Studies on the microcytotoxicity test. II. The uptake of amino acids ([3H]leucine or [75Se]methionine) but not nucleosides ([3H]thymidine or [125I]IUdR) or 51CrO24-provides a direct and quantitative measure of target cell survival in the presence of lymphoid cells

We have studied the suitability of various commonly used radioactive materials for the direct post-labeling of adherent rat target cells in long-term cytotoxicity tests. The use of nucleosides at high concentration avoids the necessity of adding fluorodeoxyuridine to enhance nucleoside uptake by target cells, and reduces the degree of non-specific inhibition of nucleoside uptake caused by products released from effector lymphoid cells. However, when [125I]iododeoxyuridine was used for labelling, such inhibition was not completely avoided even at very high nucleoside concentration, necessitating the washing of target cells prior to labelling. Similarly, without prewashing, the uptake of 51CrO24-ions frequently failed to correlate well with the numbers of surviving target cells as assessed by cell counting. On the other hand, radiolabelled amino acids, when present at semi-saturating concentrations, were taken up quantitatively by target cells under all conditions tested. Furthermore, in comparison to [125I]iododeoxyuridine, radioactive amino acids showed little if any toxicity to target cells. The use of the gamma-emitting amino acid analogue, [75Se]selenomethionine, is particularly recommended.

Clinical trial of four pancreatic scanning agents

Four 75Se-labelled amino acids have been tested in 22 patients. In 11 of the patients pancreatic function was estimated by the Lundh test. Scans at varying time intervals up to 30 hours from isotope administration were assessed by blind marking. Pancreatic visualization with all four agents was markedly poorer than would have been expected with 75Se-selenomethionine. Whole-body retention was estimated for all four agents; two showed faster 75Se clearance than selenomethionine. Radioactivity in duodenal aspirate was measured for two of the agents.

Effect of a low dose of vincristine on platelet production in mice

The effect of a single low dose (0.1 mg/kg) of vincristine (VCR) on platelet production was investigated in C57B1 mice. A parallel increase of circulating platelets and (75Se)-selenomethionine (Se-Met) uptake was observed. The total megakaryocyte count decreased insignificantly 6 hours after VCR injection, followed by an increase after 18 hours. Plasma taken from mice 24 hours after VCR injection was tested for thrombopoietic activity. The post-VCR plasma caused a significant thrombocytosis, increased Se-Met uptake and increased protein synthesis of the platelets, indicating an active overproduction of platelets. These results suggest that the thrombocytosis induced by a low dose of VCR is mediated by a thrombopoietin-like substance.

Metabolic studies in rats of 75Se incorporated in vivo into fish muscle

1. [75Se]selenite of [75Se]selenomethionine was injected in to the coelomic cavity of fish. After 2 d or 14 d the muscle portion of the fish was removed and homogenized. The long-term fate in rats of an oral dose of each labelled homogenate was compared with that of an oral dose of [75Ee]selenite or [75Se]selenomethionine mixed with unlabelled fish homogenate. 2. Urinary and faecal radioactivity were measured during the 1st week and whole-body radioactivity was determined for 10 weeks. Rats were killed at weekly intervals for 4 weeks for analysis of tissue distribution of 75Se. 3. Intestinal absorption of 75Se given as labelled fish homogenate was less complete than that of 75Se mixed with unlabelled homogenate, and the absorption of 75Se from the 14 d-labelled fish homogenate derived from [75Se]selenite was less complete than that of 75Se from the other labelled homogenates. 4. Urinary excretion of absorbed 75Se in the first 7 d was in the range 5--8% absorbed dose and was slightly greater in the rats given 75Se as selenite or derived from selenite than in those given 75Se as selenomethionine or derived from selenomethionine. Endogenous faecal excretion of absorbed Se was similar in all groups, as also were tissue distribution of retained 75Se and long-term whole-body turnover rate. 5. The results of these studies are compared with those of earlier studies of the metabolism in rats of [75Se]selenomethionine, [75Se]selenite, [75Se]selenocystine and 75Se incorporated in vivo into rabbit kidney. There were differences in the initial utilization of 75Se from these various sources but after the 1st week 75Se from all sources appeared to be metabolized similarly, suggesting that for rats dietary Se of all forms is ultimately incorporated into the same metabolic pool.

