Platelet cAMP and cGMP in essential hypertension.

Vasodilator substances released in the blood vessel wall, such as the endothelium-derived relaxing factor (EDRF) and prostacyclin (PGI2), may participate in the regulation of arterial blood pressure. However, their role in the pathogenesis of human essential hypertension to date remains unclear. For some of these factors affecting vascular smooth muscle cells, blood platelets represent a second target tissue. Thus, EDRF and PGI2 inactivate platelets by stimulation of cyclic guanosine-5'-monophosphate (cGMP) and cyclic adenosine-5'-monophosphate (cAMP) synthesis, respectively. In the present study, platelet cAMP (n = 68) and cGMP (n = 60) were determined in a control group of healthy subjects (C) and in 12 patients with untreated essential hypertension (EH). In the control group, platelet cAMP and cGMP content averaged 13.52 +/- 0.38 and 1.48 +/- 0.06 pmol/10(9) platelets and no dependence of either variable on sex or age could be established. Furthermore, cGMP levels were similar in EH as compared to the control group (1.38 +/- 0.11 pmol/10(9) platelets). However, intracellular concentrations of cAMP were significantly lower in EH as compared to C (11.22 +/- 1.37 pmol/10(9) platelets; P < .01). In addition, we investigated the stimulatory effect on cAMP of the stable PGI2 analog iloprost (10(-9), 5 x 10(-9), 10(-8), 5 x 10(-8) mol/L) in the platelets of 12 control subjects (C12) and EH.(ABSTRACT TRUNCATED AT 250 WORDS)     

Platelet cAMP and cGMP Phosphodiesterases

Platelets respond to a wide variety of exogenous agonists that bind to distinct receptors on the platelet surface resulting in the intracellular generation of second messengers or the opening of ion channels, setting off a cascade of events leading to both physical and functional changes in the platelet. The cyclic nucleotides, cyclic adenosine 3'5'-monophosphate (CAMP) and cyclic guanosine 3'5'-monophosphate (cGMP) initiate a sequence of intracellular events that modulate many of these reactions in the platelet.     

Platelet function tests in childhood. Measuring aggregation and release reaction in whole blood

Blood samples from 42 newborns, 78 infants and schoolchildren, and 81 healthy adults were tested for the parameters of primary hemostasis. Only whole blood techniques were used. Agonist-induced aggregation and release-reaction studies were performed in a whole blood lumi-aggregometer simultaneously. The release of adenosine triphosphate (ATP) was detected by the luciferin-luciferase method. The in vitro bleeding time was measured by the PFA 100 system. The results of these studies were ostensibly influenced by blood cells. Many aggregation phenomena were correlated with the platelet count. Aggregation and release reaction by collagen were inversely correlated with the hematocrit. In the PFA 100, hematocrit and leukocyte count were also inversely correlated with the closure time and the maximal blood flow velocity. Both parameters were diminished in newborns. The aggregation response to adenosine diphosphate (ADP) was similar in the three groups. The same was true for the aggregation and release reaction by arachidonic acid and for the agglutination by ristocetin. The aggregation and release reaction by collagen were diminished in the specimens from newborns. For the explanation of this transient hypofunction, only theoretical considerations exist. Beyond the postnatal period and during childhood, no remarkable differences from the adult norm were found.     

Platelet cGMP inversely correlates with age in healthy subjects

Biochemical modifications associated with the increase in platelet activity with age are not well defined. Furthermore it is well known that the nitric oxide/cyclic 3', 5'-guanosine monophosphate (cGMP) pathway regulates platelet aggregation. The aim of the present study was to examine the relationship between platelet content of cGMP and age. 120 normal subjects, evaluating the cGMP platelet concentration, 17betaE2, IGF-I, dehydroepiandrosterone sulphate (DHEAS), insulin, plasma glucose, lipid pattern, homocysteine and PAI-I antigen, were studied. The multivariate analysis in a model with cGMP as dependent variable and with age, LDL, apolipoprotein B (ApoB), DHEAS, E2 and insulin-like growth factor (IGF)-I as independent variables shows a negative correlation between cGMP and age (p<0.01, beta=-0.388). In conclusion our data suggest that the reduced efficiency of the system constitutive nitric oxide synthase (cNOS)/guanylate cyclase represents at least one of the reasons of the increased platelet aggregability with age.     

