Erythropoiesis and erythropoietin levels in renal ... - Springer Link

Klinische Wochenschrift

Klin Wochenschr (1991) 69:53-58 002321739100019B

9 Springer-Verlag 1991

Review

Erythropoiesis and Erythropoietin Levels in Renal Transplant Recipients M. Wolff and W. Jelkmann Physiologisches Institut I der Friedrich-Wilhelms-Universitfit Bonn

Summary. Moderately increased blood levels of endogenous erythropoietin (Epo) usually induce complete restoration of renal anemia after successful kidney transplantation. With good graft function erythropoiesis is maintained by normal Epo serum levels. Persistent anemia can be related to iron deficiency, low excretory graft function, and high dosage of immunosuppressive agents leading to marrow suppression or nephrotoxicity. Acute early rejection is associated with a fall in serum Epo and abrogation of reticulocytosis. A b o u t 15% of recipients fail to exhibit the normal feedback regulation and develop a mostly transient posttransplant erythrocytosis. Both an increased sensitivity of erythrocytic progenitors to Epo and inappropriate Epo secretion by the native kidneys may account for this overshooting reaction. Key words: Kidney transplantation - Erythropoietin - Erythropoiesis - Renal failure - Anemia Erythrocytosis

Erythropoietin (Epo), a sialoglycoprotein growth hormone, is produced primarily by the fetal liver and by the kidney in adults. Tissue hypoxia induced by anemia or hypoxemia is the main stimulus for Epo secretion. However, neither the cell types that produce Epo nor the mechanism by which the lack of oxygen is measured are as yet clearly known. Abbreviations: Epo=erythropoietin; rHuEpo=recombinant human erythropoietin; RIA = radioimmunoassay; ELISA = enzyme-linked immunosorbent assay; RTx=renal transplantation; CAPD= continuous ambulatory peritoneal dialysis; PTE = posttransplant erythrocytosis; Aza = azathioprine; CsA = cyclosporineA; ALG = antilymphoblast globulin

Epo acts by stimulating mitosis and differentiation of erythrocytic progenitors in the bone marrow, thus maintaining a physiological level of red cell mass to meet tissue oxygen requirements [30]. The recent cloning and expression of the human Epo gene in animal cells [29, 36] has provided the hormone in sufficient quantities for therapeutical use and to raise antisera and monoclonal antibodies. Thus, it has been possible to develop sensitive radioimmunoassays (RIA) and enzyme-linked immunoassays (ELISA) for serum Epo measurements. The present article summarizes the features of Epo production and erythropoietic activity in patients after renal transplantation. Renal Anemia Several pathogenetic factors contribute to the anemia of chronic renal failure, but their significance can only be qualitatively evaluated. It is thought that retained "uremic toxins" and osteitis fibrosa due to severe hyperparathyroidism lower the proliferation rate of red cell progenitors [9, 38, 44]. Iron deficiency due to gastrointestinal and dialysisrelated blood loss and aluminium intoxication can result in inhibited maturation of red blood cell (RBC) precursors [19, 32]. There is moderately increased hemolysis, which reduces R B C survival to about 50% of normal [44, 47]. However, the main cause of renal anemia is inadequate production of Epo by the diseased kidney [20, 22]. The ordinary inverse relationship between serum Epo and hemoglobin (Hb) levels [18] is lost in chronic renal failure [8, 15, 24, 38]. Despite the lack of excretory function the residual renal tissue maintains some endocrine potential for feedback regulation of erythropoiesis by Epo production, as observed following both acute hemorrhage and blood transfu-

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M. Wolff and W. Jelkmann: Erythropoiesis and Erythropoietin Levels

sion [15, 58]. Patients with chronic renal failure generally exhibit normal or even elevated titers of Epo [8, 11, 38, 55], but the levels are not sufficiently increased to correct the anemia. Anephric patients who depend on extrarenal Epo production usually suffer from a more severe degree of anemia, with serum Epo below normal [5, 8]. Polycystic endstage renal disease can be associated with increased Epo levels leading to normal hematocrit (Hct) and Hb concentration [5, 10]

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Erythropoiesis after Successful Transplantation

