes of cell activation, intracellular signaling, and secretion.

The internal calcium concentration of human platelets increases during chilling

Human platelets must be stored at 22 degreesC in blood banks, because of the well-known phenomenon of cold-induced activation. When human platelets are chilled below room temperature, they undergo shape change and vesicle secretion that resembles physiological agonist-mediated activation. The trigger for the cascade of events leading to platelet activation at hypothermic temperatures is not known, although an increase in the internal calcium concentration ([Ca]i) due to passage of the platelet membranes through their thermotropic phase transition has been proposed. We report here that the fluorescent calcium-sensitive probe, Indo-1, has been used to estimate the internal calcium concentration of human platelets during a reduction in temperature from 20 degreesC to 5 degreesC at a rate of 0.5 degreesC/min. An increase on the order of 100 nM was recorded. Almost all of the increase in [Ca2+]i occurs during the chilling process, as incubation of platelets for 1 h at low temperature did not lead to a continued calcium concentration increase. The increase in [Ca2+]i during chilling is likely to be due to more than a single mechanism, but might include some release of the calcium stores from the dense tubule system. Loading platelets with the calcium chelator BAPTA (1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) dramatically reduced the increase in [Ca2+]i seen during chilling. Antifreeze glycoproteins (AFGPs) isolated from the blood serum of Antarctic fishes, which are known to protect platelets from cold-induced activation, did not eliminate the rise in [Ca2+]i during chilling, suggesting that signaling mechanisms are likely to be involved in cold-induced activation.

Effect of WEB 2086-BS, an antagonist of platelet-activating factor receptors, on retinal vascularity in diabetic rats

Specific antagonists of platelet-activating factor (PAF) receptors inhibit platelet aggregation and thromboxane synthesis. These two processes have been implicated in the course of diabetic retinopathy. We assessed the effect of a specific PAF receptor antagonist, WEB 2086-BS (3-(4-(2-chlorophenyl)-9-methyl-6H-thieno(3,2-f) (1,2,4 triazolo-(4,3-a(1,4)-diazepine-2-yl)-1-(4-morpholinyl)-1-propanone) on retinal vascularity in a model of experimental streptozocin-induced diabetes in rats. Rats were divided into five experimental groups (10 animals/group): group I, non-diabetic group II, untreated diabetic group III, diabetic given 1 mg/kg per day of WEB 2086-BS (p.o.) group IV, diabetic given 5 mg/kg per day (p.o.) and group V, diabetic given 10 mg/kg per day (p.o.). After 3-month treatment, platelet aggregometry, platelet synthesis of thromboxane B2, aortic production of 6-keto-prostaglandin F1alpha, platelet and vascular lipid peroxidation, and percentage of the retinal area occupied by horseradish peroxidase-labeled vessels were measured. Untreated diabetic rats showed an increase in platelet reactivity, reduced 6-keto-prostaglandin F1alpha production, increased thromboxane B2 and lipid peroxides, and a decrease in the percentage of retinal area occupied by horseradish peroxidase-labeled vessels. WEB 2086-BS produced a decrease in platelet aggregation induced by collagen in whole blood, in thromboxane B2 synthesis and lipid peroxide production, and an increase in the percentage of retinal area occupied by horseradish peroxidase-labeled vessels (13.9+/-1.1% in group II and 9.9+/-0.8% in group V). There was a statistically significant linear correlation (Y= -0.72 + 137X, r2 = 0.7247, P < 0.0007) between thromboxane B2 values and the percentages of retinal area occupied by horseradish peroxidase-labeled vessels in the groups of animals treated with WEB 2086-BS.

Platelet prostanoid receptors

Prostaglandins (PGs) and thromboxanes are important modulators of platelet activation, and there is strong evidence to support the existence of distinct thromboxane, prostacyclin, PGD2 and PGE2 receptors on the platelet plasma membrane. In this review, each of these platelet prostanoid receptors is discussed in detail, with respect to their receptor pharmacology, molecular biology and signal transduction, and as to any therapeutic implications of the development of specific agonists and/or antagonists. In addition, it considers the possibility that there are separate vascular receptors for 8-epi PGF2 alpha, which are not present on the platelet.