Platelet von Willebrand's antigen II: active release by aggregating agents and a marker of platelet release reaction in vivo

von Willebrand's antigen II (vW AgII), a plasma and platelet antigen immunologically and biochemically distinct from factor-VIII-related antigen (VIIIR:Ag), is decreased in von Willebrand's disease. vW AgII is released from platelets during aggregation and clotting. The release of platelet vW AgII was studied in washed platelets following aggregation by thrombin, collagen, and adenosine diphosphate (ADP). Total platelet vW AgII was 3.39 U/10(9) platelets. Thrombin and collagen yielded release of 85% and 86% of platelet vW AgII, respectively, at the highest concentrations. Release of platelet vW AgII was correlated with the release of 5-hydroxytryptamine (5HT). Release of vW AgII by collagen and thrombin was inhibited by metabolic inhibitors. In addition, collagen release of vW AgII was inhibited by aspirin. In clinical syndromes associated with intravascular platelet destruction, marked elevations of plasma vW AgII were noted. Thus, vW AgII is released by a metabolically active process from platelets during aggregation. In addition, vW AgII appears to be a marker of intravascular platelet destruction.     

Platelet adhesion and intracellular calcium levels in antigen-challenged rats

There is considerable evidence that platelet activation occurs in allergic airways diseases. In this study we aimed to investigate platelet adhesion to immobilized fibrinogen and intracellular calcium levels in a rat model of allergic inflammation. Male Wistar rats were challenged with ovalbumin (OVA). At 30 min to 24 h after OVA-challenge, assays of platelet adhesion to immobilized fibrinogen and intracellular calcium levels using fura 2-AM loaded platelets were performed. The serum levels of IgE were approximately 5-fold greater in OVA-sensitized rats. A marked eosinophil influx in bronchoalveolar lavage (BAL) fluid of OVA-challenged rats at 24 h after OVA-challenge was also seen. OVA-challenge resulted in a marked thrombocytopenia, as observed within 12 h after OVA-challenge. The agonists ADP (0.5–50 μM) and thrombin (30–100 mU/ml) concentration-dependently increased platelet adhesion to immobilized fibrinogen. At an early time after OVA-challenge (30 min), platelets exhibited greater platelet adhesion compared with the non-sensitized group, whereas at a late time (24 h) they exhibited lower platelet adhesion to both agonists. Moreover, at 30 min after OVA-challenge, intracellular calcium levels to ADP (20 μM) and thrombin (100 mU/ml)-activated platelets were greater compared with non-challenged rats. As opposed, at 24 h after OVA challenge, a lower intracellular calcium level to ADP- and thrombin-activated platelets was observed. In conclusion, OVA-challenge in rats promotes a biphasic response in platelet adhesion consisting of an increased adhesion and intracellular calcium levels at an early phase (30 min), which progress to a reduction in adhesion and intracellular calcium levels at a late time (24 h) after antigen challenge.     

Platelet regulatory prostanoids and platelet release products in sickle cell disease.

Changes in platelet function have been observed for sickle cell disease (SCD). Levels of the arachidonic acid metabolites, thromboxane A2 (released by stimulated platelets) and prostacyclin (released from vascular endothelium), which stimulate and inhibit platelets, respectively, have been implicated in overall regulation of platelet function. Circulating basal levels of thromboxane and prostacyclin were determined in 1) a group of SCD volunteers (n = 21; at half-yearly steady state intervals and also at 24 hr, 72 hr, and 7 days after start of pain crisis) and 2) an age-, sex-, and race-matched control group (n = 18; single determinations). Circulating levels of beta-thromboglobulin (beta-TG), as well as thrombin (clotting)-stimulated platelet release of thromboxane, were also determined. Statistically significant decreases were found for prostacyclin, basal thromboxane, and thrombin-induced (maximal) thromboxane (alone or per platelet), for steady state SCD vs. normal controls. In addition, significant increases in maximal thromboxane were identified in crises (24, 72 hr) compared with steady state. Crisis beta-TG (24 hr) was significantly elevated compared with controls or steady state SCD. The ratio of basal thromboxane to prostacyclin was increased in crisis, but not significantly. Crisis frequency may correlate in part with changes in platelet function: steady state maximal thromboxane and released thromboxane per platelet were significantly lower in SCD volunteers who had crises during the study vs. those who did not (equivalent study time). The data support altered platelet function in SCD, possibly refractoriness (desensitization), manifest as decreased thromboxane release, to thrombin and/or other stimuli: alternate explanations are discussed.     

Platelet activation and serotonin release during colorectal surgery

In vitro studies show serotonin has a profound vasospastic effect on human mesenteric arteries. A similar response has been shown in vivo in atherosclerotic primates. If platelet serotonin stores are released as a consequence of platelet activation during colorectal surgery, a similar effect may significantly alter the perfusion of newly formed anastomoses leading to ischaemia and anastomotic breakdown. Here we have studied the effects of surgery and anaesthesia on intraplatelet and plasma serotonin levels during the peri- and postoperative period following colorectal surgery. A series of six consecutive patients undergoing colorectal resection and anastomosis were selected. Peripheral venous blood samples, taken at specified times before and after surgery and prepared in a platelet stabilizing buffer solution, were analysed using a validated enzyme immunoassay technique. Intraplatelet serotonin levels were seen to fall post-operatively, whilst plasma serotonin levels were shown to rise, implying significant platelet activation and serotonin during the peri-operative period. This study demonstrates the increased bioavailability of serotonin during the peri-operative period in colorectal surgery patients. If the in vitro effects of this amine are mirrored in vivo, increased plasma levels of serotonin may have an important role in anastomotic dehiscence secondary to ischaemia.     

