A simple,fluorescent method to internally label platelets suitable for physiological measurements

Current methods for studying platelet survival in vivo are limited by the use of radioisotopes, with their inherent safety and regulatory concerns, systemic drug administrations that produce biochemical modifications of platelet functions, or external labeling techniques, which may produce artifacts due to surface modifications. For these reasons, we sought to develop a simple, nonisotopic method for labeling platelets internally, thereby producing platelets more likely to have in vivo properties equivalent to native cells. Murine platelets in protein-free buffer were fluorescently labeled internally by incubation with 2.5 μM 5-chloromethyl fluorescein diacetate (CMFDA), and without washing, were injected into mice for platelet survival studies. CMFDA-labeled platelets were unactivated, as shown by minimal P-selectin expression. When tested in vitro for function by aggregometry, the response of CMFDA-labeled platelets to collagen and thrombin was identical to that of unlabeled platelets. Flow cytometric analysis demonstrated that CMFDA platelets were an intensely stained, unimodal population that was completely separated from unlabeled platelets. The mean half-life of labeled platelets in the murine circulation was 37.5 ± 4.5 hr (±1 SD), and the mean survival time was 3.1–3.3 days (n = 24), similar to results reported using ⁵¹Cr and ¹¹¹In. No evidence of in vivo transfer of dye from labeled platelets to unlabeled cells was observed. CMFDA produces a population of platelets that are nonradioactively, internally labeled with a highly fluorescent, stable product. The labeled platelets function equivalently to native platelets, as demonstrated by immunocytometry and aggregometry, and importantly, in vivo, by normal platelet survival. Am. J. Hematol. 56:17–25, 1997. © 1997 Wiley-Liss, Inc.     

In vitro and in vivo behavior of 111In-labelled platelets: An experimental study of healthy male volunteers

The aim of this study was to obtain a critical evaluation of a simple method for labelling platelets with ¹¹¹In-oxine. All experiments were carried out on healthy volunteers. 65 ± 7 (SD) % of the platelets in collected blood were labelled and reinjected. As compared to control experiments, only in response to a low final ADP concentration (1.0 μmol/l) did ¹¹¹In-labelled platelets show reduced in vitro aggregability. The mean platelet volume for ¹¹¹In-labelled platelets was slightly lower than the mean platelet volume in whole blood. The results for initial platelet recovery and platelet mean lifespan closely agreed with those of other studies in which considerably higher platelet extraction from whole blood was obtained. After injection, the splenic uptake and blood disappearance of ¹¹¹In-labelled platelets followed a monoexponential function with almost identical rate constants. By compartmental analysis of the equilibration of platelets between blood and spleen, the splenic blood flow was estimated to be 4.8 ± 1.9 (SD) % of the total blood volume/min; the intrasplenic platelet transit time was 9.7 ± 1.6 (SD) min, and the exchangeable splenic platelet pool 31 ± 8 (SD) %. Highly significant relationships were present between the splenic blood flow and the splenic platelet pool size, as well as between the splenic blood flow and the initial platelet recovery. It is concluded that the requirements for adequate interpretation of platelet kinetics are well met with the present method for harvesting and labelling of platelets.     

Radioactive Diisopropyl Fluorophosphate as a Platelet Label: An Evaluation of in Vitro and in Vivo Technics

A technic for the in vitro labeling of human platelets with DFP³² is presented, critically evaluated, and compared to in vivo methods employing DFP³² and to in vitro methods using Cr⁵¹. The initial recovery of platelets labeled in vitro with DFP³² averaged 79 per cent, but the survival curve was characterized by an irreversible initial loss of platelet radioactivity. Experiments in which platelets were simultaneously labeled in vitro with both DFP³² and Cr⁵¹ suggest that this is not due to elution of DFP³². The survival curve of platelets labeled in vivo with DFP³² shows an initial transient reduction in platelet radioactivity. It is suggested that both of these aberrations in initial survival are the result of platelet injury by DFP³². Significant "tailing" was observed in the survival curves obtained with DFP³², and possible explanations of this phenomenon are discussed. DFP³²-labeled platelets circulating after 5 hours apparently survive normally and disappear from the circulation as a rectilinear function over the next 6-8 days. Although both in vitro and in vivo labeling methods employing DFP³² provide a meaningful approximation of platelet lifespan, the initial and terminal aberrations of the survival curves greatly complicate further interpretation. Dextran had no detectable effect on platelet survival, and epinephrine, Mecholyl, and cutaneous vasodilatation did not alter the platelet count or the specific activity of circulating labeling platelets in human subjects. The problem of initial platelet survival and the question of an extravascular or marginal platelet pool is discussed in the light of these data.