The inhibitory effect of 2-thienyl 2'-hydroxyphenyl ketone (C85) on platelet thromboxane formation

A new synthetic compound, 2-thienyl 2'-hydroxyphenyl ketone (C85), was demonstrated as an antiplatelet agent. In rabbit washed platelets, C85 dose-dependently inhibited arachidonic acid(AA), collagen and platelet activating factor(PAF)-induced platelet aggregation and ATP release with IC50 values of 0.6 +/- 0.2, 20.5 +/- 8.3 and 145.6 +/- 28.6 microM respectively. In human platelet rich plasma(PRP), C85 selectively inhibited the second phase of platelet aggregation and ATP release induced by epinephrine. The formation of platelet thromboxane B2 (TXB2) caused by AA, collagen and thrombin was completely inhibited by C85(10 microM). C85 could significantly reduce cyclooxygenase activity as reflected by attenuation of prostaglandin E2(PGE2) formation. C85 also possess weakly inhibitory effect on thromboxane synthase as reflected by slightly inhibition of prostaglandin H2 (PGH2)-induced TXB2 formation. In Fura-2/AM loaded platelets, the rise of intracellular calcium level challenged by AA, collagen and thrombin were inhibited by C85. The C85(10 microM) also significantly suppressed the phosphoinositide breakdown induced by AA and collagen. In vivo, C85(50 micrograms/Kg i.p.) produced a marked prolongation of tail bleeding time than that treated by indomethacin in mice. In summary, the antiplatelet mechanism of C85 is mainly inhibition of platelet cyclooxygenase activity and partly inhibition of thromboxane synthase activity and lead to diminution of TXA2 formation.

Abnormal incorporation of arachidonic acid into platelets of drug-free patients with schizophrenia

Incorporation of [3H]arachidonic acid (AA) into resting platelets was studied in samples from schizophrenic patients before and after haloperidol withdrawal, and from normal subjects. Eicosanoid biosynthesis was subsequently evaluated in prelabeled platelets by sequential events of thrombin activation. The total incorporation of [3H]AA in drug-free patients was significantly lower than in the same individuals during haloperidol treatment as well as in normal volunteers. No significant difference of [3H]AA incorporation was demonstrated between relapsed and nonrelapsed drug-free patients. The majority of 3H-labeled lipids were found in platelet phospholipids, and < 10% of incorporated lipids were found in free AA, diacylglycerol (DAG), triacylglycerol, and hydroxyeicosatetraenoic acid (HETE) of normal resting platelets. After thrombin activation, however, there was an increased 3H-labeling in 12-HETE, 12-hydroxyheptadecatrienoic acid, and thromboxane B2. The thrombin-induced formation of eicosanoids was found to be significantly higher in haloperidol-treated patients than in normal volunteers. This increased formation of eicosanoids appeared to be normalized after haloperidol withdrawal. In addition, both haloperidol-treated and drug-free patients showed increased 3H-labeling in thrombin-induced DAG compared with normal volunteers. Such an increase in the second messenger formation may be due, at least in part, to an increased turnover of membrane phosphoinositides via phospholipase C reaction. The present data support our previous findings demonstrating altered membrane dynamics in schizophrenia.

Platelet activation and interactions with the microvasculature

Platelet adherence to structurally or functionally damaged endothelium and its subsequent activation are multifaceted events. Regulation of the rate and extent of platelet adhesion is under local control by the platelets and endothelium. Even in the absence of platelet adhesion to endothelium or subendothelium, there is a complex hemostatic balance of coagulation and anticoagulation. This is mediated by the manufacture, release, and inactivation of various procoagulant and anticoagulant compounds, predominantly by the platelets and endothelium. The relationship between the two in maintaining the homeostasis of coagulation and other processes is complex. This review focuses on the structure, function, and interaction of endothelium, subendothelium, and platelets and on their vasoactive and pro-/anticoagulant functions.

Disorders of platelet function

Qualitative platelet disorders are described and reviewed above. The acquired platelet function defects are very common, and sometimes result in hemorrhage, especially in association with trauma or surgery. However, the specific biochemical defect is absent, and no characterized platelet abnormalities have been recognized. On the other hand, the hereditary qualitative platelet defects are rare, but the platelet abnormalities are characteristic. The study of these patients had led to an increased understanding of the normal primary hemostatic mechanism. Recently, the molecular basis analysis of the platelet defects has been developed. This will help us understand the molecular events involved in platelet adhesion and aggregation.

