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Departments of Medicine (J.C.S., H.A.L., J.L., S.D., L.M.E., M.F.S., J.S.D.), Cardiology (J.G.), and Haematology (P.C.), University Hospital of Wales, Cardiff CF14 4XN, United Kingdom
Address all correspondence and requests for reprints to: Dr. Jamie C. Smith, Department of Diabetes and Endocrinology, Old Building, Bristol Royal Infirmary, Bristol BS2 8HW, United Kingdom. E-mail: jamie.smith{at}virgin.net.
| Abstract |
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| Introduction |
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Because hypopituitarism is a heterogeneous condition arising from a variety of underlying disorders and characterized by multiple coexisting pituitary hormonal deficiencies, it is possible that unphysiological replacement of other target hormones also contributes significantly to increased cardiovascular risk. Indeed, recent evidence from a study of more than 1000 hypopituitary adults suggests that vasopressin deficiency in hypopituitary adults is a strong predictor of poor cardiovascular outcome, being associated with a 3-fold increase in mortality, compared with the control population (3). Furthermore, patients with an underlying diagnosis of craniopharyngioma, of which as many as 60% receive vasopressin replacement in the postoperative period (12), have a particularly poor cardiovascular outcome (3, 13). The underlying mechanisms responsible for this association are unknown.
In addition to its antidiuretic effect, the long-acting synthetic vasopressin analog, desmopressin (L-deamino-8-D-arginine, DDAVP), used conventionally for vasopressin replacement in diabetes insipidus, is known to have hemostatic properties (14). In normal subjects, iv and sc administration of DDAVP increases plasma concentrations of factor VIII procoagulant and von Willebrand factor (vWF) (14, 15) as well as potentiating platelet aggregation (16) and platelet adhesion to the endothelial cell surface (17). Furthermore, given via the parenteral route in patients with coagulation disorders such as hemophilia or von Willebrands disease, DDAVP has an established therapeutic role in increasing plasma levels of endogenous clotting factors and reducing transfusion requirements during surgery or bleeding episodes (14). Reports of DDAVP infusions precipitating myocardial infarction and ischemic stroke in susceptible individuals further highlight its potent thrombotic effects (18).
The hemostatic effects of long-term oral or intranasal DDAVP therapy in hypopituitary adults have not been studied. Although the dosages used intranasally for diabetes insipidus are of a lower order of magnitude than those used either intranasally (approximately, one tenth the dose) or iv for bleeding disorders (14, 19), it is conceivable that long-term DDAVP therapy might produce some prothrombotic effect. Such an effect may adversely influence endothelial function and thus contribute to the proatherogenic state in adult hypopituitarism. The main aims of this cross-sectional study were to assess endothelium-dependent vascular reactivity and coagulant factors in stable GH-treated hypopituitary adults, compared with controls, and assess the influence of intranasal DDAVP therapy on the same parameters for those individuals with diabetes insipidus.
| Patients and Methods |
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All hypopituitary subjects were receiving full, conventional hormonal replacement therapy for other hormone deficiencies including sex hormones. The nature of the underlying pituitary pathology in group A patients was as follows: nonfunctioning pituitary tumor (six patients), craniopharyngioma (three patients), Cushings disease (cured) (one patient), acromegaly (two patients), neurosarcoidosis (one patient), and idiopathic hypopituitarism (two patients). The underlying pituitary pathology in group B patients was as follows: nonfunctioning pituitary tumor (eight patients), macroprolactinoma (two patients), pituitary irradiation (two patients), craniopharyngioma (one patient), acromegaly (one patient), and idiopathic (one patient).
Individuals within the three groups were age and sex matched and had similar body weights. There were no individuals with overt coronary heart disease, cerebrovascular disease, or diabetes mellitus. Cigarette smokers were also excluded. Ethical approval for the study was obtained and subjects gave informed consent.
Laboratory measurements
Each subject was studied following an overnight fast with measurement of IGF-I, lipid profile, plasma fibrinogen, factor VIII, and vWF antigen (VWF:Ag). Plasma fibrinogen was measured using the Clauss method on a CA6000 coagulometer (Sysmex, Milton Keynes, UK). VWF:Ag was measured using an ELISA assay using antibodies from DAKO Corp. (Glostrup, Denmark). Factor VIII was assayed on a KC10 analyzer using immunodeficient plasma (Technoclene, Vienna, Austria). Total cholesterol and triglyceride concentrations were measured enzymatically using standard techniques. High-density lipoprotein cholesterol was measured after precipitation of apolipoprotein B with phosphotungstate/magnesium. Low-density lipoprotein cholesterol was calculated using the Friedwald equation. IGF-I was measured after prior acid-ethanol extraction by polyethylene glycol-assisted second antibody RIA.