Submitted on May 24, 1966 Accepted on November 8, 1966     

Second malignancies in hairy cell leukemia (leukemic reticuloendotheliosis)

Kinetics andw quantification of the sites of destruction of ¹¹¹-Indium-oxine-labeled autologous platelets were investigated in eight patients with idiopathic thrombocytopenic purpura. The mean platelet count was 17 ± 9 × 10⁹/liter; platelets were separated by differential centrifugation and labeled with 5.6 ± 2.5 MBq ¹¹¹In. Whole body and organ ¹¹¹In-platelet distribution was quantitated with a scintillation camera and a computer-assisted imaging system acquisition matrix. Areas of interest were selected with the computer and organ ¹¹¹In-radioactivity expressed as a percentage of whole body activity. Mean platelet survival was 49.5 ± 29.6 hr and the survival curves were exponential. Equilibrium percentage organ ¹¹¹In-radioactivity was (normal values in parentheses): spleen 33.7 ± 8.8(31.1 ± 10.2); liver 16.1 ± 9.5(13.1 ± 1.3); thorax 22.8 ± 3.7(28.2 ± 5.6). Percentage organ ¹¹¹In-activity at the time when labeled platelets had disappeared from the circulation was: spleen 44.5 ± 16.4 (40 ± 16); liver 16.0 ± 11.5 (32.4 ± 7.2); thorax 19.7 ± 6.0 (17.7 ± 10.3). Thorax activity corresponds to bone marrow radioactivity. Three patterns of platelet sequestration were evident. Three patients had mainly splenic sequestration, two mainly hepatic sequestration, and three diffuse reticuloendothelial system sequestration with a major component of platelets destroyed in the bone marrow. Splenectomy was performed in two patients. The pattern of ¹¹¹In-platelet sequestration was not predictive of response of glucocorticoid therapy or indicative of the necessity for splenectomy. Quantitative ¹¹¹In-labeled autologous platelet kinetic studies provide a new tool for the investigation of platelet disorders.     

Organ distribution and fate of human platelets: studies of asplenic and splenomegalic patients

Little is known about the organ distribution and fate of human platelets. We investigated the kinetics, organ distribution, and fate of autologous 111In-oxine-labeled platelets in 12 normal volunteers, four asplenic subjects, and four patients with splenomegaly. The initial recovery of infused 111In-platelets from the circulation was 97.8 +/- 9.8% (means +/- SD) for asplenic subjects and 26.3 +/- 5.9% for splenomegalic patients as compared to 59.2 +/- 9.3% for normal controls. The mean platelet survival times as derived from the multiple-hit model were 9.2 +/- 1.0 days for asplenics and 6.2 +/- 0.6 days for splenomegalic subjects (8.4 +/- 0.8 days for normals). At 30 min postinfusion, 79.4 +/- 19.2% of the infused 111In-platelets pooled in the spleen of splenomegalic subjects and 42.7 +/- 12.2% in normal controls. There was 7.1 +/- 2.0, 12.6 +/- 3.7, and 29.3 +/- 8.4% pooling in the liver of splenomegalic, normal, and asplenic subjects, respectively. At 10 days postinfusion, 37 and 24% of the 111In-platelets were sequestered in the spleen and liver of normal control subjects, respectively. Similar figures for splenomegalic subjects were 71 and 14%, respectively. In asplenic subjects, 89% was sequestered in the liver. We conclude that spleen and liver are the primary sites of platelet destruction, accounting for 61% of infused 111In-platelets in normal volunteers and 85% in splenomegalics, while the liver is the primary site of platelet destruction, accounting for 89% in asplenic subjects.     

