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Biomedical subjects

L P McKeown

Publications and source records attributed to L P McKeown.

At least 19 recordsLinked to original sources
PubMed2004

Caregivers of people with multiple sclerosis: experiences of support.

The aim of this phenomenological study was to gain an understanding of the experiences of a group of caregivers of people with multiple sclerosis (MS). Sixteen caregivers from Northern Ireland and the Republic of Ireland participated in focus group interviews. The theme of support, either sought or received, emerged as a major aspect of the experiences described. Caregivers' feelings about, and experiences of, support appeared to change over time. Four common phases that caregivers experienced in relation to support were identified as: 'rejecting', 'resisting, 'seeking' and 'accepting' support. This paper will present and discuss these four phases. The study findings highlight the complexity of issues surrounding a caregiver's decision to seek and accept support. It is hoped that the phases identified within this study are useful in depicting how caregivers of people with MS may progress through stages in their desire for, and acceptance of support. Findings from this study are useful to healthcare professionals who work with people with MS and their caregivers by increasing awareness that a caregiver's attitude toward and acceptance of support changes over time.

Adolescent↗
PubMed2003

The needs and experiences of caregivers of individuals with multiple sclerosis: a systematic review.

PRIMARY OBJECTIVE: To appraise recent studies regarding the needs and experiences of caregivers of individuals with multiple sclerosis (MS). DESIGN: The following computerized databases were searched: CINAHL, BIDS IBSS, ASSIA, MEDLINE, PSYCHINFO, British Nursing Index, ISI Web of Science, Zetoc, AMED (1990-April 2002). The computer-based search was supplemented by manual searches of the reference lists of all retrieved studies and review articles. Inclusion and exclusion criteria were formulated. RESULTS: Twenty-four studies from across the world that met the inclusion criteria were reviewed. The majority of studies were descriptive in nature. The studies covered a variety of topics, including how carers assist people with MS, the effect of providing care on a carer's physical and psychological well-being, social life, financial situation and overall quality of life, and how carers cope with the stresses of providing care. CONCLUSIONS: Providing care for a person with MS has a major impact on all areas of the caregiver's life. Perceived social support has been shown to have a beneficial impact on the caregiver. Limitations in design and variation in methodology of studies limits the generalizability of findings. There is a need for further research, in particular the development of reliable and valid disease-specific caregiver assessment instruments.

Caregivers↗
PubMed1998

Platelet von Willebrand factor in Hermansky-Pudlak syndrome.

The Hermansky-Pudlak Syndrome (HPS) is an autosomal recessive inherited disorder characterized by oculocutaneous albinism, tissue accumulation of ceroid pigment, and a mild to moderate bleeding diathesis attributed to storage-pool deficient (SPD) platlets. Patients have platelet aggregation and release abnormalities. In addition, low levels of plasma von Willebrand factor (vWF) antigen in some HPS patients have been associated with a greater bleeding tendency than would be predicted from either condition alone. Other HPS patients have severe bleeding despite normal levels of plasma vWF, suggesting that at least one additional factor is responsible for their bleeding diathesis. Because platelet vWF levels have been well correlated with clinical bleeding times in patients with von Willebrand's disease, we have measured the platelet vWF activity and antigen levels in 30 HPS patients and have attempted to correlate their clinical bleeding with these values. The platelet vWF activity levels in patients was significantly lower than that of normal subjects (P < 0.0001). The patients as a group also had slightly lower values of plasma vWF activity when compared with normals (P-0.03). In 11 of the HPS patients, the multimeric structure of plasma vWF showed a decrease in the high molecular weight multimers and an increase in the low molecular weight multimers. In correlating the platelet and plasma vWF values with the bleeding histories, we were not able to show a predictable relationship in the majority of the patients.

Adenosine Triphosphate↗
PubMed1996

Platelet adhesion at high shear rates: the roles of von Willebrand factor/GPIb and the beta 1 integrin alpha 2 beta 1.

We have previously described a monomeric rvWf fragment, Leu504-Lys728 that contains one disulfide bond linking Cys509-Cys695. This fragment, VCL, has previously been shown to inhibit vWf-ristocetin, asialo-vWf, and botrocetin-induced vWf binding and aggregation of platelets. VCL inhibited 50% of vWf binding to heparin, but it did not inhibit vWf binding to type I collagen. At a high shear force (2600-1 sec), VCL inhibited platelet adhesion to the subendothelial surface of human umbilical arteries. The maximum inhibition of platelet adhesion was 83 +/- 4% at a VCL concentration of 7.6 mumol/L. Various monoclonal anti-Very Late Activation antigens (VLA) antibodies were added to the VCL and tested for their ability to enhance the inhibition of platelet adhesion at high shear forces. Of all of the VLA antibodies tested, only the anti-VLA-2 antibody (176D7) inhibited platelet aggregation in the absence of VCL and enhanced the inhibition of platelet adhesion in the presence of VCL. The VLA-2 antibody and VCL together inhibited 96 +/- 4% of platelet adhesion at high shear forces.

Adult↗
PubMed1996

1-Desamino-8-arginine-vasopressin corrects the hemostatic defects in type 2B von Willebrand's disease.

DDAVP is effective treatment in most types of von Willebrand's disease; however, in type 2B von Willebrand's disease the use of DDAVP has been contraindicated due to DDAVP-induced thrombocytopenia. Several reports have confirmed the thrombocytopenic effects of DDAVP and the presence of circulating platelet aggregates in type 2B von Willebrand's disease. We have infused three type 2B patients with DDAVP. The three patients had different mutations of their vWf. All three patients had a missense mutation which resulted in a single amino acid substitution in the disulfide loop of the A1 domain. Administration of 20 micrograms of DDAVP resulted in significant elevations of factor VIII, vWf antigen, and ristocetin cofactor levels. In contrast to other studies, DDAVP did not induce or enhance thrombocytopenia in these three patients. When blood was obtained by fingerstick and diluted into sodium oxalate (Unopette) or EDTA (Microvette), the platelet counts did not change over 4 hr. In contrast, blood collected directly into evacuated tubes containing sodium citrate, lithium heparin, or EDTA consistently demonstrated varying degrees of thrombocytopenia and platelet clumping. We also observed a shortening of the pre-infusion bleeding time over the 4 hr period. All three patients have been studied twice and each has shown consistent results. DDAVP appears to be a useful form of treatment in type 2B vWd.

Adult↗
PubMed1996

Glycoprotein Ib alpha peptides inhibit thrombin and SFLLRN-induced platelet aggregation.

The platelet glycoprotein Ib(alpha) (GPIb(alpha)) receptor contains a high-affinity binding site for thrombin that, when occupied, augments platelet activation and aggregation in part via the 7-transmembrane domain receptor (7-TMDR). We have constructed a series of peptides derived from GPIb(alpha) that encompass the amino acid sequence F216-T240. We have studied the effect(s) of these peptides on platelet aggregation induced by thrombin or by the 7-TMDR peptide SFLLRN. Twenty-four peptides were synthesized from the peptide sequence F216-T240. Several of the peptides derived from the sequence W219-V227 of GPIb(alpha) inhibited platelet aggregation, which was primarily dependent on the presence of the amino acid sequence A224-N226 (AEN). These data suggest that a region within the GPIb(alpha) chain modulates the platelet aggregation induced by alpha-thrombin. These GPIb(alpha) peptides did not interfere with platelet aggregation induced by other agonists--for example, collagen, ristocetin, calcium ionophore, or botrocetin--which indicates that these GPIb(alpha) peptide-platelet interaction(s) are specific. Our studies provide another potential mechanism for modulating platelet activation and aggregation via synthetic and natural peptides.

Amino Acid Sequence↗
PubMed1995

Activated platelets in paroxysmal nocturnal haemoglobinuria.

One of the major causes of morbidity and mortality in paroxysmal nocturnal haemoglobinuria (PNH) is venous thrombosis. We have studied fibrinolysis, coagulation and platelets in 11 patients with PNH in an attempt to identify the possible mechanism(s) of thrombosis in PNH. In this study we did not identify any fibrinolytic defects, evidence of coagulation activation, nor reduction in coagulation inhibitors. In contrast, in this cohort of 11 PNH patients we have identified varying degrees of platelet activation as defined by the surface expression of activation-dependent proteins and the binding of adhesive proteins to the platelet surface. The thrombotic events in PNH usually occur in the venous system. Our studies and previous experimental studies suggest that anti-platelet therapy may be efficacious in reducing the incidence and severity of venous thrombosis in PNH.

Adolescent↗
PubMed1994

High-affinity alpha-thrombin binding to platelet glycoprotein Ib alpha: identification of two binding domains.

alpha-Thrombin binding to and activation of platelets are of major importance in the initiation of physiologic thrombi and in the genesis of arterial thrombus formation. We have studied the site(s) and affinity of thrombin binding to human platelets. Our studies of the peptide inhibition of thrombin binding indicate that the glycoprotein Ib alpha binding site is of high affinity, Kd approximately 10(-10) M, while the seven-transmembrane-domain site is a moderate-affinity thrombin binding site, Kd approximately 10(-8) M. Further studies to modulate the high- or moderate-affinity thrombin binding can be directed to a specific class of sites. This would allow partial or total inhibition of specific thrombin-platelet interaction(s) in different clinical settings.

Amino Acid Sequence↗
PubMed1994

Purification and characterization of human platelet von Willebrand factor.

Platelet von Willebrand factor (vWf) was purified from human platelet concentrates. The multimeric structure of the purified platelet vWf was similar to that observed in the initial platelet lysate, and, like the platelet lysate, the purified platelet vWf contained higher molecular weight multimers than plasma vWf. The apparent molecular weight of the reduced platelet vWf subunit was similar to the plasma vWf subunit. The N-terminal amino acid of the purified platelet and plasma vWf was blocked. In concentration dependent binding to botrocetin- or ristocetin-stimulated platelets, 125I-plasma vWf bound with a higher affinity than platelet. The ristocetin cofactor activity per mg of purified plasma vWf was 5-fold greater than the platelet vWf activity. Platelet and plasma vWf bound to collagen with similar affinities; however, platelet vWf bound to thrombin-stimulated platelets and to heparin with a higher affinity than plasma vWf. The differences in the binding affinity(s) of plasma and platelet vWf to platelet GPIb and GPIIb/IIIa and extracellular matrix proteins may reflect different roles for plasma and platelet vWf in the initial stages of haemostasis and thrombosis.

Amino Acid Sequence↗
PubMed1993

Alpha granule proteins in type I von Willebrand's disease.

Platelet von Willebrand factor (vWf) is located in the alpha granules. Individuals with type I von Willebrand's disease (vWd) with prolonged bleeding times are best discriminated from those who have normal bleeding times by the normal level of platelet vWf ristocetin cofactor activity (vWf activity) and, to a lesser extent, by their platelet vWf antigen content. We have studied the content of adhesive proteins and platelet factor-4 (PF-4), and beta-thromboglobulin (beta TG) in the platelet alpha granules of types I and III vWd patients to determine if other alterations in alpha granule contents of proteins occur in vWd. We found that type I vWd patients with prolonged or normal bleeding times could not be differentiated on the basis of their platelet levels of beta TG, PF-4, fibronectin, or fibrinogen. The levels of the alpha granule constituents in the type I vWd patient were similar to normal except for the platelet fibrinogen concentration. Patients with type I vWd, regardless of the level of platelet vWf activity of antigen, had increased levels of platelet fibrinogen. The patients with type III vWd who had undetectable levels of platelet and plasma vWf also had increased levels of platelet fibrinogen. In our study we could not attribute the variation in the platelet vWf activity and antigen in type I vWd to the size of the alpha granule pool as determined by the measurement of other alpha granule proteins. The mechanism(s) of increased platelet fibrinogen in these vWd patients is at present unknown.

Antigens↗
PubMed1993

Failure to mobilize intracellular calcium in response to thrombin in a patient with familial thrombocytopathy characterized by macrothrombocytopenia and abnormal platelet membrane complexes.

We report a mother and son who were found to have macrothrombocytopenia, prolonged bleeding time, and abnormal platelet responses to thrombin. Transmission electron microscopy performed on the son's platelets demonstrated an unusual arrangement of membrane complexes formed by association of the open canalicular and dense tubular systems. Number and appearance of platelet alpha-granules, dense bodies, and mitochondria were normal. These platelets demonstrated normal agonist-induced Ca2+ flux in response to collagen and supranormal responses to arachidonic acid but displayed no increase in intracellular free Ca2+ in response to thrombin. Platelet surface glycoproteins IIb-IIIa, Ib, and granular membrane protein-140 measured by fluorescence-activated flow cytometry, along with platelet content of von Willebrand factor and fibrinogen, were normal. The von Willebrand factor binding function of GP-Ib on these platelets was also normal. We believe that this family demonstrates a unique macrothrombocytopenia syndrome characterized by deficient Ca2+ mobilization in response to thrombin that is not related to a defect in GP-Ib.

Adolescent↗
PubMed1993

A human chimera for von Willebrand disease following bone marrow transplantation.

PURPOSE: von Willebrand disease (vWD) is a common bleeding disorder characterized by quantitative or qualitative defects in von Willebrand factor (vWF), a protein important for coagulation and platelet adhesion. There are two forms of vWF currently recognized: plasma vWF synthesized by endothelial cells and platelet vWF produced within megakaryocytes. Although both plasma and platelet vWF play important roles in overall hemostasis, the relative contribution of each form is not clear. PATIENTS AND METHODS: We report the results of bone marrow transplantation from a donor with Type I vWD into a hemostatically normal recipient. RESULTS: Following engraftment, the recipient appears to be a human chimera for vWD, with normal amounts of plasma vWF from endogenous endothelial cells, but low levels of platelet vWF derived from donor megakaryocytes. CONCLUSIONS: Although the vWD chimerism has not been associated with any clinical manifestations, the bleeding time has become prolonged, suggesting that platelet vWF is important for normalization of the bleeding time.

Adolescent↗
PubMed1992

Absence of the largest platelet-von Willebrand multimers in a patient with lactoferrin deficiency and a bleeding tendency.

We have studied a young male with lactoferrin deficiency and a bleeding tendency responsive to cryoprecipitate. This child has had increased bleeding following surgical procedures and a variably prolonged template bleeding time. The patient has a normal platelet count, normal in vitro platelet ATP secretion and aggregation in response to a variety of agonists, and normal concentration of plasma-von Willebrand factor ristocetin cofactor activity and antigen. Analysis of plasma-vWf multimers by agarose gel electrophoresis consistently demonstrated a subtle decrease in the largest vWf multimers. In contrast, analysis of the patient's platelet-vWf revealed normal vWf:Ag, decreased vWf ristocetin cofactor activity, and a striking absence of the high and intermediate size molecular weight vWf multimers. Analysis of surface bound platelet-vWf demonstrated normal amounts on the surface of unstimulated platelets, but after thrombin stimulation the platelet-vWf surface expression did not increase. This lack of increased platelet-vWf surface expression resulted from decreased binding of secreted platelet-vWf to be surface of stimulated platelets. These data suggest that the patient's bleeding tendency may be related to a defect in his platelet-vWf structure and/or mobilization. This case represents a unique demonstration of an abnormality of platelet-vWf in the presence of normal plasma-vWf, and supports the data indicating an important role for platelet-vWf in primary hemostasis.

Biopolymers↗
PubMed1992

Effect of platelet count on the DDAVP-induced shortening of the bleeding time in thrombocytopenic Gaucher's patients.