[Studies on amino acid kinetics in rats without and with interference of the pregnancy using 75-Se-methionine (author's transl)]

A collective of 60 rats was divided into 3 groups of 20 non-pregnant, 20 pregnant without interference of the pregnancies and 20 pregant with ligature of the uterine vessels to induce an intrauterine growth retardation. The organ distribution of 100 muC intravenously injected 75-Se-methionine, which is actively transported from the placenta into the fetal circulation, was computer-scintigraphically measured. After 30 minutes and 6 hours typical differences, amongst the 3 groups, between the liver and foetoplacental unit methionine uptake were demonstrated both visually and quantitatively. On the basis of this experimental animal model, it should be attempted to measure quantitatively the placental disorders which are not caused primarily by perfusion of the uteroplacental unit.

Specific thrombopoietic inhibition by syngeneic platelet homogenates

Injections of 1 to 2.5 X 10(8) syngeneic, uninjured platelets did not diminish the circulating platelet count nor the bone marrow megakaryocyte content in mice, and did not influence the incorporation of 75selenomethionine into circulating platelets. However, platelet homogenates, prepared by repeated freezing and thawing of identical amounts of syngeneic paltelets induced a dose-dependent thrombocytopenia along with a diminution on bone marrow megakaryocyte content, and a decrease in 75 selenomethionine incorporation. Other circulating blood cell counts were not diminished after platelet homogenates, and three intravenous doses of homogenates prepared from 0.7 to 1 X 10(5) syngeneic buffy coat cells failed to influence circulating platelet count or 75 selenomethionine incorporation.

The effects of hemorrhage, hypoxia, and a preparation of erythropoietin on thrombopoiesis

The effects of hemorrhage, hypoxia, or a preparation of erythropoietin on platelet production were investigated by measuring incorporation of selenomethionine-75Se (75SeM) into the platelets of rabbits or mice. Rabbits that were bled daily for 5 days had a significant increase in the platelet count, 48 hours after cessation of hemmorrhage, that coincided with a threefold increase in isotope incorporation into platelets. Mice that were bled daily for 3 days also had significantly higher platelet counts and a 38 per cent increase in incorporation of isotope into platelets, 3 days after the last hemorrhage. Normal rabbits, injected with plasma from repeatedly bled, anemic, and moderately thrombocytopenic rabbits, had a 58 per cent greater maximum incorporation of 75SeM than did control animals. Mice exposed to hypoxia for 6 days had a mean platelet count 23 per cent lower than normal controls, but no change in incorporation of 75SeM into platelets. Plasma from hypoxic mice did not stimulate platelet production when injected into normal mice. A preparation of human urinary erythropoietin (15 to 30 U. per mouse or 30 to 120 U. per rabbit) significantly increased incorporation of isotope into the platelets of normal mice and rabbits. The results demonstrated that hemorrhage, but not hypoxia, is associated with increased thrombopoietic activity in plasma. However, large doses of preparations of human erythropoietin contained detectable thrombopoietic activity.

Further clinical evaluation of the selenomethionine uptake test

A review of the selenomethionine uptake ratio as a measure of placental function is presented. Attempts at the early identification of the poorly growing fetus were unsuccessful, and the previously published close correlation of results with clinical findings in later pregnancy has not been confirmed. The theoretical basis of the test has been re-examined. Although modification much reduced the incidence of technically unsatisfactory readings, other difficulties prevented the test from being clinically useful. To some extent the aim of the test has been superseded by concurrent developments and advances in care of the premature baby.