Earlier studies have shown that successful renal transplantation (RTx), including both immediate and delayed onset of graft function, leads to correction of renal anemia over an 8- to 10-week period [27]. One reason may be the removal of the various inhibitors of erythropoiesis which accumulate in the serum during renal failure [9]. The increase in Hb concentration following conversion from hemodialysis to continuous ambulatory peritoneal dialysis (CAPD), with its more intense clearance of "uremic middle molecules" (mol wt 500-5000), supports this reasoning [68, 51, 37]. Epo levels in CAPD-treated patients, however, have been reported to increase [62, 10] or to remain unchanged [17, 37]. Additionally, as in CAPD, the increase in Hb and Hct after RTx may be due to both a fall in plasma volume and an increase in red cell mass, with improved red cell survival [511. Most authors agree that the major factor contributing to the correction of anemia is the production of Epo by the donor kidney. Striking elevation of Epo following RTx and coinciding with the onset of erythropoietic response was observed in earlier studies using in vitro and in vivo bioassays [1, 16, 39, 41]. These studies were limited, however, by the insensitivity of the assays, which were inappropriate for estimating pretransplant levels in uremic patients and in normal control subjects. More recently, serum Epo concentrations have been measured in renal transplant recipients by sensitive radioimmunoassays, which are unlikely to be invalidated by the presence of inhibitors of erythropoiesis present in uremic sera [11]. Figure 1 shows the typical course of erythropoietic indices after successful renal transplantation. An early postoperative Epo peak has been demonstrated by several groups of investigators who collected serum samples at short intervals after RTx [7, 46, 52]. Within the first 4 days Epo increases about ninefold and thereafter normalizes at least by the 7th postoperative day. This early

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Fig. 1. Temporal course of serum erythropoietin, hematocrit, and serum creatinine after 31 successful renal transplantations (initially non-functioning grafts included). Erythropoietin levels in normal subjects without anemia ranged from 0 to 25 mU/ml. The first erythropoietin peak was observed in 70% of the patients. From Sun et al. 1989 [52], with kind permission

Epo peak is not correlated to recovery of excretory renal function or blood loss, nor is it followed by reticulocytosis. Four to eight days after serum creatinine starts to decrease, with improved graft function, Epo again increases, with a plateau of about twice the pretransplant values [6, 7, 33, 52, 63]. This sustained Epo elevation is followed by reticulocytosis after 3-7 days and by a permanent rise in Hct and Hb after 1-2 weeks [33, 52, 63]. After 1-3 months Epo levels decline to normal (5-30 mU/ml), when the Hct reaches about 32% [52]. Nevertheless, the resolution of anemia continues, thus indicating the return of the orderly feedback regulation of erythropoiesis [6, 33, 46,

521. The cause of the initial Epo peak is not clear, but it has been suggested that Epo accumulates in the graft during ischemic storage and is flushed out by reperfusion [52]. Brown et al. [7], however, did not find a correlation between the duration of warm or total ischemia and the size of the early Epo peak. As this peak has been noted only in patients with delayed onset of graft excretory function, the authors assume that transient hypoxia of tubular cells stimulates production of Epo. Delayed graft function or so-called acute tubular necrosis (ATN) retards the onset of erythropoiesis


until excretory function commences, without inhibiting the long-term correction of anemia [5]. Besarab et al. [6] have also noted a biphasic Epo response in recipients with polycystic kidneys who were treated with cyclosporine A (CsA). In this study Epo peak levels were not altered by concurrent blood transfusions. Neither was a difference observed when living-related and cadaver-donor grafting was compared. The highest endogenous serum Epo levels sufficient to resolve anemia following RTx are only about twice the normal level, while much higher levels are necessary in the treatment of renal anemia by recombinant human (rHu) Epo [21, 65]. This observation indicates that the restoration of renal excretory function improves the erythropoietic response to Epo. Epo levels remain high in patients with functional iron deficiency after RTx, characterized by rapidly falling serum ferritin levels and persistent anemia. As in rHu Epo treatment of endstage renal anemia, this iron depletion occurs with high frequency [21, 33], and for prevention a pretransplant ferritin concentration of at least 250 ~tg/1 has been recommended [52]. In about 70% of the patients the Hb concentration normalizes 2-6 months after successful RTx and the reestablishment of ordinary feedback regulation of erythropoiesis [6, 33, 46, 52]. The degree o f correction of anemia is ultimately limited by the stage of graft function attained and correlates with the serum creatinine concentration.