Antiplatelet actions of 2-(4-[1-(2-chlorophenyl) piperazinyl]) methyl-2,3-dihydroimidazo[1,2-c]quinazolin-5(6H)-one compound

The new quinazolinone derivative, 2-(4-[1-(2-chlorophenyl)piperazinyl]) methyl-2,3-dihydroimidazo[1,2-c]quinazolin-5(6H)-one(CK53 ), inhibited platelet aggregation and ATP release induced by arachidonic acid, collagen, PAF and U46619 in washed rabbit platelets. In human platelet-rich plasma CK53 also significantly suppressed the platelet aggregation and ATP release challenged by epinephrine and ADP. The thromboxane B2 formation of rabbit washed platelets caused by collagen and thrombin was reduced by CK53 but that induced by arachidonic acid. CK53 inhibited the intracellular calcium increase stimulated by collagen and thrombin in quin-2/AM-loaded rabbit platelets. Phosphoinositides breakdown caused by collagen, U46619, PAF and thrombin was inhibited by CK53. CK53 also suppressed the aggregation of elastase-treated human platelets induced by fibrinogen but no alteration in platelet cyclic-AMP level. In conclusion, these data indicate that antiplatelet effect of CK53 may be mainly due to the direct inhibition of phosphoinositides breakdown.

Regulation of inositol 1,4,5-trisphosphate-induced calcium release by inositol 1,4,5-trisphosphate and calcium in human platelets

Inositol 1,4,5-trisphosphate (IP3)-induced release of Ca2+, its regulation by IP3 and Ca2+, and the metabolism of IP3 was analyzed in saponin-permeabilized human platelets. At 37 degrees C successive sub-maximal aliquots of IP3 induce a biphasic release of part of the IP3-sensitive Ca2+ pool each time. Although the IP3-induced Ca2+ flux rapidly ceases after addition of IP3, the IP3-sensitive Ca2+ stores regain their sensitivity shortly after stimulation with IP3. Under these conditions IP3 is rapidly hydrolyzed to inositol 1,4-bisphosphate which is unable to release Ca2+. The cooperativity of IP3-induced Ca2+ release and the metabolism of IP3 can account for both the rapid recovery of sensitivity of the IP3-sensitive Ca2+ pool and the partial release induced by sub-maximal concentrations of IP3. The data of the present study favour a steady state rather than an all-or-none mechanism of Ca2+ release.

Distribution of plasma membrane Ca(2+)-ATPase and inositol 1,4,5-trisphosphate receptor in human platelet membranes

Human platelet plasma membranes were prepared by the glycerol lysis method of Harmon et al. [Harmon JT. Greco NJ. Jamieson GA. (1992) Isolation of human platelet plasma membranes by glycerol lysis. Meth. Enzymol., 215, 32-36]. The membranes were observed to contain a Ca(2+)-ATPase with different properties than those of internal membranes. The specific activity of Ca(2+)-ATPase was lower in plasma membranes (10-40 nmol ATP hydrolyzed/min/mg), but the ATPase was less sensitive to thapsigargin (41% inhibition at 500 nM) and more sensitive to vanadate (50% inhibition at 4 microM) than the Ca(2+)-ATPase in internal platelet membranes. The plasma membranes contained a Ca(2+)-ATPase detectable by monoclonal and polyclonal antibodies against erythrocyte Ca(2+)-ATPase that had a molecular mass of 144 kD. However, an anti-peptide antibody against an N-terminal sequence of the inositol 1,4,5-trisphosphate receptor recognized this protein in internal membranes, but not plasma membranes.

Calpain-induced down-regulation of protein kinase C inhibits dense-granule secretion in human platelets. Inhibition of platelet aggregation or calpain activity preserves protein kinase C and restores full secretion

The relationship between platelet aggregation, calpain activation, PKC activities and the secretory response have been examined in PMA-and ionomycin-stimulated platelets. Co-addition of PMA and ionomycin resulted in a maximal synergistic secretion of [14C]5-hydroxytryptamine ([14C]5-HT) from platelet dense granules. However, prior addition of PMA for 5 or 10 min resulted in a reduction of this secretory response. Inclusion of either RGDS (to inhibit platelet aggregation) or E64-d (to inhibit calpain activity) resulted in full restoration of the secretory response. In experiments to determine the activity status of PKC, PMA was found to induce a loss in cytosolic and total PKC activity without an increase in membrane-associated activities during this time period. Inhibition of either platelet aggregation or calpain activity resulted in preservation of total and cytosolic activities with a measurable increase in membrane translocated activity. PMA-induced phosphorylation of a number of PKC substrates was measured in 32P-labelled platelets. PMA induced potent phosphorylation of the 45 and 20 kDa species and also proteins of the molecular masses 66, 80, 97 and 119 kDa. Phosphorylation was maximal at either 1 or 2 min after which dephosphorylation occurred. Inclusion of either RGDS or E64-d resulted in a reduction of the dephosphorylation rates, and sustained phosphorylation of the 66, 80, 97 and 119 kDa proteins. These studies suggest that the activity status of PKC is an important factor in the level of secretion obtained and that platelet aggregation is involved in calpain-initiated down-regulation of PKC.