Incorporation of 75Se-selenomethionine into human apoproteins. III. Kinetic behavior of isotopically labeled plasma apoprotein in man

The metabolism of lipoprotein-apoprotein was examined in four subjects with normal lipid metabolism and in one subject with type II hyperlipemia by means of isotopic tracer methodology. Studies were performed after intravenous injection of a radioactive amino acid precursor for apoprotein synthesis (75Se-selenomethionine), in both the basal state and following the acute injection of intravenous heparin. Computer technics were used to evaluate a series of multicompartmental models, and a general model is proposed that yields optimum fitting of experimental data for serum free amino acid precursor, very-low-density lipoprotein-apoprotein (VLD-apoprotein), and low-density lipoprotein-apoprotein (LDL-apoprotein) in man. The analysis demonstrates that approximately half of the transport of 75Se-apoVLDL from the plasma VLDL pool is converted to 75Se-apoLDL. The acute injection of heparin in two normal subjects results in a two-and-a-half-fold increase in this rate of conversion of 75Se-apoVLDL to 75Se-apoLDL. 75Se-apoLDL is metabolized by rapid transport into a recycling extravascular pool and by irreversible catabolism. The fractional rate of recycling is large relative to the fractional rate of catabolism of apoLDL (3.7:1.0), suggesting extravascular recycling as a potential site of regulation of the plasma concentration of apoLDL. In a patient with type II hyperlipemia, the extravascular recycling pathway is reduced and is not corrected with D-thyroxine therapy. However, this therapy did reduce conversion of apoVLDL to apoLDL in this type II patient. The kinetic data support the validity of the compartmental model in simulating both normal and pathologic apoprotein metabolism and that perturbation of physiology seen with heparin injection and D-thyroxine therapy. These data support a quantitative role of apoVLDL as a precursor of apoLDL and identify an important recycling pathway of apoLDL metabolism in addition to that of catabolism.

The metabolism of [75Se]selenomethionine in four women

1. The long-term fate of an oral dose [75Se]selenomethionine was studied in four women. 2. Urinary and faecal excretion, respiratory losses and whole-body retention of 75Se were measured, and also 75Se turnover in whole body, plasma and erythrocyte during a period of 33-34 weeks. 3. Intestinal absorption of [75Se]selenomethionine by the four subjects was 95.5-97.3% of the administered dose. 4. Urinary excretion accounted for 6-9% of absorbed 75Se in the first 2 weeks. No radioactivity was detected in expired air.

Biological potency of selenium from sodium selenite, selenomethionine, and selenocystine in the chick

Experiments were conducted to determine the relative effectiveness of selenium (Se) from sodium selenite, selenomethionine and selenocystine for promoting weight gain and preventing exudative diathesis. The chicks used were hatched from eggs low in Se. They were fed a basal diet made up mostly of corn (low in Se) and torula yeast or the basal diet supplemented with various levels of Se from sodium selenite, selenomethionine, or selenocystine. At 10 mug. of added Se per kg of diet, sodium selenite and selenocystine were about equal in promoting weight gain and preventing exudative diathesis. Selenomethionine was less effective. Tissues from chicks fed the various Se sources providing 60 mug. Se per kg of diet for four weeks were analyzed for Se. The content of tissues from chicks fed sodium selenite or selenocystine was similar. Chicks fed selenomethionine had a higher concentration of Se in the pancreas and breast muscle than chicks fed the other two Se sources, but a lower concentration in the kidney, liver, and heart. The level of Se in the kidney, liver, or heart which a Se source produces seems to be more important for preventing exudative diathesis than that which is found in the pancreas or muscle.

Use of (75Se)L-Selenomethionine as a label for lymphoid cells

[75Se]L-selenomethionine can be employed as a label for following lymphocytes in vivo. Its properties are broadly similar to those of 51Cr with regard to uptake, release of the isotope on cell death and reutilization; thus relatively high levels of 75Se in lymphoid tissue indicate the presence of viable labelled cells. It differs from 51Cr in that active release of selenium occurs from the labelled cells when the ambient cell concentration is low. No significant reutilization of leaked label occurs. Leakage of 75Se label occurs in vivo; this differs in the spleen and lymph nodes. Evidence is presented to suggest that 75Se losses reflect the transit times of cells through lymphoid organs.