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Fig. 2. Effect of acute early rejection in 11 renal graft recipients (14 separate events). Rejection suppresses erythropoietic indices, which recover 16 days following successful treatment. Hematocrit is not significantly altered. From Besarab et al. 1987 [6], with kind permission

Chronic Graft Rejection Acute Graft Rejection Early acute rejection occurs by definition in the first 3 postoperative weeks. Diagnosis is made by observation of deteriorating excretory renal function, by graft biopsy, or ex iuvantibus by steroid treatment. Earlier in vivo or in vitro bioassay measurements showed Epo levels to be elevated during episodes of acute rejection [1, 16, 31, 34, 39]. However, data based on sensitive RIAs revealed a striking fall in plasma Epo at the height of the rejection process [6, 46]. The normal negative feedback mechanism of erythropoiesis is interrupted by a drop in reticulocyte counts during rejection, recovering only with successful reversal of rejection (Fig. 2). In 14 episodes studied by Besarab et al. [6] there was no correlation between the serum creatinine concentration and hematopoietic indices. Therefore, it is more likely that mediators generated by the rejection process itself, rather than retention of toxins, cause suppression of Epo synthesis and abrogation of erythropoiesis.

Severe anemia is a common feature in patients with chronic rejection. Eventually, both chronic rejection and chronic CsA-nephrotoxicity lead to the same relative Epo deficiency in endstage renal failure as other causes do. Many of these patients require regular blood transfusions. Aside from the risk of infections and iron overload, sensitization to histocompatibility antigens reduces the chance of another succesful RTx. Winearls et al. [65] first evaluated the beneficial effects of rHu Epo on the anemia of recipients with chronic rejection before hemodialysis was started. Yoshimura et al. [67] demonstrated a reduced need for transfusions and their risks under rHu Epo treatment in these patients. The therapy with rHu Epo has no deleterious effect on morbidity and transplant outcome in patients who will subsequently undergo RTx [42]. Moreover, there is no evidence for bone marrow unresponsiveness to endogenous Epo. On the other hand, the findings of Wahlberg et al. [57] indicate that a low

56


Hct of 25 to 29% is supportive for graft reperfusion and early function.

Effects of Immunosuppression CsA-treated recipients show an earlier but blunted increase in erythropoiesis compared to conventionally immunosuppressed patients, with reticulocytosis even during the period of delayed graft function [5]. CsA can acutely reduce ultrafiltration pressure in normal kidneys by constriction of the afferent glomerular arterioles [12]. CsA-induced vasoconstriction could lead to renal hypoxia and increased synthesis of prostaglandins. Prostaglandins E 2 and I2, in turn, have been shown to be mediators of renal Epo production [30]. Additionally, in vitro studies [23, 45] revealed that CsA is able to enhance the growth rate of bone marrow stem cells, possibly by inhibiting an endogenous, T-cell mediated, suppressor mechanism. In contrast, antilymphoblast globulin (ALG) seems to delay the onset of reticulocytosis [5]. This corresponds to the in vitro finding that A L G also has an antiproliferative effect on erythroid progenitor cells which possibly share antigenic determinants with T-cells [2]. Besarab et al. [5] stated that neither conventional immunosuppression with prednisone and azathiowine (Aza) nor immunosuppression with the newer agents A L G and CsA per se limits the restoration of anemia, although their effect on ultimate renal function does. However, the more long-term effects of Aza, prednisone, and CsA on erythropoiesis are still not well defined [48, 64].

Posttransplant Erythrocytosis Erythrocytosis, defined by Hct > 51% and Hb > 165 g/l, is a common complication observed since the early days of renal transplant surgery [40, 41, 60, 66]. Wickre et al. [61] observed a 17.3% incidence of posttransplant erythrocytosis (PTE). Developing 3 to 90 months post transplantation, PTE persisted for an average of 20 months. In smaller series a mostly transient PTE in 6 to 18% of kidney recipients has been reported [4, 13, 31, 33, 40, 50, 52, 59]. The pathogenesis of PTE is still unclear, but probably multifactorial. Theoretically, the cause of PTE can be linked to three different conditions. First, kidney oxygenation may be impaired, resulting in reactively elevated serum Epo. Second, a sensitized progenitor cell pool may increase red cell production at low Epo levels. Third, Epo production can be increased autonomously (as in some patients with hypernephroma) or due to an ele-