Siderophore electrochemistry: relation to intracellular iron release mechanism.

Previous studies have shown that there is a major difference between the iron release mechanism of enterobactin, a catechol-based siderophore, and that of the hydroxamate-based siderophores such as ferrichrome. For ferric enterobactin there is an esterase that hydrolyzes the ligand during iron release. In contrast, iron is released by the hydroxamate-based siderophores and the ligands are reused in subsequent iron transport. It has been suggested that release of iron by hydroxamates occurs by reduction to the ferrous complex, a process that does not occur for ferric enterobactin. Cyclic voltammograms of ferrichrome A and ferrioxamine B exhibit reversible one-electron waves with pH-independent formal potentials (Ef-vs. the normal hydrogen electrode) -446 and -454 mV, respectively, within the range of physiological reductants. Ferric enterobactin also shows a reversible one-electron wave (at pH greater than 10) with Ef = -986 mV vs. the normal hydrogen electrode. From the pH dependence of this potential we estimate a reduction potential of -750 mV at pH 7. In sharp contrast to the value for the ferric hydroxamates, this value is well below the range of physiological reducing agents. The results demonstrate that the observed hydrolysis of enterobactin is a necessary prerequisite to in vivo release of iron from the siderophore via ferric ion reduction.     

Platelet regulation by NO/cGMP signaling and NAD(P)H oxidase-generated ROS

Platelets play a crucial role in the physiology of primary hemostasis and pathophysiological processes such as arterial thrombosis. Accumulating evidence suggests a key regulatory role of both NO and reactive oxygen species (ROS) in platelets. While the inhibitory role of NO/cGMP signaling in both murine and human platelets is well established, recent data suggest that intracellular ROS generation is involved in platelet activation. Thrombin-induced intracellular ROS production was inhibited by NAD(P)H oxidase inhibitors (DPI and apocynin), cyclooxygenase inhibitor (acetylsalicylic acid), and superoxide scavengers (tiron and MnTMPyP). Furthermore, thrombin (Trap6)-induced platelet aggregation and thrombus formation on collagen under high shear was inhibited by NAD(P)H oxidase inhibitors (DPI and apocynin), whereas secretion and platelet shape change were not affected. Inhibition of alphaIIbbeta3 activation by NAD(P)H oxidase inhibitors and superoxide scavengers was independent of NO/cGMP signaling demonstrating a direct role of platelet NAD(P)H oxidase-generated ROS for integrin alphaIIbbeta3 activation.     

Lung cGMP release subsequent to NO inhalation in pulmonary hypertension: responders versus nonresponders.

Inhalation of nitric oxide (NO) is widely employed for the assessment of pulmonary vasoresponsiveness in pulmonary hypertension (PH). However, the reasons for the huge differences in vascular reactivity to NO between patients are unknown, and the role of NO-induced cyclic guanosine monophosphate (cGMP) is unclear. Twenty patients with severe precapillary PH were investigated. Thirty-six Swan-Ganz catheter investigations were performed and the study subjects were tested for responses to NO inhalation. This included an assessment of pulmonary and systemic arterial plasma cGMP and atrial natriuretic peptide (ANP) levels. A significant NO response (pulmonary vascular resistance (PVR) decrease >20%) was noted in nine of 20 patients (45%) during the first catheterization. A highly significant correlation between baseline plasma cGMP and ANP levels with PVR was observed (r=0.62 and r=0.66, respectively; p<0.0001). In response to NO, systemic and mixed venous cGMP levels increased from 13.9 +/- 1.28 nM and 12.75 +/- 0.99 nM to 79.23 +/- 4.99 nM and 55.25 +/- 4.41 nM (p<0.001), respectively, accompanied by the appearance of a marked transpulmonary cGMP gradient. Although in the responder group ANP levels were significantly reduced after NO inhalation, no significant correlation was observed to the extent of PVR reduction. The magnitude of the NO-elicited cGMP response did not discriminate between haemodynamic responders and nonresponders. This study concludes that plasma cyclic guanosine monophosphate levels are significantly correlated with the severity of disease in pulmonary arterial hypertension. Nitric oxide inhalation provokes a prompt increase in cyclic guanosine monophosphate secretion, but the magnitude of this release is not linked with a decrease in pulmonary vascular resistance.     