External detection of axoplasmic flow using radionuclides

In order to investigate the possibilities of studying the axoplasmic flow in vivo by means of radionuclide scanning 75Se-methionine was injected into the L6, L7 or S1 spinal ganglia of cats and the spread of the isotope recorded with a gamma camera. -2. After 4 hours the area of isotope distribution extended 1-2 cm cranially and caudally in the spinal cord and a further movement of 1-2 cm was noted during the following 2-5 days. -3. Scanning of removed specimens of the spinal cord, roots, ganglia and nerves confirmed the intramedullary uptake and in addition revealed an uptake in the dorsal root. Determination of the specific radioactivity showed the presence of the isotope in the ventral root and spinal nerve also, however, sufficient for scintigraphy. -4. Microautoradiographic studies with 3H-methionine demonstrated isotope uptake in axons of the dorsal roots and root entry zone of the spinal cord. Hence, the spread of the isotope was likely to have occurred by axoplasmic flow. This seems to provide a basis for further in vivo studies of this phenomenon.

Incorporation of 75Se-selenomethionine into human apoproteins. II. Characterization of metabolism of very-low-density and low-density lipoproteins in vivo and in vitro

This investigation is designed to explore the potential role of apo VLDL as a precursor of a polypeptide component of human LDL. Attention was directed to the chromatography-defined Sf-I polypeptide fraction of apo VLDL, which has been previously shown to be immunologically and chemically indistinguishable from the major component of apoLDL.1-3 In VLDL isolated from bloow drawn within two hours following 75Se-SM injection, the Sf-I polypeptide fraction of apo VLDL was highly enriched with isotope, providing an appropriate preparation for in-vitro tracer studies. Conversion of 75Se-VLDL to 75Se-LDL occurred in vitro in the presence of normal plasma at 37 degrees C., and this conversion was augmented by post-heparin plasma. No conversion to HDL lipoproteins could be detected. Injection of heparin in vivo resulted in acute reciprocal changes in the radioactivity contained within serum apo VLDL and apoLDL. These findings suggest that a component of the Sf-I polypeptide fraction of apo VLDL can be metabolized into the apoprotein of LDL in man. Thus, the biochemical and immunologic similarities between the Sf-I fractions of apoVLDL and apoLDL may result from a physiologic "precursor-product" relationship between the apoprotein moieties of these two lipoprotein species. A method for further investigation of the metabolism of human apoprotein is suggested.

Incorporation of 75Se-selenomethionine into human apoproteins. I. Characterization of specificity in very-low-density and low-density lipoproteins

Characterization of specificity of incorporation of 75Se-selenomethionine into human apoproteins following in-vivo injection of the radioactive amino acid precursor is reported. Apoprotein fractionation was attained by serial ultracentrifugation, anhydrous partial delipidization, decyl sulfate solubilization with Sephadex G-150 gel filtration into polypeptide fractions, Sf-I, and Sf-II, for VLDL, and LDL subfractions. Similarity of the Sf-I fractions isolated from VLDL, LDL1, and LDL2 was defined by gel exclusion volume, disk-gel electrophoresis, immunodiffusion, and amino acid composition. Enrichment of the Sf-I fractions with 75Se-SM in VLDL and LDL subfractions was observed, suggesting a possible use for this isotopic method for the investigation of apoprotein metabolism.

Metabolic studies of [75Se]selenocystine and [75Se]selenomethionine in the rat

1. The long-term fate in rats of an oral dose of [75Se]selenocystine was compared with that of an oral dose of [75Se]selenomethionine. 2. Urinary and faecal radioactivities were measured during the 1st week and whole-body radioactivity was determined for 10 weeks. Rats were killed at weekly intervals for 4 weeks and at weeks 6 and 10 for analysis of tissue distribution of 75Se. 3. Intestinal absorption of [75Se]selenocystine was 81% of the administered dose; that of [75Se]selenomethionine was 86%. Urinary excretion of absorbed [75Se]selenocystine was 13-9% and that of [75Se]selenomethionine was 5-8% in the 1st week. 4. Whole-body retention of 75Se was greater for [75Se]selenomethionine than for [75Se]selenocystine but after the 1st week it decreased at a similar rate in both groups. Tissue distribution of retained 75Se was also similar in both groups. 5. The initial utilization of [75Se]selenocystine was different from that of [75Se]selenomethionine. However, after the 1st week 75Se from both sources appeared to be metabolized similarly, suggesting that dietary Se of both forms is ultimately incorporated into the same metabolic pool. 6. When these findings were compared with those of earlier studies with [75Se]selenite and 75Se incorporated in vivo into rabbit kidney (RK-64Se) (Thomson, Stewart & Robinson, 1975) the metabolism of [75Se]selenocystine resembled that of [75Se]selenite and RK-75Se, rather than that of [75Se]selenomethionine.