Platelet aggregation and dense granule secretion in schizophrenia

The functional state of platelets and their possible impairment in response to various stimuli were assessed in saline-diluted citrated blood samples of normal male control subjects (n = 27), and in schizophrenic patients with (n = 34) and without (n = 23) haloperidol treatment. In response to collagen, but not to arachidonic acid (AA) and adenosine diphosphate, platelet aggregation (as measured by changes in impedance) was significantly higher in both haloperidol-treated and drug-free schizophrenic patients than in normal control subjects. Comparison of the secretion traces, however, indicated that only AA-induced adenosine triphosphate (ATP) release was significantly lower in haloperidol-treated schizophrenic patients than in normal control subjects. In response to thrombin, collagen, and AA, the mean values of ATP release from drug-free patients were significantly higher than those from the same individuals when they were receiving haloperidol. Furthermore, there was a trend toward increased ATP release (in response to thrombin or collagen) in the nonrelapsed group of drug-free schizophrenic patients as compared with the relapsed group. The collagen-induced platelet aggregation or dense granule secretion in drug-free patients was correlated significantly and negatively with psychosis ratings. Such changes in platelet function of schizophrenic patients were not correlated significantly with daily haloperidol dose, plasma haloperidol levels, age of subjects, age of onset, or duration of illness.

A haemorrhagic platelet disorder associated with altered stimulus-response coupling and abnormal membrane phospholipid composition

Haemorrhagic diatheses due to platelet function defects are a heterogenous and poorly understood group of conditions. We report the investigation of a female with a lifelong history of epistaxes, haemarthroses, menorrhagia and persistent iron-deficiency anaemia. Although platelet numbers and morphology were normal, platelet function was abnormal both in vivo and in vitro. Skin bleeding time was prolonged and aggregation thresholds in platelet-rich plasma to a variety of weak and strong agonists were increased. Platelet granule contents were normal and membrane glycoproteins GpIb and GpIIIa were present in normal amounts. Polyphosphoinositide metabolism and phosphatidic acid generation were diminished in thrombin-stimulated platelets, as was phosphorylation of the 47 kD substrate for protein kinase C and the 20 kD protein myosin light chain kinase, indicating impaired generation of the intracellular second messengers diacylglycerol and inositol trisphosphate due to diminished stimulated phospholipase C activity. Although intracellular free calcium, calmodulin activity and basal cAMP concentrations were normal, washed platelets showed increased cAMP accumulation following stimulation with prostaglandin E1 and forskolin. Platelet membrane lipid analysis revealed a reduction in plasmalogen phosphatidylethanolamine content. It is suggested that the membrane phospholipid abnormalities cause the abnormal platelet reactivity by interfering with signal transduction from platelet receptor, via intermediary G proteins, to phospholipase C and adenylate cylase. The bleeding tendency is likely to be a consequence of the altered stimulus-response coupling.

Platelet intracellular calcium in patients with recurrent affective disorders

Forty-four subjects with a history of a major recurrent affective disorder in remission and who were either on no medication or taking a single dose of psychotropic medication were conscripted together with matched controls. The fluorescent indicator fura 2 was used to measure intracellular calcium in platelets and estimations were made of total serum and ionised calcium as well as of whole blood serotonin. Intracellular calcium was measured in the resting state as well as after stimulation with thrombin, platelet activating factor and serotonin. No significant differences were found between the 17 subjects with a diagnosis of bipolar disorder or the 27 subjects with recurrent unipolar depression and their matched controls. Intracellular calcium measures were significantly higher in the lithium treated group after stimulation with 5HT, whereas the subjects taking tricyclic antidepressants did not differ significantly from their controls on any measure. Serum calcium was found to be significantly higher in those subjects taking lithium. These findings suggest that the measurement of intracellular calcium is not a useful trait marker in affective disorders. Lithium appears to enhance the 5HT induced rise of intracellular calcium.