The in vivo kinetics of 111In- and 51Cr-labelled platelets: a comparative study using both stored and fresh platelets

The in vivo kinetics of simultaneously injected 51Cr- and 111In-labelled platelets was investigated in 20 healthy male volunteers. The studies were carried out using both fresh platelets and platelets stored for 5 d at 22 degrees C; the disappearance of platelet-bound radioactivity was measured on whole blood samples as well as platelet pellets. Compared to 111In, the labelling efficiency was notably lower for 51Cr, and a higher amount of 51Cr was bound to contaminating red cells. As regards the fresh platelets, only small dissimilarities were observed in the in vivo kinetics obtained with the two labels. 51Cr yielded a slightly higher platelet recovery and longer T1/2 than 111In, when whole blood samples were used for calculations; no differences were seen when using platelet pellets. When stored platelets were studied, 51Cr gave significantly longer T1/2 and mean life-span (MLS) than did 111In. This difference was present for all mathematical models used for the calculation of MLS, and when whole blood samples as well as platelet pellets were employed. It was demonstrated that the estimation of MLS was also highly dependent upon techniques of blood sampling and curve fitting. Calculation, in which the initial data points were excluded, gave consistently longer MLS (P less than 0.0001), as compared to when all data points were included. Furthermore, when all survival studies were grouped together, calculations using platelet pellets gave a significantly (P less than 0.001) shorter platelet MLS than calculations using whole blood samples. It is concluded that both 51Cr and 111In are acceptable as radiolabels for both fresh and stored platelets. However, it appears that the viability of stored platelets may be influenced by the choice of label, and caution must be taken when these isotopes are used together in dual tracer experiments. Also, our results show that a meticulously standardized processing of blood samples and experimental data is required to enable interlaboratory comparisons of the results.     

Comparative Studies of the in Vivo Kinetics of Simultaneously Injected 111In- and 51Cr-Labelled Human Platelets

The kinetics of simultaneously injected ¹¹¹In- and ⁵¹Cr-labelled platelets have been assessed in 40 subjects, 13 of them thrombocytopenic. 4 platelet survival models were applied. The mean life-time (MLT) of ⁵¹Cr-platelets from non-thrombocytopenic individuals was found to be slightly, but significantly, longer than that of ¹¹¹In-platelets by applying linear and exponential models for data fitting. The in vivo recovery (IVR) of ¹¹¹In-platelets was significantly higher than that of ⁵¹Cr-platelets in this patient group when using all 4 models. In the group of thrombocytopenic patients no statistically significant differences in MLT or IVR were found between ¹¹¹In- and ⁵¹Cr-platelets. However, for each of the 11 ⁵¹Cr-labelled platelet suspensions with the shortest MLT, a longer MLT was observed in the corresponding ¹¹¹In-platelets, a finding probably related to antibody-induced elution of ⁵¹Cr-activity. The same mechanism might be responsible for an increasing ¹¹¹In-/⁵¹Cr-recovery ratio in the early post-injection period. The efficiency of platelet isolation from blood prior to labelling seemed to influence the IVR, inasmuch as the difference in IVR between ¹¹¹In- and ⁵¹Cr-platelets was eliminated in the group where the yield of ¹¹¹In-platelets surpassed that of the ⁵¹Cr-platelets by more than 15 %.     