The effect of massive liver resection on platelet kinetics in the rat

The effect of partial hepatectomy on platelet kinetics has been assessed in the rat. Four-fifths hepatectomy resulted in thrombocytopenia reaching 50 per cent of control platelet counts on the fifth postoperative day. Platelets were found to accumulate in the sinusoids of the liver remnant after partial hepatectomy. Once correction was made for changing blood volume, it was found that during the first two postoperative days 50 per cent of platelets had disappeared from the blood. After this, platelet numbers fell slowly for a further three days following which recovery commenced. Neither 51Chromium labeled platelet survival nor total organ pooling of platelets varied from the normal after four-fifths hepatectomy. Although platelets were shown to accumulate in the liver remnant the numbers involved did not contribute significantly to the thrombocytopenia. The mechanism responsible for the thrombocytopenia was suppression of entry of platelets into the peripheral blood. The observed effect was specific for platelets; erythropoiesis was not suppressed. Megakaryocytes showed a decrease in average diameter and ploidy value, a change seen also after physiologic suppression of platelet production; these findings raise the possibility that the liver is concerned in the elaboration of thrombopoietin. However, as four-fifths hepatectomy results in immediate cessation of entry of platelets into the peripheral circulation, a phase of platelet production in which the principal physiologic regulator has hitherto been considered to have no effect, a second mechanism must be invoked affecting the final step of platelet production.

Determination of platelet and fibrinogen half-life with [75Se]selenomethionine: studies in normal and in diabetic subjects

1. Simultaneous platelet and fibrinogen survival studies were carried out on a group of seven normal persons and eleven diabetic patients. 2. Survival time was calculated: (a) by measuring the interval between 50% of the maximum radioactivity in the anabolic phase and 50% of the maximum radioactivity in the catabolic phase, and (b) by fitting an exponential function to the decay phase of the curve. 3. With the first method, the normal group had a mean plaetelet survival which did not differ significantly from that in the diabetic group. With the second method, however, the platelet half-life in the diabetic patients was significantly shorter than in the normal subjects. 4. Fibrinogen survival was significantly shorter in the diabetic group with either method. 5. It is concluded that there is an increased utilization of platelets and fibrinogen in diabetic subjects.

A comparison of the uptake of [75Se]selenite, [75Se]selenomethionine and [35S]methionine by tissues of ewes and lambs

The fate of selenium, given as Na2(75)SeO3, or [75Se]selenomethionine, and of [35S]methionine administered intravenously to ewes and lambs, has been examined. The main intention was to follow the incorporation of selenium into protein in a number of tissues, including liver and kidney, and to measure the extent of that incorporation of selenoamino acid, particularly with respect to the administration of selenite. The ewes chosen were lactating ewes with lambs at foot, and the lambs were animals which had been weaned on to fodder low in selenium and were recovering from white muscle disease with selenium therapy. These two experimental situations were chosen as they offered conditions under which selenium incorporation might be considered to be maximal. Entry of isotope into milk was rapid and was greater when 75Se was given as the selenoamino acid than as selenite. In both ewes and lambs greater amounts of activity, derived from selenite, were bound to plasma proteins than to the proteins of milk. This was particularly evident in samples taken some hours after administration. This ability of the plasma to bind selenium was demonstrated by alkaline dialysis. Small, though significant amounts of selenium, derived from Na2(75)SeO3, were incorporated as selenoamino acids into the proteins of liver, kidney and pancreas, as well as into the proteins of milk and plasma. In ewes, both selenomethionine and selenocystine were identified chromatographically in enzyme digests of defatted liver and kidney. Some differences occurred in the distribution of labelled compounds in organs from lactating ewes and recovering lambs. The incorporation of selenium into protein is discussed briefly in relation to the recent findings of an association between selenium and the enzyme glutathione peroxidase.