Calcium signalling in platelets and other nonexcitable cells

By virtue of their biological simplicity and widespread availability, platelets frequently have been used as a model system to study signal transduction. Such studies have revealed that changes in intracellular free calcium concentration are central to platelet functioning. The following article reviews current concepts of platelet structure and function, with particular emphasis on the mechanisms involved in platelet Ca2+ signalling.

Intracellular calcium mobilization is triggered by clustering of membrane glycoproteins in concanavalin A-stimulated platelets

Stimulation of human platelets with concanavalin A resulted in a significant increase in the concentration of cytoplasmic free Ca2+. This effect was due to two different processes: Ca2+ mobilization from internal stores and Ca2+ influx from the extracellular medium. Kinetic analysis revealed that the release of Ca2+ from internal storage sites occurred sooner than the opening of plasma membrane Ca2+ channels. The ability of concanavalin A to induce a sustained increase in cytoplasmic Ca2+ concentration was antagonized and reversed by methyl alpha-D-mannopyranoside, demonstrating that it was promoted by the interaction of the lectin with cell surface glycoproteins. Succinyl-concanavalin A, a dimeric derivative of the lectin, that does not promote patching/capping of the receptor, was able to bind to the platelet surface, and antagonized the effects of native concanavalin A. In addition, succinyl-concanavalin A, per se, was unable to induce Ca2+ mobilization in human platelets. Therefore, the action of the native concanavalin A was mediated by receptor clustering events. Concanavalin A mobilized Ca2+ from the same internal stores from which Ca2+ was mobilized in response to strong platelet agonists, such as thrombin and arachidonic acid. However, while thrombin was ineffective in inducing Ca2+ release after stimulation of platelets with ConA, ConA was able to cause a full discharge of Ca2+ from internal stores even in platelets previously stimulated with thrombin. These results demonstrate for the first time that the clustering of specific membrane glycoproteins can trigger platelet activation. The physiological implications during platelet aggregation are discussed.

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.

Defective signal transduction in platelets from cirrhotics is associated with increased cyclic nucleotides

BACKGROUND

Patients with advanced cirrhosis show defective platelet aggregation, which is dependent, at least in part, on intrinsic platelet abnormalities. The aim of this study was to evaluate the activating and inhibitory pathways of platelet signal transduction in cirrhotic patients.

METHODS

Twelve cirrhotic patients and 12 control subjects participated in this study. Measurements were performed on washed platelets.

RESULTS

Thrombin-stimulated inositol 1,4,5-trisphosphate production was reduced fivefold, and the increase in cytosolic calcium concentration was significantly lower in platelets from cirrhotic patients following stimulation with thrombin, platelet activating factor, or U-46619. In addition, the activity of the platelet Na+/H+ antiporter, evaluated after an acid load, was significantly lower in platelets from cirrhotic patients (0.90 +/- 0.19 vs. 1.37 +/- 0.16 delta pHi/min, P = 0.07). Cirrhotic patients also showed a significantly increased basal intraplatelet content of both 5'-cyclic adenosine monophosphate (cAMP) (2724 +/- 330 vs. 1561 +/- 258 fmol/10(8) platelets, P < 0.05) and 5'-cyclic guanosine monophosphate (cGMP) (217 +/- 18 vs. 159 +/- 29 fmol/10(8) platelets, P < 0.05).

CONCLUSIONS

Our results indicate that in platelets from cirrhotic patients, defective early signal transduction is associated with an increase in platelet cAMP and cGMP, thus revealing new mechanisms contributing to the defective platelet function in this disease.

Chemotactic stimulation of aggregation-stage Dictyostelium cells induces rapid changes in energy metabolism, as measured by succinic thiokinase phosphorylation

In crude mitochondrial fractions of the cellular slime mold Dictyostelium discoideum, a 38-kDa protein can be detected in phosphorylation assays under autophosphorylation conditions in SDS polyacrylamide gels. p38 can be phosphorylated in vitro using either ATP or GTP as phosphoryl donors. After stimulation of aggregation competent cells with the chemoattractant cAMP, p38 phosphorylation pattern changes rapidly. Caffeine, a known inhibitor of cAMP relay in D. discoideum inhibits cAMP induced changes in p38 phosphorylation. The rapid changes in p38 phosphorylation after cAMP stimulation reflect changes in energy metabolism and these changes are most likely mediated by changes in internal calcium concentrations. The mitochondrial localization and other data presented on the characterization of this protein led us to the conclusion that p38 is the alpha subunit of succinic thiokinase. Data showing a correlation between in-vitro p38 phosphorylation and the metabolic state of the cells at the moment of the cell lysis are included.