vated threshold of the O2-sensing mechanism with a shift of normal feedback regulation. As to the first of these mechanisms, suggested etiologic factors for renal hypoxia are acute or chronic rejection [31, 40, 53, 60], transplant renal artery stenosis [3, 48], and hydronephrosis. However, these disorders do not occur more frequently in patients with PTE than in recipients with normal Hct [61]. Additionally, Pagel et al. [43] have shown in rat experiments that the reduction of renal blood supply is no major stimulus for Epo secretion. Using in-vitro marrow cultures from graft recipients with PTE, Lamperi et al. [35] and Heilmann et al. [26] have demonstrated an increased sensitivity and proliferation rate of erythrocytic progenitors to which Epo was added. Hence, it has been proposed that prior to transplantation chronic uremia causes resetting of the myeloid erythrocytic response via upregulation of the Epo receptors [26]. Restoration of an almost normal milieu interne by transplantation could then result in excess erythropoiesis from sensitized progenitor cells. Administration of cytotoxic immunosuppressive agents may mask this effect. This hypothesis is supported by the inverse relationship between azathioprine dosage and occurrence of PTE reported by Webb et al. [59]. Indeed, elevated serum Epo levels decreased and reticulocytosis developed in two patients when azathioprine therapy was stopped [40]. On the other hand, it was suggested earlier that in PTE there is increased Epo formation by the native diseased kidneys [1, 13, 16, 28, 56, 66]. Selective catheterization of the renal veins unexpectedly revealed that the diseased kidneys are capable of producing remarkably high levels of Epo [3, 13, 52]. However, an increased Epo concentration in the venous renal outflow does not necessarily indicate an absolute increase in Epo production, since blood flow in endstage renal disease is diminished. Obviously, the native kidneys in some recipients escape from the feedback regulation, or their feedback works at a higher level of Hct even after resolution of uremia. The increase of Epo levels after phlebotomy in these patients is suggestive of a preserved feedback regulation and provides evidence for a shifted threshold of the oxygen sensor or a lowered oxygen tension at the sensor site due to fibrosis [54]. Removal of the remaining diseased kidney has been found to lower Epo levels and normalize Hct [14, 25]. It must be emphasized, however, that bilateral nephrectomy is indicated in PTE only when the serum Epo concentration is elevated. PTE is not invariably associated with increased Epo levels. Moreover, PTE may develop even in bilaterally nephrectomized patients [58, 60].


Although transient and easily managed by phlebotomy, PTE is of clinical importance because it frequently causes thromboembolic complications [53, 60, 61] and may compromise cardiovascular functions. Recently, alternative treatment of PTE with theophylline, a nonselective adenosine antagonist which attenuates the formation of Epo, has been proposed [4]. References 1. Abbrecht PW, Greene JA (1966) Serum erythropoietin after renal homotransplantation. Ann Int Med 65:908-92l 2. Arnold R, Schmeiser T, HeR W, Kohne E, Goldmann SF, Heimpel H, Kubanek B (1983) Treatment of severe acute GvDH with high dose ATG. Exp Hematol 11 [Suppl 13] :21-23 3. Bacon BR, Rothman SA, Ricanati ES, Rashad FA (1980) Renal artery stenosis with erythrocytosis after renal transplantation. Arch Int Med 140:1206-1211 4. Bakris GL, Sauter ER, Hussey JL, Fisher JW, Gaber AO, Winsett R (1990) Effect of theophylline on erythropoietin production in normal subjects and in patients with erythrocytosis after renal transplantation. N Engl J Meal 323 : 86-90 5. Besarab A, Caro J, Jarrell B, Burke J, Francos G, Mallon E, Karsch R (i 985) Effect of cyclosporine and delayed graft function on posttransplantation erythropoiesis. Transplantation 40: 624-631 6. Besarab A, Caro J, Jarrell BE, Francos G, Erslev AJ (1987) Dynamics of erythropoiesis following renal transplantation. Kidney Int 32: 526.536 7. Brown JH, Lappin TRJ, Elder GE, Taylor TN, Bridges JM, McGeown MG (1989) The initiation of erythropoiesis following renal transplantation. Nephrol Dial Transplant 4:1076.1079 8. Caro J, Brown S, Miller O, Murray T, Erslev AJ (1979) Erythropoietin levels in uremic, nephric and anephric patients. J Lab Clin Med 43:449-458 9. Caro J, Erslev AJ (1985) Uremic inhibitors of erythropoiesis. Semin Nephrol 5 : 128-132 10. Chandra M, Miller ME, Garcia JF, Mossey RT, McVicar M (1985) Serum immunoreactive erythropoietin levels in patients with polycystic kidney disease as compared with other hemodialysis patients. Nephron 39:26.29 1~. Cotes PM (1982) Immunoreactive erythropoietin in serum. Br J Haematol 50:427438 12. Curtis J J, Luke RG, Dubrowsky E, Diethelm AG, Whelchel JD, Jones P (1986) Cyclosporin in therapeutic doses increases renal allograft vascular resistance. Lancet 2: 477-479 13. Dagher F J, Ramos E, Erslev AJ, Alongi SV, Karmi SA, Caro J (1979) Are the native kidneys responsible for erythrocytosis in renal allorecipients ? Transplantation 28:496-498 14. Dagher FJ, Ramos E, Erslev A, Karmi S, Alongi SV (1980) Erythrocytosis after renal allotransplantation: treatment by removal of the native kidneys. South Med J 73 : 940-942 15. DeKlerk G, Wilmink JM, Rosengarten PCJ, Vet RJWM, Goudsmit R (1982) Serum erythropoietin (ESF) titers in anemia of chronic renal failure. J Lab Clin Med 100:720-734 16. Denny WF, Flanigan WJ, Zukoski CF (1966) Serial erythropoietin studies in patients undergoing renal transplantation. J Lab Clin Med 67:386-397 17. DePaepe MBJ, Schelstraete KHG, Ringoir SMG, Lameire NH (1983) Influence of continuous ambulatory peritoneal