Platelet survival, atherosclerotic intermittent claudication and ticlopidine

Platelet survival times were measured twice within 6 months using autologous 111-Indium-labelled platelets in 13 male patients diagnosed as stable intermittent claudicants. Linear, exponential, weighted mean and gamma function (multiple-hit) analyses were carried out on the data. Of these methods, exact mathematical models such as linear and exponential are unsuitable for the comparison of curves which may alter with treatment or disease progression. Weighted mean platelet survival correlated (r = 0.96) with platelet survival calculated by the precision reference method of analysis, gamma function. Statistics were expressed from gamma function analyses unless otherwise stated. Severity of claudication was measured by doppler; the ratio of brachial to mean ankle pressures was directly related to platelet survival (r = 0.84). In 7 patients, where duration of claudication was greater than 5 years, platelet survival was significantly reduced (P less than 0.02) compared with 6 newly diagnosed claudicants of less than 18 months duration. Six randomly selected patients were treated with 250 mg b.d. ticlopidine during one of the study periods. Ticlopidine significantly increased platelet survival (P less than 0.02) in all patients irrespective of disease duration or severity. The involvement of platelets in atherosclerosis observed in these claudicants, can be reduced by therapy with ticlopidine, however, the lack of clinical improvement following long-term drug treatment only confirms the platelet's secondary role in this disabling disease.     

Evaluation of Apheresis Platelet Concentrates Collected with a Reduced (30-ml) Collection Chamber with Resuspension and Storage in a Synthetic Medium

Recently, the CS-3000® Plus Blood Cell Separator with the TNX-6 platelet separation chamber insert has been furnished with a small-volume (30-ml) collection chamber. In this study, a platelet synthetic medium containing glucose and bicarbonate (PSM) was used for resuspension and storage of this highly concentrated platelet product. Eighteen donors participated in a paired study design where each participant donated platelets on two occasions, once following collection in a standard chamber with resuspension and storage in plasma and once following collection in the new chamber with resuspension and storage in PSM. Substantially higher total platelet counts were obtained using platelets collected in the small chamber and stored in PSM as compared to control (4.4±0.9times10¹¹ vs. 3.5±0.9times10¹¹ platelets, p<0.01 by paired t test). After 5 days of storage, PSM-stored platelets demonstrated higher ATP levels, less lactate dehydrogenase in the supernatant and increased lactate production with resulting lower pH at day 5 of storage (6.94±0.15 vs. 7.08±0.09, p<0.05). There were no statistically significant differences of the survival by multiple-hit estimation of PSM-stored as compared to plasma-stored platelets as determined by ¹¹¹In labeling and infusion. A slight decrease in the initial percent recovery with the additive-suspended as compared to suspended plasma cells was noted: 50±8 versus 54±9%, respectively (p<0.05). In conclusion, the CS-3000 Plus/TNX-6 apheresis system with a new reduced-volume collection chamber and an additive solution provides a plasma-poor and highly concentrated platelet product with satisfactory in vivo viability and in vitro functional characteristics after 5 days of storage.     

Comparative Studies of the Function and Morphology of 111In-and 51Cr-Labelled Human Platelets

Comparative studies of the aggregability in vitro and ex vivo, and of the surface/volume ratio of ¹¹¹In- and ⁵¹Cr-labelled human platelets were carried out. The ADP-induced aggregation in vitro of ¹¹¹In-platelets was superior to that of ⁵¹Cr-platelets, as was that of ⁵¹Cr-platelets labelled in plasma as compared to ⁵¹Cr-platelets labelled in buffer. These differences seemed to be reversed in vivo, as identical collagen-induced aggregation responses were observed ex vivo when comparisons were made between ¹¹¹In- and ⁵¹Cr-platelets, and between labelled and unmanipulated platelets. Morphometric determination of the surface/volume ratios of the labelled platelets indicated a higher degree of platelet activation of ⁵¹Cr-platelets labelled in buffer as compared to those labelled in plasma. In this respect, no difference seemed to be present between ¹¹¹In- and ⁵¹Cr-platelets. The results of the ex vivo aggregation studies were unaffected by the time spent by the platelets in the circulation within 24 h post-injection, by platelet isolation yield, and by the medium used in ⁵¹Cr-labelling. Our results indicate that it will be possible to conduct comparative studies of simultaneously induced aggregation ex vivo of different platelet populations labelled with ¹¹¹In and ⁵¹Cr.     