Carrageenan-induced activation of human platelets is dependent on the phospholipase C pathway

Stimulation of washed human platelets by the pro-inflammatory polysaccharide carrageenan is accompanied by shape change, aggregation and release of granule contents and unaccompanied by thromboxane A2 synthesis. Carrageenan triggers platelet activation through a prostaglandin synthetase-independent mechanism. The phospholipase A2 (PLA2) inhibitor, p-bromophenacyl bromide suppresses platelet responses to carrageenan (Vargaftig et al, 1980) probably by mechanism(s) other than those which involve PLA2 activity. Exposure of platelets to carrageenan (2-25 micrograms/ml) induced inositol phosphate formation in a time- and concentration-dependent manner, the level of inositol phosphate formation correlating with the intensity of aggregation. Neomycin, an aminoglycoside antibiotic which inhibits the phospholipase C-mediated phosphatidylinositol 4,5-bisphosphate breakdown, suppressed both platelet activation and inositol phosphate formation. Inhibition was concentration-dependent with an IC50 value of about 180 microM. Platelet-activating factor (PAF) is not responsible for carrageenan-induced platelet activation and inositol phosphate formation, since exposure of platelets to carrageenan (25 micrograms/ml) in the presence of compound WEB 2086 (100 microM), a PAF antagonist, failed to inhibit carrageenan responses. However, compound Ro 19-3704, a structurally related antagonist of PAF reported to be also an inhibitor of phospholipases A2 and C, inhibited concentration-dependently (0.1-10 microM) aggregation and ATP release induced by carrageenan (25 micrograms/ml). These findings indicate that carrageenan activates human platelets through a phospholipase C-dependent mechanism and show that neomycin, at low concentrations, can be a selective inhibitor of phospholipase C-mediated PIP2-breakdown.

Effects of nitric oxide-containing compounds on increases in cytosolic ionized Ca2+ and on aggregation of human platelets

The present study was undertaken to determine the modulatory effects of nitric oxide (NO)-releasing compounds on increases in cytosolic ionized calcium ([Ca2+]i) and on aggregation of gel-filtered human platelets induced via diverse agonists. We used various sydnonimines and organic nitrates as donors of NO. Gel-filtered and fura-2-loaded platelets were stimulated with ADP (4-8 microM), collagen (2-10 micrograms/ml) or thrombin (0.02-0.05 IU/ml), respectively. Half-maximal inhibiting effects of sydnonimines on agonist-evoked increases in [Ca2+]i were observed between 30 and 1000 nM, while half-maximal inhibiting effects of the compounds on aggregation were between 3 and 500 nM. The compound C 87-3754, which is the bioactive metabolite of pirsidomine, was a much stronger inhibitor of increases in [Ca2+]i than of platelet aggregation. This was due to an enhanced NO release from this compound exposed to ultraviolet light during Ca2+ measurement. The organic nitrates isosorbide 5-mono-nitrate and nicorandil inhibited both aggregation and increase of cytosolic ionized calcium in stimulated platelets at half-maximal concentrations of approximately 200 microM. The present results suggest that some of the effects of NO on platelets are independent of cytosolic ionized calcium. The results also suggest that some of the inhibitory effects of NO-releasing compounds correspond rather to the presence of the A forms (NO-containing intermediates) than to the presence of free NO.

Comparison and characterization of retinal pericytes and retinal pigment epithelial cells on subcellular IP3-sensitive Ca2+ pools

A comparative study of inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ mobilization in bovine retinal capillary pericytes (BRCP) and bovine retinal pigment epithelial cells (BRPE) was carried out. Both cells were permeabilized with saponin. The two cell types had similar basal levels of [Ca2+]i (130 nM for BRCP, 132 nM for BRPE) and responded to IP3 in a dose-dependent manner. However, when stimulated by various concentrations of IP3 (1-10 microM), the increase in [Ca2+]i of BRCP was always two- to threefold higher than that in BRPE. Subcellular-fractionation studies showed that a single population of IP3 binding site with a high affinity and high specificity of IP3 mainly localized to plasma membrane in these two cell types. Although the dissociation constant of specific [32P]-IP3 binding sites (Kd 1.9-2.8 nM) was similar, the profile of maximal binding capacity (Bmax) of each fraction was markedly different. In comparison, plasma membrane fractions of BRCP were with Bmax of 165 fmol/mg protein versus 90 fmol/mg protein for BRPE membranes. The ATP-dependent Ca2+ uptake and IP3-dependent Ca2+ release were observed in the both plasma membrane fractions. With quantitative correlation, the membrane fraction (2 mg) of BRCP released 0.2 nmol Ca2+ whereas BRPE only released 0.07 nmol Ca2+ with the same dose of IP3 (5 microM). The selectively higher density of IP3 binding sites in coupling to the larger Ca(2+)-release in the membrane of BRCP suggests that the quantity of Ca2+ mobilized is determined by the spatially preferential distribution of membrane-associated IP3 binding sites. These findings may provide an explanation for the differences observed between BRCP and BRPE in IP3-induced DNA replication.

Effect of staurosporine on fMet-Leu-Phe-stimulated human neutrophils: dissociated release of inositol 1,4,5-trisphosphate, diacylglycerol and intracellular calcium

Staurosporine, a microbial alkaloid, enhances inositol 1,4,5-trisphosphate (IP3) and 1,2-diacylglycerol (DG) production rapidly and dose-dependently in fMet-Leu-Phe (FMLP)-stimulated human neutrophils showing maximal effects at 1 microM concentration. The IP3 increase was specific for staurosporine as three other putative protein kinase C (PKC) inhibitors, H7, sphingosine and palmitoylcarnitine were unable to enhance the IP3 generation in FMLP-stimulated human neutrophils. Staurosporine, at concentrations 0.3-1.0 microM, did not affect the initial mobilization of FMLP-induced intracellular Ca2+ (Ca2+i), although a sustained elevation of cytosolic Ca2+ level was observed within 5 min. This effect could not be suppressed, even by 1 microM phorbol-myristate 12,13-acetate (PMA). Whereas lower concentrations of staurosporine (less than or equal to 100 nM) were unable to affect FMLP-induced IP3 production, DG accumulation and Ca2+i, the PMA-inhibited initial Ca2+i signal and IP3 formation triggered by FMLP were almost completely restored. At higher concentrations (greater than or equal to 300 nM) staurosporine reversed the inhibitory effect of other protein kinases, distinct from the PMA-inducible one, which may be responsible for the phosphatidyl inositol 4,5-bisphosphate (PIP2) breakdown, thus causing accumulation of IP3 and DG and an elevation of C2+i level. Whereas IP3 declined to basal level within 5 min, the DG level remained elevated during the same period. This phenomenon is attributed to phospholipase D (PLD) stimulation by staurosporine, which augments the DG synthesis, in part through PA degradation via phosphatidic acid (PA) phosphohydrolase.

Anti-platelet action of isoliquiritigenin, an aldose reductase inhibitor in licorice

The mechanism was studied by which isoliquiritigenin, a new aldose reductase inhibitor purified from licorice (Glycyrrhizae radix), inhibits platelet aggregation. This new agent significantly inhibited the phosphorylation of 40,000- and 20,000-dalton proteins, and inhibited the formation of 12 (S)-hydroxy-5,8,10-heptadecatrienoic acid, 12-hydroxyeicosatetraenoic acid and thromboxane B2. The inhibitory effect of isoliquiritigenin on platelet aggregation in vitro was comparable to that of aspirin. Our findings may indicate that isoliquiritigenin elicits an anti-platelet action by inhibiting not only cyclooxygenase but also lipoxygenase or peroxidase activity in platelets. Isoliquiritigenin also showed an anti-platelet action in vivo. Isoliquiritigenin appears to be the only aldose reductase inhibitor with a significant anti-platelet action. Since the hyperaggregability of platelets has been implicated in the pathogenesis of diabetic complications, isoliquiritigenin may offer a unique benefit as an aldose reductase inhibitor.

Calcium mobilization in platelets from schizophrenic and healthy subjects. Regulation by lithium and neuroleptics

Intracellular free calcium concentrations ([Ca(2+)](1)) were measured in platelets from healthy volunteers before and after adding thrombin, chlorpromazine, haloperidol and/or lithium, and in platelets from DSM-III-R diagnosed schizophrenic patients receiving neuroleptic medication. Thrombin increased [Ca(2+)]( 1) in a dose- dependent fashion. Chlorpromazine and haloperidol also mobilized Ca(2+) in a dose-dependent fashion, and augmented the response to low doses of thrombin without changing the maximal response to thrombin. The effects of all three drugs were not additive, suggesting that they affected the same intraplatelet calcium pool; most likely the dense tubular system. Lithium also increased [Ca(2+) ] but without affecting the response to thrombin, chlorpromazine or haloperidol. The effects of the latter three drugs were additive to that of lithium, suggesting that lithium was acting on a different calcium pool. The response to thrombin was significantly lower in platelets from schizophrenic patients than in platelets from healthy volunteers. Further studies are required to explore potential causes for this observation. Such causes include schizophrenia per se and chronic neuroleptic treatment.