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dialysis on the anemia of endstage renal disease. Kidney Int 23 : 744-748 18. Erslev AJ, Wilson J, Caro J (1987) Erythropoietin titers in anemic, nonuremic patients. J Lab Clin Med 109:429M33 19. Eschbach JW, Cook JD, Scribner BH, Finch CA (1977) Iron balance in hemodialysis patients. Ann Int Med 87:710-713 20. Eschbach JW, Adamson JW (1985) Anemia of end-stage renal disease (ESRD). Kidney Int 28 : i-5 21. Eschbach JW, Abdulhadi MH, Browne JK, Delano BG, Downing MR, Egrie JC, Evans RW, et al. (1989) Recombinant human erythropoietin in anemia patients with endstage renal disease. Ann Int Med 111:992-1000 22. Fisher JW (1980) Mechanism of the anemia of chronic renal failure. Nephron 25 : 106-111 23. Frassoni F, Bacigalupo A, Piaggio G, Podesta M, Repetto M, Marmont AM (1985) Effect of cyclosporin A (CyA) on the in vitro growth of hemopoietic progenitors from normal marrow. Exp Hematol 13 : 1084-1088 24. Fukushima Y, Fukuda M, Yoshida K, Yamaguchi A, Nakamoto L, Miura AB, Harada T, Tsuchida S (1986) Serum erythropoietin levels and inhibitors of erythropoiesis in patients with chronic renal failure. Tohoku J Exp Med 150:1-15 25. Fyhrquist F, Renlund M, Kuhlb/ick B (1975) Erythropoietin and renin after renal transplantation. Scand J Urol Nephrol [Suppl 29]: 143-145 26. Heilmann F, Gottschalk D, Gottschalk I, Lison AE (1983) Studies in polycythemia after kidney transplantation. Clin Nephrol 20 : 94-97 27. Hoffman GC (1968) Human erythropoiesis following kidney transplantation. Ann NY Acad Sci 149:504-508 28. Ianhez LE, DaFonseca JA, Chocair PR, Maspes V, Sabbaga E (1977) Polycythemia after kidney transplantation. Influence of the native kidneys on the production of hemoglobin. Urol Int 32:382-392 29. Jacobs K, Shoemaker C, Rudersdorf R, Neill SD, Kaufman RJ, Mufson A, Seedra J, et al. (1985) Isolation and characterization of genomic and cDNA clones of human erythropoietin. Nature 313 : 806.809 30. Jelkmann W (1986) Renal erythropoietin: Properties and production. Rev Physiol Biochem Pharmacol 104:139-215 31. Jepson JH, DeLeenw NKM, Gault MH, Dossetor JB, Manasc B (1971) Characteristic of erythropoietin following human renal homotransplantation. Transplant Proc 3:353-357 32. Kaiser L, Schwartz KA (1985) Aluminium-inducedanemia. Am J Kidney Dis 6:348-351 33. Keusch G, Kurtz A, Fehr J, Eckhardt K-U, Frei D, