Mechanism of action of thyroid hormones on erythrocyte 2,3-diphosphoglyceric acid synthesis

Molecular Interactions between Erythrocytes and the Endocrine System

Hormones are secreted by the endocrine glands and reach their targets after circulating in the blood. Many studies have documented that erythrocytes can bind hormones, and possible interactions have been reported. Erythrocytes are responsive to signaling initiated after binding of epinephrine, norepinephrine, estrogen, progesterone, thyroid hormones, parathyroid hormone, and angiotensin. Signaling results in regulation of cellular metabolism and membrane fluidity. In addition, erythrocytes are circulating pools for dopamine, thyroid hormones, cortisol, and aldosterone. Erythrocyte function and structure are regulated by endocrine signals, while erythrocytes are important constituents for the transport of hormones in the body.

Changes in serum hepcidin according to thyrometabolic status in patients with Graves' disease

INTRODUCTION

Hepcidin is an acute-phase protein and a key regulator of iron homeostasis. Anaemia frequently occurs in patients with thyroid dysfunction, and hepcidin may be a potential link.

OBJECTIVES

Prospective assessment of hepcidin serum concentration and other parameters related to Fe homeostasis in hyperthyroid patients in the course of GD at diagnosis and during remission.

PATIENTS AND METHODS

Out of 70 patients recruited, 42 (32 women, 10 men), aged 42.5±15.1 years, met the inclusion criteria. Clinical and biochemical assessment, including hepcidin measurement by ELISA, was performed at baseline (T0) and after restoration of euthyroidism (T1).

RESULTS

Hepcidin concentration at T0 in the 24 patients who completed the study was significantly higher than the value during euthyroidism (28.7 (8.1-39.4) ng/mL vs. 7.9 (4.3-12.9) ng/mL, p<0.001). Hepcidin level was most significantly correlated with ferritin (rho = 0.723) in women at T0. In both men (377 (171-411) vs. 165 (84-237) ng/mL, p=0.001) and women (84 (23-104) vs. 35 (16-64) ng/mL, p=0.001), a significant decrease in ferritin level was demonstrated following therapy. A significant (p<0.001) increase in mean corpuscular volume (MCV) (83.5 (82.5-87.1) vs. 89.5 (88.8-90.0) fL) and mean concentration of haemoglobin (MCH) (29.0 (28.0-29.4) vs. 30.4 (29.5-31.1) pg) was observed.

CONCLUSIONS

Hepcidin and ferritin decrease significantly during the transition from a hyperthyroid state to euthyroidism in patients with GD. The observed changes occur in parallel to iron homeostasis fluctuations. During the transition from the hyperthyroid state to euthyroidism, the improvement of haematological status is reflected mainly by the increase in MCV and MCH.

Diabetes, trekking and high altitude: recognizing and preparing for the risks

Although regular physical activity is encouraged for individuals with diabetes, exercise at high altitude increases risk for a number of potential complications. This review highlights our current understanding of the key physiological and clinical issues that accompany high-altitude travel and proposes basic clinical strategies to help overcome obstacles faced by trekkers with Type 1 or Type 2 diabetes. Although individuals with diabetes have adaptations to the hypoxia of high altitude (increased ventilation, heart rate, blood pressure and hormonal responses), elevated counter-regulatory hormones can impair glycaemic control, particularly if mountain sickness occurs. Moreover, high-altitude-induced anorexia and increased energy expenditure can predispose individuals to dysglycaemia unless careful adjustments in medication are performed. Frequent blood glucose monitoring is imperative, and results must be interpreted with caution because capillary blood glucose meter results may be less accurate at high elevations and low temperatures. It is also important to undergo pre-travel screening to rule out possible contraindications owing to chronic diabetes complications and make well-informed decisions about risks. Despite the risks, healthy, physically fit and well-prepared individuals with Type 1 or Type 2 diabetes who are capable of advanced self-management can be encouraged to participate in these activities and attain their summit goals. Moreover, trekking at high altitude can serve as an effective means to engage in physical activity and to increase confidence with fundamental diabetes self-management skills.