Brachial artery studies
Flow-mediated dilatation (FMD) was measured by ultrasonic wall tracking as reported previously (10). The system comprises an adapted duplex color flow echo machine (Diasonics Spectra, Bedford, UK) with a 7.5-MHz linear phased-array transducer (giving high axial resolution). After obtaining a longitudinal B-mode image of the brachial artery, the radiofrequency signals from the M-mode output are digitized and relayed to the wall-tracking software (Vadirec; Medical Systems Arnhem, Oosterbeek, The Netherlands). Following a 10-sec period of data acquisition, vessel wall movements are tracked using the stored radiofrequency signals to produce displacement waveforms of the anterior and posterior walls together with the distension waveform (diameter change as a function of time) (20). This allows determination of end-diastolic diameter for each beat providing a theoretical resolution of ± 3 µm. Brachial artery blood flow was measured throughout the study using an 8-MHz continuous-wave Doppler probe. The Doppler signals were analyzed by a spectrum analyzer (SciMed Dopstation, Bristol, UK). All studies were performed during the morning in a temperature-controlled room with subjects fasted and supine following a 15-min rest. Measurements were performed at baseline and at 1-min intervals for a period of 5 min during reactive hand hyperemia (produced by releasing a pneumatic wrist cuff inflated for 5 min to suprasystolic pressure). A second cuff inflation was performed and repeat measurements were obtained during reactive hand hyperemia to assess operator reproducibility.
Reproducibility of this technique for a single operator, determined by calculating the mean absolute difference ± SD between paired measurements of FMD, was 0.58% ± 1.7%.
Statistical analysis
All statistical analyses were performed using SPSS for Windows (version 9.0; SPSS, Inc., Chicago, IL). Data are expressed as mean values ± SD. Statistical comparisons among groups were made using ANOVA. Q-Q plots and Bloms proportional estimation formula were used to determine whether a parameter was normally distributed. The variables IGF-I, triglyceride, and body mass index followed a nonnormal distribution. Correlation among variables was evaluated using Spearmans and Pearsons correlation coefficients and multiple regression analysis. A two-sided P value of less than 0.05 was considered significant.
| Results |
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To assess acute effects of DDAVP on the parameters studied, group A subjects were further subdivided into those who had and had not received a DDAVP dose within 4 h of being studied. Of the 15 subjects in group A, six had received DDAVP within 4 h of being studied. Patients within this subgroup were receiving DDAVP as part of a twice-daily dosage regimen. Comparing patients in this latter subgroup vs. those receiving only an evening dose (n = 9), concentrations of fibrinogen were 3.3 ± 0.9 g/liter vs. 3.3 ± 1.1 g/liter (P = NS); factor VIII, 134 ± 36% vs. 128 ± 28% (P = NS); and VWF:Ag, 123 ± 45% vs. 124 ± 30% (P = NS). There were therefore no significant differences in terms of coagulant factors between these two subgroups. In addition, there was no correlation within the whole group between the size of the daily DDAVP dose and coagulant factors or FMD.
| Discussion |
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In agreement with earlier studies performed at our center (9, 10, 21), the present study demonstrates that endothelium-dependent dilation of the brachial artery in hypopituitary adults receiving stable GH replacement is of a similar magnitude to that of comparable healthy controls. This contrasts with observations in GH-deficient hypopituitary subjects in whom FMD is significantly impaired (9, 10, 21). Thus, stimulated endothelium-derived nitric oxide release in GH-treated hypopituitary adults is intact and might imply healthy large artery function. However, we have also demonstrated elevated levels of vWF, factor VIII, and fibrinogen in these individuals despite stable physiological GH replacement, suggesting the persistence of a prothrombotic tendency.
Following endothelial injury, activation of the coagulation cascade together with platelet activation and adhesion are early events that are prerequisites for thrombogenesis in atheromatous vessels. VWF is a multimeric glycoprotein, synthesized in endothelial cells and platelets (22). Because VWF release is increased when endothelial cells are damaged, plasma levels of VWF have been widely regarded as a marker indicating the extent of endothelial damage and thus the susceptibility to atheromatous complications (22). The validity of this concept has been borne out by results of prospective studies, which have shown that in individuals with overt cardiovascular disease, high plasma levels of VWF are highly predictive of future cardiovascular events including myocardial infarction and death (23, 24). However, rather than merely acting as a marker of endothelial dysfunction, VWF may also play a pivotal role in the pathophysiology of thrombogenesis in atheromatous arteries. At sites of endothelial damage, VWF promotes hemostasis by binding to the exposed subendothelium and bridging this surface with platelet plugs through interactions with glycoprotein complexes on platelet membranes (25). Indeed, VWF-mediated platelet adhesion to the injured endothelium is considered to be an initiating step in physiological thrombus formation (22). Under conditions of high shear stress, similar to those that might occur in diseased arteries, large multimeric forms of VWF stimulate spontaneous platelet aggregation. In addition, VWF stabilizes factor VIII by forming noncovalent complexes with the latter in plasma, thus protecting it from proteolytic inactivation (22, 26). Increased levels of factor VIII and associated acute phase reactants including fibrinogen are also implicated in the development of atherosclerotic disease (27, 28).
