Clearance and In Vivo Release by Heparin of Human Platelet Factor 4 (PF4) in the Rabbit

13 male New Zealand rabbits were injected with two different doses (25 μg/Kg and 100 μg/Kg) of human platelet factor 4 antigen (PF4). The disappearance of the protein was extremely fast with an half-life for the fast component of 1.07 ± 0.16 and 1.76 ± 0.11 min respectively. The half-life for the slow component, detectable only with the highest dosage, was 18.8 min.

The administration of 2500 I.U. of heparin 30 min after PF4 administration induced a partial release of the injected protein and its clearance from plasma was slow, with half-life of 23.3 ± 5.9 min and 30.9 ± 2.19 min respectively.

Fluid intake and hydration status in obese vs normal weight children

BACKGROUND/OBJECTIVES

Little is known on the relationship between obesity and hydration levels in children. This study assessed whether and by which mechanisms hydration status differs between obese and non-obese children.

SUBJECTS/METHODS

Hydration levels of 86 obese and 89 normal weight children (age: 7-11 years) were compared. Hydration was measured as the average free water reserve (FWR=urine output/24 h minus the obligatory urine output [total 24 h excreted solutes/97th percentile of urine osmolality of children with adequate water intake, that is, 830 mOsm/kg]) over 2 days. Three days of weighed dietary and fluid intakes were recorded. Non-parametric tests were used to compare variables that were skewed and to assess which variables correlated with hydration. Variables mediating the different hydration levels of obese and normal weight children were assessed by co-variance analysis.

RESULTS

Obese children were less hydrated than normal weight peers [FWR=median (IQR): 0.80 (-0.80-2.80) hg/day vs 2.10 (0.10-4.45) hg/day, P<0.02; 32% of obese children vs 20% of non-obese peers had negative FWR, P<0.001]. Body mass index (BMI) z-score (z-BMI) and water intake from fluids correlated with FWR (ρ=-0.18 and 0.45, respectively, both P<0.05). Water intake from fluids completely explained the different hydration between obese and normal weight children [FWR adjusted for water from fluids and z-BMI=2.44 (0.44) hg vs 2.10 (0.50) hg, P=NS; B coefficient of co-variation between FWR (hg/day) and water intake from fluids (hg/day)=0.47, P<0.001].

CONCLUSIONS

Obese children were less hydrated than normal weight ones because, taking into account their z-BMI, they drank less. Future prospective studies are needed to explore possible causal relationships between hydration and obesity.

Disappearance of human platelet factor 4 (PF4) in rabbits: does an immediate component exist?

Twelve male New Zealand rabbits were injected with 21 micrograms/kg of human platelet factor 4 antigen (PF4). The decay of the protein followed a monoexponential curve for the first 5 mins, with a half-life (t 1/2) of 1.94 mins and a calculated concentration at 0 time (CO) of 79.4 ng/ml. Five rabbits were pre-treated with heparin (2.500 I.U. i.v.) and 3 mins later were injected with the same amount of PF4. PF4 decay followed a monoexponential curve with a t 1/2 of 25.3 mins, and with CO of 380.8 ng/ml. This value is not greatly different from the one calculated assuming an immediate and uniform distribution in plasma (482.7 ng/ml for a plasma volume of 43.5 ml/kg). The 12 rabbits injected with PF4 were divided in 3 groups, in which heparin was given at 10', 30' or 60' after PF4, respectively. After heparin the peak levels of PF4 were 139.9 ng/ml, 65.3 ng/ml and 52.7 ng/ml, respectively. The following monoexponential PF4 decay had t 1/2 of 20.7, 25.6 and 26.0 mins, respectively. In a separate group of 4 animals, we studied heparin decay after an intravenous bolus of 2.500 I.U. Heparin decay could not be described by a monoexponential equation and was different from the decay of PF4 injected after heparin. On the basis of the present data we suggest the presence of an immediate component of PF4 decay, most likely due to uptake by the tissues. Heparin pretreatment may avoid this uptake process.

Clearance and in vivo release by heparin of human platelet factor 4 (PF4) in the rabbit

13 male New Zealand rabbits were injected with two different doses (25 micrograms/Kg and 100 micrograms/Kg) of human platelet factor 4 antigen (PF4). The disappearance of the protein was extremely fast with an half-life for the fast component of 1.07 +/- 0.16 and 1.76 +/- 0.11 min respectively. The half-life for the slow component, detectable only with the highest dosage, was 18.8 min. The administration of 2500 I.U. of heparin 30 min after PF4 administration induced a partial release of the injected protein and its clearance from plasma was slow, with half-life of 23.3 +/- 5.9 min and 30.9 +/- 2.19 min respectively.

Effects of low-molecular-weight heparin on platelets as compared with commercial heparin

Five subjects were injected with 5,000 IU of commercial heparin and low-molecular-weight heparin at an interval of 20 days after each injection. Both heparins produced the same platelet factor 4 release immediately after administration (commercial heparin 114.6 +/- 21.6 ng/ml, low-molecular-weight heparin 113.1 +/- 22.1 ng/ml). However, commercial heparin induced a more evident potentiating effect on ADP-induced platelet aggregation and was still present 60 min after injection. Low-molecular-weight heparin had a higher anti-Xa-specific activity than that determined by activated partial thromboplastin time. The opposite was true for the commercial preparation.

Are platelet factor 4 and beta-thromboglobulin markers of cardiovascular disorders?

Beta-thromboglobulin and platelet factor 4 are the two best characterized platelet-specific proteins. They are stored in the platelet alpha-granules and released during platelet activation. Their physiological function is unknown. PF4 has high anti-heparin activity, whilst beta-TG does not. Certain factors can affect the plasma level of one or both of these two proteins and these must be borne in mind whenever the evaluation of beta-TG and PF4 are thought to represent true in vivo platelet activation: their artificial release due to sample collection and processing, the in vivo release of PF4 induced by heparin, and the elevation of beta-TG due to renal failure. What really represents an abnormal level of beta-TG and PF4 is unknown, since we do not know their pathophysiology. At present, however, the platelet-specific proteins, even if they are considered as 'markers' of platelet activation, do not necessarily reflect the severity of the cardiovascular disorders nor do they signal thrombus formation, as thrombosis is a consequence of several interacting factors.

A new family with classical factor X deficiency as demonstrated by electroimmunoassay

A new family with classical factor X deficiency is described. The proposita is a 6 year old girl who presented with occasional epistaxis and a hematoma after an intramuscular injection. The main laboratory features consisted in a prolongation of partial thromboplastin, prothrombin and Stypven clotting times corrected by the addition of normal serum. Factor X activity varied between 3 and 6%. Factor X amydolytic activity was 15% of normal. Electroimmunoassay failed to show the presence of factor X antigen. No inhibitor was found in the proposita plasma. Parents and other family members showed intermediate levels of factor X activity and antigen and were considered to be heterozygotes. No consanguineity was found in the family.