Effects of high-altitude hypoxia on the hormonal response to hypothalamic factors

Acute and chronic exposure to high altitude induces various physiological changes, including activation or inhibition of various hormonal systems. In response to activation processes, a desensitization of several pathways has been described, especially in the adrenergic system. In the present study, we aimed to assess whether the hypophyseal hormones are also subjected to a hypoxia-induced decrease in their response to hypothalamic factors. Basal levels of hormones and the responses of TSH, thyroid hormones, prolactin, sex hormones, and growth hormone to the injection of TRH, gonadotropin-releasing hormone, and growth hormone-releasing hormone (GHRH) were studied in eight men in normoxia and on prolonged exposure (3-4 days) to an altitude of 4,350 m. Thyroid hormones were elevated at altitude (+16 to +21%), while TSH levels were unchanged, and follicle-stimulating hormone and prolactin decreased, while leutinizing hormone was unchanged. Norepinephrine and cortisol levels were elevated, while no change was observed in levels of epinephrine, dopamine, growth hormone (GH), IGF-1, and IGFBP-3. The mean response to hypothalamic factors was similar in both altitudes for all studied hormones, although total T4 was lower in hypoxia during 45 to 60 min after injection. The effect of hypoxia on the hypophyseal response to hypothalamic factors was similar among subjects, except for the GH response to GHRH administration. We conclude that prolonged exposure to high-altitude hypoxia induces contrasted changes in hormonal levels, but the hypophyseal response to hypothalamic factors does not appear to be blunted.

Pathogenesis of mental deficiency in trisomy 21

In trisomy 21, pathogenesis of mental retardation is still poorly understood although the knowledge of the genic content of chromosome 21 is steadily increasing. Short of discovering how to silence selectively one of the 3 chromosomes 21, no rational medication can be envisaged before pathogenesis has been unraveled, at least partially. A biochemical scheme of impairment of mental efficiency is presented. Secondarily, the possible deleterious effects of a given gene overdose are discussed. Cu/Zn SOD, cystathionine beta synthase, S 100 beta protein, phosphofructokinase, purine synthesis and adenosine pharmacology, thyroid disturbance, and elevated TSH with low rT3 as well as biopterine metabolism interferences are reviewed. It is observed that the metabolic paths controlled by these genes, although unrelated at first glance, are in fact tightly related by their effects, just as if synteny was in some way related to biochemical cooperation or mutually controlled regulation. Experiments in vitro have demonstrated a peculiar sensitivity of trisomic 21 lymphocytes to methotrexate. From this starting point, systematic research of special sensitivities has begun. Clinical observations and relevant statistical methods allow study of the speed of mental development under various medications. The interest of regulating thyroid metabolism, when needed, is exemplified. Reequilibration of monocarbon metabolism is discussed and the seemingly favourable effect of folinic acid medication in pseudo-Alzheimer complication is presented.

Effects of thyroid hormone and hypoxia on 2,3-bisphosphoglycerate, bisphosphoglycerate synthase and phosphoglycerate mutase in rabbit erythroblasts and reticulocytes in vivo

OBJECTIVES

The effects of triiodothyronine (T(3)) and hypoxia on 2,3-bisphosphoglycerate (2,3-BPG) studied in vitro are unclear. To clarify these effects we selected a more physiologic approach: the in vivo study in rabbits. We also present the changes produced by T(3) and hypoxia on phosphoglycerate mutase (PGAM), which requires 2,3-BPG as a cofactor, and 2,3-BPG synthase (BPGS), the enzyme responsible for 2,3-BPG synthesis in erythroblasts and reticulocytes.

METHODS

Hyperthyroidism was induced by daily T(3) injection (250 microg/kg), hypoxia by a mixture of 90% nitrogen and 10% oxygen and hypothyroidism by propylthiouracil (PTU) added to drinking water.

RESULTS

Both T(3) administration and hypoxic conditions increased 2,3-BPG levels and BPGS mRNA levels and activity in erythroblasts but not in reticulocytes. Unlike BPGS, both PGAM mRNA levels and activity were increased in erythroblasts and reticulocytes under hyperthyrodism and hypoxia. The antihormone PTU produced opposite effects to T(3).

CONCLUSION

The results presented here suggest that both hyperthyroidism and hypoxia modulate in vivo red cell 2,3-BPG content by changes in the expression of BPGS. Similarly, the changes in PGAM activity are also explained by changes in its expression.

Free and total thyroid hormones in humans at extreme altitude

Alterations in circulatory levels of total T4 (TT4), total T3 (TT3), free T4 (FT4), free T3 (FT3), thyrotropin (TSH) and T3 uptake (T3U) were studied in male and female sea-level residents (SLR) at sea level, in Armed forces personnel staying at high altitude (3750 m) for prolonged duration (acclimatized low-landers, ALL) and in high-altitude natives (HAN). Identical studies were also performed on male ALL who trekked to an extreme altitude of 5080 m and stayed at an altitude of more than 6300 m for about 6 months. The total as well as free thyroid hormones were found to be significantly higher in ALL and HAN as compared to SLR values. Both male as well as female HAN had higher levels of thyroid hormones. The rise in hormone levels in different ALL ethnic groups drawn from amongst the southern and northern parts of the country was more or less identical. In both HAN and ALL a decline in FT3 and FT4 occurred when these subjects trekked at subzero temperatures to extreme altitude of 5080 m but the levels were found to be higher in ALL who stayed at 6300 m for a prolonged duration. Plasma TSH did not show any appreciable change at lower altitudes but was found to be decreased at extreme altitude. The increase in thyroid hormones at high altitude was not due to an increase in hormone binding proteins, since T3U was found to be higher at high altitudes.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of thyroid hormones on oxygen and glucose metabolism in the sheep fetus during late gestation

1. The effects of thyroid hormones on fetal metabolism during late gestation were examined by measuring the rates of glucose and oxygen utilization rates in chronically catheterized sheep fetuses made hypothyroid by either fetal thyroidectomy (TX) or hypophysectomy (HX). The values were compared with those in intact fetuses and in thyroxine (T4)-treated TX and HX fetuses. 2. Umbilical O2 uptake expressed on a weight-specific basis was reduced by 20-30% in the hypothyroid fetuses and was restored to normal values when plasma T4 levels were maintained in the TX and HX fetuses by exogenous T4 administration. 3. The low O2 consumption rates of the untreated hypothyroid fetuses were accompanied by fetal growth retardation, an abnormal blood gas status, and in the TX fetuses, by significant reductions in the rates of glucose oxidation, CO2 production from glucose carbon and O2 utilization for glucose oxidation. 4. When T4 levels were maintained in the TX fetuses, these metabolic rates and fetal blood gas status were restored to their normal values. Replacement of T4 also sustained growth in TX but not in HX fetuses. 5. When the data from all fetuses were combined irrespective of treatment, there were significant positive correlations between plasma levels of T4 (but not triiodothyronine (T3)) and the rates of umbilical O2 uptake, glucose oxidation, CO2 production from glucose carbon and O2 utilization for glucose oxidation in the individual fetuses. 6. These findings demonstrate that T4 is a physiological regulator of O2 utilization by the sheep fetus close to term.

[In vitro and in vivo effects of propofol on the hemoglobin dissociation curve]

This study assessed in vivo and in vitro the effects of propofol on the affinity of hemoglobin for oxygen in seven ASA 1 adults. For the in vivo study, venous blood samples were withdrawn before and after premedication, after the injection of 2.5 mg.kg-1 of propofol and after 15 minutes of maintenance at an infusion rate of 0.2 mg.kg-1.min-1. For the in vitro study, propofol was added to the blood withdrawn before premedication in order to obtain two samples at a concentration of 1 microgram.mL-1 and 2 micrograms.mL-1 respectively. Propofol changed neither in vivo, nor in vitro, the P50 and the number of Hill indicating the angle of the slope of the dissociation curve, nor in vivo the concentration of 2,3-DPG.

Effect of sevoflurane on P50 and on measurement of oxygen tension

Fresh samples of heparinized human blood from 10 healthy nonsmoking volunteers were used to study the effect of the inhaled anesthetic sevoflurane on the oxygen half-saturation pressure of hemoglobin (P50) and on polarographic measurements of oxygen tension at low values. Control samples had a baseline P50 of 26.9 +/- 0.2 mm Hg. When the blood samples were exposed to 1.75% (1 minimum alveolar concentration, MAC), 2.75%, and 3.5% (2 MAC) of sevoflurane, the P50 values were 27.0 +/- 0.5 mm Hg, 27.1 +/- 0.4 mm Hg, and 26.9 +/- 0.5 mm Hg, respectively. Our present data show that 1 to 2 MAC sevoflurane has no significant effect on P50 (P greater than 0.05). Our data also show that sevoflurane did not interfere with polarographic measurements of oxygen tension (P greater than 0.05). Other inhaled agents--halothane, enflurane, and isoflurane--do interfere with these measurements, and we cannot explain the difference.

Androgen therapy in haemodialysis patients. II. Effects on red cell metabolism

Activities of the red cell enzymes hexokinase, glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, lactic dehydrogenase and aspartate aminotransferase were measured in 17 chronic haemodialysis patients receiving androgen therapy, 15 untreated chronic haemodialysis patients and 15 normal subjects. Compared to normal subjects, untreated haemodialysis patients had similar reticulocyte counts but significantly increased levels of all five enzymes studied. This finding suggests the presence of a younger red cell population in the peripheral blood and is consistent with the shortened red cell survival known to occur in this clinical setting. Red cell enzyme activities in untreated haemodialysis patients were significantly correlated with one another and with the serum phosphate level. Moreover, in this population, red cell DPG content was directly related to hexokinase and glucose 6-phosphate dehydrogenase activities while haemoglobin-oxygen affinity (P50) was inversely related to all five enzyme activities. In contrast, in androgen-treated haemodialysis patients, despite higher reticulocyte counts, red cell enzyme activities were the same or lower than those in the untreated haemodialysis group and only slightly higher than those in normal subjects, suggesting an overall older red cell population. Moreover, relationships of red cell enzymes to one another, to serum phosphate levels and to both red cell DPG content and haemoglobin-oxygen affinity were significantly different in androgen-treated subjects than in the untreated haemodialysis group. These changes are consistent with a direct effect of androgens on red cell metabolism and an improved red cell survival during androgen therapy.

Androgen therapy in hemodialysis patients: I. Effects on red cell oxygen transport

Measures related to red cell oxygen transport were evaluated in 17 hemodialysis patients receiving androgen therapy, 15 untreated hemodialysis patients and 15 normal subjects. Hemoglobin levels were higher in androgen-treated patients than in the untreated population and were directly related to the reticulocyte index. Hill coefficients were normal, and in vivo P5O values were increased to the same degree in both dialysis groups. However, DPG and serum phosphate explained 70% and 12%, respectively, of the variance in P5O in untreated patients but only 2% and 5% in androgen-treated subjects. In contrast, sample pH explained 34% of the variance in P5O in the androgen-treated group and less than 1% in the untreated dialysis population. Despite the relative importance of pH as a determinant of P5O in patients on androgen therapy, the Bohr coefficient in this group was only about half of that in untreated dialysis subjects. Androgen-treated patients also had lower red cell ATP levels. Finally, the expected correlation of MCHC with pH was noted in untreated dialysis subjects but not in patients receiving androgens. We conclude that androgen therapy in hemodialysis patients in addition to increasing red cell production, directly alters red cell metabolism. Moreover, although the androgen regimens used did not change the net oxygen transport characteristics of hemoglobin, they decreased the responsiveness of hemoglobin-oxygen affinity to changes in pH, DPG and phosphate. Thus, red cell adaptation to changes in oxygen supply and/or demand may be limited in androgen treated patients, and the improvement in clinical performance expected from androgen-stimulated erythropoiesis may not be realized.

2,3-diphosphoglyceric acid changes in uremia and during hemodialysis

The present study is an investigation of the mechanism of changes in erythrocyte 2,3-diphosphoglyceric acid (2,3-DPG) in patients with chronic renal failure during hemodialysis. The study was conducted on 7 Korean and 6 American patients on maintenance hemodialysis. The plasma pH of Korean patients was 7.31±0.02 before hemodialysis and 7.40±0.04 after hemodialysis (p<0.001). The pH of erythrocyte also increased from 7.13±0.02 to 7.20±0.03. The concentration of hemoglobin 2,3-DPG in Korean patients was 10.86±2.89 μmol/g before hemodialysis and 19.93±2.89 μmol/g after hemodialysis (p<0.001). Similar results were obtained in American patients. Hemoglobin 2,3-DPG was 12.54±2.53 and 18.76±6.73 μmol/g before and after dialysis respectively. Despite the presence of substantial anemia, hemoglobin 2,3-DPG prior to hemodialysis was significantly lower than the values obtained in the normal controls (17.45±4.3 μmol/g). The blood glucose increased from 93.3±8.5 mg/dl before dialysis to 117.1±6.1 mg/dl after hemodialysis in Korean patients but no significant change was detected in American patients. The increased blood glucose with hemodialysis observed in Korean patients was probably attributable to the lower baseline glucose concentration and the gain of glucose from dialysate, which had a glucose concentration of 186±34.1 mg/dl. The results suggest that the increase in 2,3-DPG with hemodialysis is probably caused by an increase of pH and an increased glucose utilization.