Release of platelets into the circulation induced by gamma globulin treatment in a case of idiopathic thrombocytopenic purpura

Summary The kinetics and distribution in vivo of ¹¹¹In-labelled platelets prior to and during intravenous high-dose gamma globulin therapy in a 3-months-old child with idiopathic thrombocytopenic purpura were evaluated.Concomitantly with a prompt increase in platelet concentration we found evidence for release of platelets into the circulation, presumably to a large extent originating from the spleen and liver. Our results support the concept that, in response to high-dose gamma globulin therapy, platelets are not only forced to remain in but are also released into the circulation, presumably as a result of mobilization of platelets adhering to cells of the monocyte-macrophage system.     

Concurrent label method with 111In and 51Cr allows accurate evaluation of platelet viability of stored platelet concentrates

Summary. The precision and reproducibility of ¹¹¹In and ⁵¹Cr platelet radiolabel agents for in vivo kinetic studies of stored platelet concentrates (PC) were investigated. The objective was to develop a precise method with concurrent labelling of two platelet populations using different isotopes, which would allow identification of small differences in in vivo platelet quality. Identical labelling procedures were used to investigate the effects of PC storage age, different methods of red cell (RBC) and white cell (WBC) contamination correction, and label elution correction on the results of ¹¹¹In and ⁵¹Cr kinetic studies. ¹¹¹In and ⁵¹Cr platelet survival curves from the same PC, even when uncorrected for elution and RBC contamination, exhibited excellent correlation, irrespective of the age of the concentrate and its viability. However, slightly higher, but statistically significant, post-infusion per cent recoveries with ⁵¹Cr labelled platelets were found. Two factors were identified as the cause for this difference. There was a higher affinity of contaminating RBC/WBC in PC for ⁵¹Cr than for ¹¹¹In. With determination of RBC/WBC activity by centrifugation/density separation, RBC/WBC fractions from the injectate were found to contain 12.6 ± 3·8%v 7·1 ± 3·6% of total ⁵¹Cr and ¹¹¹In activity, respectively, in 20 studies. In addition, there was a significantly higher ¹¹¹In activity in plasma immediately post-infusion than with ⁵¹Cr, 5·2 ± 1·3%v 2·8 ± 1·6%, respectively, suggesting more label elution or carryover. After correction for the activity of RBC/WBC and for elution or carryover, essentially identical ⁵¹Cr/¹¹¹In platelet survival curves were found. In 31 stored PC studies, the absolute average difference between ⁵¹Cr and ¹¹¹In per cent recoveries was only 4 ± 3% in a group of donors whose platelet recoveries ranged from 10% to 80%. Similarly, the average difference between ⁵¹Cr and ¹¹¹In survival was only 8±4 h within a range of survivals from 40 to 220 h. In conclusion, after correction for elution and contaminating RBC/WBC binding, these studies show that ⁵¹Cr and ¹¹¹In may be used interchangeably for labelling of stored PC, and that small differences between test and control platelets could be reliably detected using concurrent labelling with simultaneous infusion.     

Function ex vivo of 111 In-Labelled Human Platelets Simultaneous Aggregation of Labelled and Unlabelled Platelets Induced by Collagen

The function of ¹¹¹In-labelled platelets has been assessed by collagen-induced aggregation of platelets in samples of whole blood. The blood samples were drawn after injection of autologous ¹¹¹In-labelled platelets in 19 subjects undergoing platelet kinetic studies. It was thus possible to measure the aggregability of labelled and unmanipulated platelets simultaneously. ¹¹¹In-labelled platelets aggregated to the same extent as unmanipulated platelets when tested from 10 min to 24 h after injection of the labelled platelets. The results confirm the assumption that minimal damage is inflicted on the platelets during the isolation and labelling procedures, and support the concept that platelets manipulated in vitro may recover in vivo within a few minutes after reinjection.     

Mechanisms of thrombocytopenia induced by interferon therapy for chronic hepatitis B

To clarify the mechanisms of thrombocytopenia observed in patients with chronic hepatitis B treated with interferon. We studied six patients with chronic active hepatitis B who received intramuscular injections of natural interferon-alpha (3 or 5 million IU/day) for 4 weeks. Peripheral blood platelet counts, bone marrow findings, and platelet kinetics, determined using¹¹¹In-labeled platelets, were analyzed. Platelets decreased significantly 1 week after the beginning of treatment and remained decreased until the completion of treatment. The number of nucleated cells and megakaryocytes in bone marrow decreased in three of five patients studied during treatment. The kinetic study showed platelet survival time to be 8.1±1.3 days (range, 5.8–10.0). One day after platelet injection, platelets accumulated predominantly in the splenic area in all patients, whereas hepatic accumulation was predominant 7 days after injection in three of the six patients. Thrombocytopenia during interferon treatment arises from the inhibition of stem cell proliferation and differentiation in the bone marrow and from the capture of platelets by the liver.     

Platelet survival studies in man with diisopropylphosphorofluoridate (DF32P). Some observations in patients with hypocoagulability, 'hypercoagulability' and platelet disorders

Diisopropylphosphorofluoridate (DF³²P) acts as a cell label by combining irreversibly with esterases in the cell membrane. It was first used in man for tagging red cells (Cohen and Warringa, 1954) and later for platelets (Leeksma and Cohen, 1956) and leucocytes (Athens, Mauer, Ashenbrucker, Cartwright and Wintrobe, 1959). Since DF³²P allows platelets to be labelled directly in vivo it has obvious advantages over the ⁵¹Cr method (Aas and Gardner, 1958) which necessitates removal of platelets from the body for labelling and subsequent re-injection. The precise form of the normal platelet survival curve in man and the relative importance of intrinsic factors such as cell age, and extrinsic factors such as variations in the concentration of plasma coagulation factors, remains to be established. As part of an investigation into some of these factors, DF³²P has been used in a small comparative study of platelet survival in some hypocoagulable and some ‘hypercoagulable’ states. During the course of this study the opportunity arose to apply the method to patients with qualitative and quantitative platelet abnormalities. These studies are presented and discussed in this paper.     

Platelet kinetics with indium-111 platelets: comparison with chromium-51 platelets

The application of 111In-oxine to platelet labeling has contributed to the understanding of platelet kinetics along three lines: 1. It allows the measurement of new parameters of splenic function, such as the intrasplenic platelet transit time, which has shed new light on the physiology of splenic blood cell handling. 2. It facilitates the measurement of platelet life span in conditions, such as ITP, in which 51Cr may undergo undesirable elution from the platelet as a result of platelet-antibody interaction. 3. It allows the determination of the fate of platelets, that is, the site of platelet destruction in conditions in which reduced platelet life span is associated with abnormal platelet consumption, as a result of either premature destruction of "abnormal" platelets by the RE system, or the consumption (or destruction) of normal platelets after their interaction with an abnormal vasculature. Future research using 111In platelets may yield further valuable information on the control as well as the significance of intrasplenic platelet pooling, on the role of platelets in the development of chronic vascular lesions, and on the sites of platelet destruction in ITP. With regard to the latter, methods will have to be developed for harvesting sufficient platelets representative of the total circulating platelet population from severely thrombocytopenic patients for autologous platelet labeling. This would avoid the use of homologous platelets, which is likely to be responsible for some of the contradictory data relating to the use of radiolabeled platelet studies for the prediction of the response of patients with ITP to splenectomy.     

111In oxine-labeled rabbit platelets: in vivo distribution and sites of destruction

We have studied the kinetics, biodistribution, and fate of autologous platelets labeled with 111In-oxine in rabbits. The initial recovery was 75% and mean survival time was 2.8 days when the data were analyzed by the multiple-hit gamma function model. Using a modified geometric mean for correction of attenuation, there was good correlation between the values obtained by in vivo quantification and those obtained by postmortem measurements of the radioactivity in the liver and the spleen (i.e., r = 0.854 and 0.899, respectively, n = 32). Using this method, it was shown that after infusion, the 111In-platelets rapidly accumulated in these two organs reaching 35% and 12% of the injected dose in the liver and spleen, respectively, by 1 day. Thereafter, there was little subsequent change. On the sixth day, when essentially all of the 111In-platelets had cleared from the circulation, a total of 82% of the injected dose was deposited in the three major reticuloendothelial organs: liver (40%), spleen (14%), and bone marrow (28%). Our results suggest that in addition to liver and spleen, bone marrow played an important role in sequestering platelets in rabbits.     

Platelet turnover and kinetics in immune thrombocytopenic purpura: results with autologous 111In-labeled platelets and homologous 51Cr- labeled platelets differ

Mean platelet survival and turnover were simultaneously determined with autologous 111In-labeled platelets (111In-AP) and homologous 51Cr- labeled platelets (51Cr-HP) in ten patients with chronic immune thrombocytopenic purpura (ITP). In vivo redistribution of the 111In-AP was quantitated with a scintillation camera and computer-assisted image analysis. The patients were divided into two groups: those with splenic platelet sequestration (spleen-liver 111In activity ratio greater than 1.4), and those with diffuse sequestration in the reticuloendothelial system. The latter patients had more severe ITP reflected by pronounced thrombocytopenia, decreased platelet turnover, and prominent early hepatic platelet sequestration. Mean platelet life span estimated with 51Cr-HP was consistently shorter than that of 111In-AP. Platelet turnover determined with 51Cr-HP was thus over-estimated. The difference in results with the two isotope labels was apparently due to greater in vivo elution of 51Cr. Although the limitations of the techniques should be taken into account, these findings indicate that platelet turnover is not always normal or increased in ITP, but is low in severe disease. We suggest that this may be ascribed to damage to megakaryocytes by antiplatelet antibody. The physical characteristics in 111In clearly make this radionuclide superior to 51Cr for the study of platelet kinetics in ITP.     

Platelet kinetics in stable atopic asthmatic patients

The kinetics of platelets labeled with indium-111 were investigated in 13 healthy subjects as well as in 9 patients in the asymptomatic interattack stage of asthma. The survival times of platelets in healthy subjects was 8.9 +/- 1 days; in asthmatic subjects it was 4.7 +/- 1.3 days (p less than 0.001). The survival curve is of a biexponential form in asthmatics, thus suggesting the presence of 2 populations: one with a short life span (23 +/- 7 h), representing a third of the total population (33 +/- 9%), and the other with a normal life span. No single preferred site of platelet sequestration was found. These results suggest the presence of functional or anatomic lesions of platelets in asthmatic patients, which can be explained only hypothetically at the present time.     

The relation of platelet density to platelet age: survival of low- and high-density 111indium-labeled platelets in baboons

The relationship between platelet density and platelet age has been studied using continuous linear Percoll density gradients and 111In- labeling of autologous platelets in baboons. To investigate changes in platelet density during senescence in the circulation, baboons were infused with 111In-labeled autologous platelets, and blood was collected at one hour postinfusion and twice daily thereafter for six days. Platelets were isolated from these samples in high yield (greater than 95%) and separated in continuous linear Percoll density gradients following density equilibrium centrifugation. Although at one hour postinfusion the density distribution of radiolabeled platelets coincided closely with the distribution of the total platelet population, a detectable symmetrical shift toward higher densities was observed after five days. The relative specific radioactivity (RSR) of high-density platelets (1.064 to 1.067 g/mL) decreased at a slower rate than that of the total platelet population (platelets of all densities), whereas the RSR of low-density platelets (1.053 to 1.056 g/mL) showed a more immediate and rapid decrease. These results give rise to one of two interpretations: (1) low-density platelets have a shorter survival time than more dense platelets and are therefore cleared from the circulation at a faster rate, or (2) platelets of all densities increase in density upon aging in the circulation. To determine the explanation for changing RSR of different density fractions we studied the in vivo disappearance characteristics of low- and high-density 111In-labeled platelets. There were no significant differences between the mean survival times of low-density platelets (5.0 +/- 0.49 days, +/- 1 SD, n = 6), high-density platelets (4.9 +/- 0.56 days, n = 6), or control platelets representing platelets of all densities (4.9 +/- 0.38 days, n = 6). Although a slight increase in the density of all platelets during platelet senescence is indicated by these studies, we conclude that platelet density heterogeneity is not primarily a consequence of age-related changes in platelet density.