Inhibition by the PAF antagonist WEB 2086 of PAF induced inositol-1,4,5-trisphosphate production in human platelets

Platelet-activating factor (PAF) activates human platelets by binding to a putative PAF receptor which evokes the rapid formation of inositol-1,4,5-trisphosphate (IP3) by phospholipase C mediated phosphatidylinositol-4,5-bisphosphate (PIP2) hydrolysis. Stimulation of [3H]inositol-labeled human platelets by PAF (1 nM-1 microM) resulted in a concentration-dependent increase of intracellular IP3, IP2 and inositolmonophosphate (IP1). IP1 levels increased up to three-fold upon maximum stimulation by 100 nM PAF. The EC50 concentration for PAF was 1.2 +/- 0.3 nM. Addition of the hetrazepinoic PAF antagonist, WEB 2086, inhibited PAF stimulated hydrolysis of PIP2 in a dose-dependent manner. WEB 2086 (100 microM) blocked inositol-1,4,5-trisphosphate formation down to baseline levels (IC50 = 33 +/- 12 microM WEB 2086). In thrombin and ADP stimulated platelets, inositol phosphate (IP) generation was not influenced by WEB 2086. It is concluded that WEB 2086 selectively antagonizes PAF-induced increases in IP and does not interfere directly with intracellular signal transduction. Instead, WEB 2086, which has been shown to bind specifically and with high affinity (Ki 15 nM) to human platelets, acts as a competitive antagonist at the PAF receptor level.

Production of platelet-activating factor in photocoagulated retinas

Platelet-activating factor (PAF), an 1-0-alkyl-2-acetyl-sn-glycero-3-phosphocholine, plays an important role in tissue inflammation and ischemia. Previous results from our laboratory have shown that PAF is synthesized in the cornea after injury and that PAF antagonists reduced inflammation in an experimental model of anterior uveitis. This study was conducted to determine the effect of photocoagulation on PAF levels in the retina. Dutch belted pigmented rabbits underwent panretinal photocoagulation in the right eye with an argon blue-green laser. The left eye of each animal served as the control. Four hours later, the animals were killed. PAF was isolated from retinal extracts using high-performance liquid chromatography (HPLC) and was quantitated by platelet aggregation activity. In each animal the level of PAF in the photocoagulated retina was one-and-one-half to four times higher than in the control retina. The specific PAF antagonist BN52021 completely inhibited PAF activity in each sample. Due to the proinflammatory properties of PAF, its increase after laser application may be implicated in some of the clinical side effects seen after this therapy.

Compartmental analysis of 45Ca2+ efflux in perfused rat liver: effects of hormonal stimulation

The kinetics of calcium movements in the isolated perfused rat liver were examined using compartmental analysis of the efflux profiles of 45Ca2+ from 45Ca(2+)-equilibrated livers under a variety of calcium concentrations and hormonal treatments. From the 45Ca2+ efflux profiles, we determined that a three compartment model was appropriate to describe the movements of calcium in the liver on the time scale of the experiments. Hormonal treatment with the alpha-adrenergic agonist, phenylephrine, or the vasoactive peptide, vasopressin, during the efflux period lowered significantly the rate of transfer of Ca2+ between the internal compartments at all of the calcium concentrations employed. Also, phenylephrine treatment leads to increased transfer of Ca2+ into the liver from the perfusate. The temporal characteristics of the phenylephrine and vasopressin sensitive Ca2+ pools were examined by pulsing livers, loaded for variable periods of time with 45Ca2+, with the two hormones during the efflux of 45Ca2+ to measure the kinetics of Ca2+ exchange in the hormone-sensitive pools. Results from these experiments indicate that the rate of unstimulated Ca2+ efflux, k2, for the phenylephrine and vasopressin sensitive Ca2+ pools, modeled as a one compartment system, are the same, 0.074 and 0.078 min-1 for phenylephrine and vasopressin respectively, corresponding to half times for turnover of the pool(s) of 9.3 and 8.9 min, respectively.