The GH/IGF-I system may be directly involved in the regulation of factor VIII and VWF levels (29, 30). Abnormalities of coagulation and fibrinolysis with elevated plasma concentrations of several hemostatic markers have been previously reported in GH-deficient adults (31, 32) but results of earlier studies are inconsistent (5). More recent data in hypopituitary adults with GH deficiency do suggest elevated levels of both fibrinogen (6) and VWF (33) with positive correlations observed between other surrogate markers of endothelial dysfunction (33). Conversely, acutely blocking GH action in healthy males using the GH receptor antagonist, pegvisomant, led to a reduction in VWF levels (34). There has been relatively less study of the effects of GH replacement on hemostatic markers, and these studies have yielded variable results (6, 32). Our observations that hypopituitary adults continue to have elevated levels of the hemostatic markers fibrinogen, VWF, and factor VIII despite stable GH replacement suggest a persistent perturbation of endothelial function and prothrombogenicity in these individuals.
Besides GH deficiency, other target hormonal deficiencies and/or their unphysiological replacement probably contribute to cardiovascular risk in hypopituitarism. Specifically, vasopressin deficiency, necessitating DDAVP treatment, has been identified as a predictor of poor cardiovascular outcome (3). Furthermore, the diagnosis of craniopharyngioma is independently associated with increased cardiovascular mortality (3, 13). Because patients with craniopharyngioma often have posterior pituitary insufficiency, DDAVP replacement therapy is frequently required in this condition. In addition to its antidiuretic effect, it possesses potent hemostatic properties (14). When infused iv or administered sc or intranasally in relatively high doses [approximately 10-fold higher than those used for diabetes insipidus (19)], DDAVP induces a rapid 2- to 3-fold increase in plasma VWF in healthy individuals that lasts 46 h (14, 15). The mechanism for this increase in VWF induced by DDAVP probably occurs through direct release of stored factors within endothelial cells (35) and leads to enhanced platelet aggregation and platelet adhesion to the endothelial cell surface.
In addition, release of factor VIII, a cofactor for factor IX in the process of fibrin formation (14), further enhances coagulability. In animal models, DDAVP also exerts direct activating effects on endothelial cells leading to expression of adhesion molecules (P selectin) and margination of leukocytes (36). Although a possible association between venous thromboembolism and low-dose intranasal DDAVP treatment has been reported previously in a patient with diabetes insipidus (37), the influence of DDAVP therapy on coagulation and arterial function in hypopituitary adults has not been examined previously. Our findings suggest that DDAVP treatment using conventional intranasal doses does not affect either endothelium-dependent dilation or VWF concentrations in hypopituitary adults receiving GH replacement. Thus, any hemostatic effect of DDAVP at the conventional doses used to treat diabetes insipidus in hypopituitarism is likely to be minimal and probably has no influence on the susceptibility to atherosclerotic complications.
There are several limitations to the present study. The cross-sectional study design did not allow us to prospectively evaluate the effects of DDAVP therapy on the study parameters over a time period. The numbers of subjects in each group were also relatively small, so small differences that may exist among groups might not have been detected. Despite this, previous experience at our center using the technique of FMD (9, 10, 21, 38) suggests that we would have been able to detect differences in FMD of the order of 23% with statistical significance using the chosen sample size. In addition, determining vascular function and cardiovascular risk in hypopituitary adults is complicated because of the heterogeneous nature of hypopituitarism. The imbalance among the groups in terms of nature of underlying pituitary disease could conceivably have influenced endothelial function and coagulation profiles of the subjects.
In conclusion, although endothelium-dependent vasodilation is intact in GH-treated hypopituitary adults, elevated concentrations of hemostatic markers suggest the persistence of a prothrombotic tendency and endothelial dysfunction. Intranasal DDAVP does not appear to influence this proatherogenic profile in hypopituitary adults with vasopressin deficiency. The mechanisms underlying the reported relative increase in cardiovascular risk associated with vasopressin deficiency and craniopharyngioma remain unexplained.
| Acknowledgments |
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| Footnotes |
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Received October 17, 2002.
Accepted February 10, 2003.
| References |
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