As part of an investigative protocol requiring serial liver biopsies in patients with Type I Gaucher's disease who were receiving enzyme replacement therapy, we gave 11 patients a total of 15 infusions of DDAVP (0.3 micrograms/kg IV) and measured the Simplate bleeding time pre- and postin-fusion. All patients were thrombocytopenic (one patient 100-150,000; seven 66-99,000; and three 50-65,000/microliters). Nine of 15 infusions resulted in at least a 2-min shortening of the bleeding time; in 6/11 infusions in which the platelet count was greater than 65,000, the bleeding time shortened to less than or equal to 10 min, in 1/11 it shortened greater than or equal to 2 min but not to less than or equal to 10 min. In only 2/4 infusions given to patients with platelet counts less than 65,000 did the bleeding time demonstrate any significant shortening. None of the six liver biopsies performed in the patients whose bleeding time shortened to less than or equal to 10 min resulted in any significant bleeding; blood products were not transfused either pre- or postprocedure. These limited data demonstrate that a high percentage of patients with platelet counts of 65-100,000/microliters can manifest a significant shortening of bleeding time following a standard infusion of DDAVP, and that in approximately half of these patients the bleeding time will shorten enough to safely allow the performance of a liver biopsy without the need for prophylactic transfusion of blood products.

Adolescent↗
PubMed1991

Platelet von Willebrand factor.

von Willebrand factor (vWF) circulates in the blood in two distinct compartments. One, plasma vWF, is synthesized and released from endothelial cells; the second, synthesized by megakaryocytes, circulates in platelets primarily stored in the alpha granules. Recent experimental and clinical studies of von Willebrand's disease (vWD) indicate that platelet vWF plays an important role in the bleeding time determination and the degree of clinical bleeding in vWD. Platelet vWF is released from the platelet alpha granules by various agonists and then rebinds to the glycoprotein IIb/IIIa complex. Fibrinogen or monoclonal antibodies against this complex inhibit 60 to 70% of the expression of platelet vWF. Aspirin inhibits 80% of the adenosine diphosphate-induced platelet vWF surface expression, and the platelet vWF surface expression that is not inhibited by aspirin can be almost totally inhibited by disruption of the platelet cytoskeleton. Platelet vWF may, in part, be expressed in the open canalicular system prebound to a receptor. Transfusion studies have shown that correction of the bleeding time in severe vWD requires both plasma and platelet vWF. On the basis of numerous studies, we hypothesize that platelet vWF plays an important role in platelet interaction with the subendothelial surfaces under conditions of high shear stress. After platelet contact, platelet vWF is released, binds to the glycoprotein IIb/IIIa complex, and forms a bridge between the subendothelial surface and the platelet, which initiates and supports platelet spreading. Platelet vWF also acts as an intercellular bridge between platelets and thereby promotes platelet aggregation. This process is important not only in the initial steps of hemostasis but also in the process of thrombosis.

Animals↗
PubMed1991

Platelet activation and alpha granule secretion in type IIB von Willebrand's disease.

Type IIB von Willebrand disease is characterized by enhanced ristocetin-induced platelet aggregation, spontaneous platelet aggregation, thrombocytopenia and the absence of the largest plasma von Willebrand factor (vWf) multimers. The absence of the largest plasma vWf multimers is related to their enhanced binding to platelets. The abnormal affinity of the IIB von Willebrand factor to platelets results in thrombocytopenia, but the mechanism is not known. We have studied the platelets from three patients with type IIB von Willebrand disease and have found evidence of platelet activation and alpha granule secretion as defined by increased amounts of von Willebrand factor, fibrinogen and the alpha granule protein PADGEM/GMP-140 on the surface of these platelets. The degree of thrombocytopenia appears to be directly related to the number of platelets with fibrinogen bound to the surface. PADGEM/GMP-140, an alpha granule membrane protein, fuses with the platelet plasma membrane after activation and is a site on platelets which binds to neutrophils or monocytes. This alpha granule protein may play an additional role in platelet clearance and thrombocytopenia in type IIB von Willebrand disease. This may, in part, explain the absence of thromboembolic phenomena despite the presence of activated platelets in patients with type IIB von Willebrand disease.

Blood Platelets↗