Platelet thromboxane synthesis and release reactions in myeloproliferative disorders

A group of patients with myeloproliferative disorders was studies with respect to platelet aggregation responses, release of beta-thromboglobulin and incorporated 5-hydroxy-tryptamine, and synthesis of thromboxane b 2. In all patients the resting plasma beta-thrombo-globulin was elevated. Aggregation responses were frequently impaired to adrenaline, arachidonic acid, A23187 and the prostaglandin endoperoxide analogue, U44069. Both 5-hydroxy-tryptamine and beta-thromboglobulin release were greater with patients' platelets than with those of controls in response to adrenaline, ADP and U44069. The patients' platelets produced more thromboxane B2 than did controls, irrespective of the agonist used, yet those aggregating agents which are thought to act by generating thromboxane A2 were relatively ineffective in causing aggregation. This might reflect resistance to thromboxane A2 action in these patients, which is met by increased thromboxane formation.     

Platelet release abnormality associated with a variant of von Willebrand's disease.

A family with a platelet release abnormality (PRA) is described. The only son also showed a reduced rate of platelet aggregation in response to ristocetin, markedly reduced levels of von Willebrand's factor (vWf, ristocetin cofactor), and increased mobility of factor VIII-like antigen, features which were suggestive of von Willebrand's disease (vWd). No inhibition of vWf was found in his plasma. Family studies showed no evidence of vWd in the mother. The father's investigations showed a low rate of ristocetin aggregation on one of the two occasions when it was tested and low vWf on two of four occasions. Despite repeated testing, the findings in the father did not conclusively rule out the possibility of mild vWd, and it was impossible to determine whether the vWd in the son was inherited or arose as a mutation. The findings in this family suggest a possible relationship between abnormalities of the factor VIII complex and defective platelet function.     

Platelet transfusion reaction associated with interdonor HLA incompatibility

An HLA-compatible platelet transfusion was followed by chills, fever, and severe respiratory distress in a multitransfused patient with chronic lymphocytic leukemia. During the previous 7 days the patient had received blood products without incident, including 8 units of red blood cells (RBC), 24 units of pooled random donor platelet concentrates, and five HLA-compatible platelet pheresis products. The patient had no demonstrable RBC, HLA lymphocytotoxic, platelet or granulocyte antibodies. The platelet donor, a multiparous female, had no granulocyte or RBC antibodies but had lymphocytotoxic antibodies against HLA-A2 CREG (cross-reacting group A2, A28, A23, A24) which reacted not with lymphocytes of the patient but with lymphocytes of the donor whose RBC were transfused 24 h prior to the platelet transfusion reaction and whose HLA type is A23, A24; B44, B57. No RBC donors had HLA lymphocytotoxic, granulocyte, or platelet antibodies against the platelet donor. The patient received three subsequent platelet transfusions from the same donor after removal of the antibody-laden plasma with no adverse reaction. These data suggest an interdonor reaction caused by the presence of cells from the RBC donor received by the patient 24 h prior to the transfusion of donor lymphocytotoxic antibody to HLA-A2 CREG antigens.     

Liposome--lymphocyte interaction: saturable sites for transfer and intracellular release of liposome contents.

The water-soluble dye 6-carboxyfluorescein was trapped in the internal aqueous compartments of small sonicated dioleoyl lecithin vesicles and used to assess the kinetics of transfer of vesicle contents to human lymphocytes. By using flow microfluorometry, the initial rate of dye transfer to the cells was measured as a function of the concentration of vesicles in the external medium. The rate of transfer consists of at least two components, one of which saturates at high vesicle concentration and the other of which does not saturate in the range of concentrations explored. The saturable component was competitively inhibited by vesicles not containing dye. Both the saturable and nonsaturable components of transfer were inhibited by fetal calf serum or bovine serum albumin but neither component was affected by bovine IgG, choline chloride, or heparin. Pretreatment of the lymphocytes with trypsin or Pronase had no effect on either component. The saturable component can be interpreted in terms of a two-step process in which vesicles bind reversely to sites on the cell surface, and dye is then transferred into the cell from the vesicle-site complex.     

Platelet aggregation and release of ATP in patients with hepatic cirrhosis

The purpose of this study was to assess the platelet aggregation and releasable platelet ATP in patients with alcoholic cirrhosis (n = 10) and primary biliary cirrhosis (n = 10). In patients with liver disease a significant decrease was found in both adenosine diphosphate-induced aggregation (P less than 0.01) and collagen-induced aggregation (P less than 0.001) compared with that of controls, but there was no significant difference between the two groups of patients. Patients with primary biliary cirrhosis release smaller amounts of ATP than patients with alcoholic cirrhosis, and compared with the controls there was a significant (P less than 0.001) decrease in the releasable ATP in the patient groups. These results suggest that platelets are damaged during an intravascular activation (loss of granules), which gives rise to their subsequent hypo-function when tested in vitro.