Elevated serum angiotensin-converting enzyme in hyperthyroidism

Serum angiotensin-converting enzyme was elevated in patients with hyperthyroidism (72 +/- 31 nmol/minute/ml, n = 12, p less than 0.001) but not in patients with hypothyroidism (38 +/- 3, n = 3) or thyroiditis (26, n = 1), and was positively correlated in 23 patients with serum thyroxine concentration (r = 0.60, p less than 0.01) and triiodothyronine resin uptake (r = 0.56, p less than 0.01). Triiodothyronine failed to enhance the synthesis of angiotensin-converting enzyme in rabbit alveolar macrophages or in human monocytes in culture, suggesting that the increased serum enzyme is a consequence of an effect other than increased angiotensin-converting enzyme synthesis. Hyperthyroidism should be considered in the evaluation of serum angiotensin-converting enzyme for the diagnosis and management of sarcoidosis.

Effect of 1,25-dihydroxycholecalciferol and 1,25-dihydroxycholecalciferol glycoside on 2,3-diphosphoglycerate levels of the rat erythrocyte

The erythrocytes of rats treated with 1, 25-dihydroxycholecalciferol or 1, 25-dihydroxycholecalciferol glycoside showed decreased levels of 2, 3-diphosphoglycerate. The same result has been obtained in vitro, indicating a direct effect of the sterol on the red cell. The glycoside is less active than the free sterol in vivo and more active in vitro. The decreased levels of diphosphoglycerate induced tissue hypoxia as shown by a higher plasma lactate/pyruvate ratio and a three fold increase in plasma erythropoietin concentration.

2,3-Diphosphoglycerate: its role in health and disease

2,3-Diphosphoglycerate (2,3-DPG) was first discovered and isolated in 1925. However, it was not until 1967 that the function of 2,3-DPG was explained. This resulted in multiple research projects devoted to elucidating the mechanism by which 2,3-DPG exerts it effect on the oxygen affinity of hemoglobin. In addition, a vast amount of research has been devoted to assessing the role of 2,3-DPG in oxygen transport in various physiological and pathophysiological states. In many instances, the results of this research have produced conflicting data which have dampened the initial enthusiasm which followed the discovery of the function of 2,3-DPG. However, much of this conflicting data can be explained by the fact that 2,3-DPG is only one of a number of factors influencing the transport of oxygen to the tissues. Several of these factors influence oxygen transport independently as well as by altering the synthesis of 2,3-DPG and modifying its effect on hemoglobin. In spite of the conflicting results, the overall data gathered thus far appears to be sound enough to warrant the extensive research now being done, particularly in the area of blood storage and transfusion therapy.

Effect of thyroid hormone on the levels of erythrocyte carbonic anhydrase isozymes and 2,3-diphosphoglycerate in rabbits

Levels of rabbit erythrocyte carbonic anhydrase B and C isozymes were determined in experimental hyperthyroidism using a quantitative immunologic technique. Levels of erythrocyte 2,3-diphosphoglycerate and protein binding iodine were simultaneously determined. Thyroxine and 3,5,3'-triiodothyronine were administered to rabbits orally for 30 days. A significant decrease in carbonic anhydrase B type was observed after 30 days, although no significant change was observed in carbonic anhydrase C type. These findings suggest that the steady state level of carbonic anhydrase B type in red cells is affected by thyroid hormone more readily than that of carbonic anhydrase C type. The level of red cell 2,3-diphosphoglycerate increased markedly after 10 days of treatment, corresponding to the increase of protein binding iodine. The clinical or pathologic significances were discussed in relation to the changes in the levels of these isozymes and 2,3-diphosphglycerate in red cells.

Altered hemoglobin-oxygen affinity with long-term propranolol therapy in patients with coronary artery disease

Pharmacologic agents that alter hemoglobin affinity for oxygen may affect systemic or myocardial oxygen delivery. In vitro, and in normal man propranolol shifts the oxyhemoglobin equilibrium curve to the right, thus increasing the partial pressure of oxygen at which hemoglobin is 50% saturated (P50) and enhancing oxygen delivery. The effect of propranolol on hemoglobin P50 was evaluated in 12 patients with angina pectoris and documented coronary artery disease. Determinations were made during oral propranolol therapy (mean daily dose 152 mg) of at least 3 months' duration and after administration of propranolol had been discontinued for at least 4 days. Hemoglobin P50 and erythrocyte 2,3-diphosphoglycerate (DPG) were measured. Data in 12 patients were: the mean P50 after discontinuation of propranolol was 28.2 mm Hg+/-0.9 (standard error of the mean) and during propranolol therapy 31.7+/-0.7; P less than 0.001; red blood cell 2,3-DPG did not change to explain the increase in P50. This demonstrated shift could increase systemic oxygen delivery and thus benefit marginally perfused myocardium while sparing coronary flow. Propranolol, in addition to its negative chronotropic and inotropic effects, may increase tissue oxygen delivery in patients with the anginal syndrome.

The action of thyroid hormone

Thyroid hormone affects both developmental and metabolic processes. It has a relatively specific effect on the synthesis of a number of enzymes and other proteins. The fundamental cellular mechanism of action seems to be at the level of genetic regulation. It involves interaction with nuclear receptors, leading to an activation of the protein synthesizing machinery. How binding to receptors is coupled to genetic activation is completely unknown. At least part of the metabolic effects of thyroid hormone could be mediated through an interaction with mitochondria and cell membrane, and with some enzymatic systems such as adenylcyclase.

2,3 diphosphoglycerate in Parkinson's disease

The red cell 2,3 DPG, the most important factor for oxygen delivery in the tissues, was found to be increased in Parkinsonism patients compared with controls. The aging process seems not to be a factor in the increased 2,3 DPG concentration. Other factors relevant to raised 2,3 DPG level such as physical activity, increased oxygen requirements, and metabolic changes are discussed.

Genetic aspects of quantitative variation in the carbonic anhydrases of the pig-tailed macaque, Macaca nemestrina. I. Response to thyroxine

Effects of thyroxine on incorporation of L-serine-C14 into four carbonic anhydrase isozymes (CA II, CA Ia, CA Ib, CA Ic) and hemoglobin were quantified in reticulocytes of Macaca nemestrian in vitro. Response to thyroxine differed significantly between CA Ia and two allelic variants (CA Ib and CA Ic) and the nonallelic isozyme (CA II). The effects of thyroxine on serine incorporation into hemoglobin and three of the carbonic anhydrase isozymes were shown to be nonlinear with thyroxine concentration.

Anemia in thyroid diseases

Thyroid hormones generally stimulate erythropoiesis. These agents also increase erythrocyte 2,3-DPG concentrations, which serve to enhance the delivery of oxygen to tissues. In the absence of thyroid hormones, anemia frequently develops and may be normocytic, hypochromic-microcytic, or macrocytic. Anemia is an uncommon finding in hyperthyroidism but when present may be morphologically similar to that observed in hypothyroidism. Pernicious anemia has been strongly associated with hypothyroidism, hyperthyroidism, and thyroiditis. Complete correction of anemia often requires restoration of thyroid function as well as specific hematinic therapy. Continued attention to hematologic status is essential in the management of patients with thyroid diseases.

Studies with electron microscopic autoradiography of thyroxine 125 I in organotypic cultures of the CNS. II. Sites of cellular localization of thyroxine 125 I

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Cultivation techniques for the erythrocytic stages of malaria parasites

The study of the biochemistry, physiology, and immunology of plasmodia has been restricted by the difficulty of maintaining the parasites in isolation from the host. Some success has been achieved in cultivating them in vitro, using tissue cultures and chick embryo techniques to study exoerythrocytic states and the sporogonic cycle, but no completely successful method has been found for studying the asexual and sexual stages of plasmodia in circulating red blood cells. The relative slowness with which techniques for continuous in vitro cultivation have been developed is the result of inadequate knowledge of the biochemistry of the parasites and of the blood and its constituents. However, radioactive labelling techniques applied to P. knowlesi cultures are beginning to yield data of fundamental importance. Existing methods for the short-term in vitro cultivation of plasmodia are potentially very useful for analysing malarial antigens, for developing vaccines, and for screening and studying antimalarial drugs. Investigations of the physicochemical requirements for the in vitro preservation of red blood cells are required, and more emphasis should be given to the study of plasmodia with longer cycles. Differences between the metabolism of plasmodia in vivo and in vitro should be studied and the growth factors in normal plasma identified. Studies of the membrane of the parasites and of the red blood cells, of the immune response, and of extracellular methods for the cultivation of plasmodia should be extended.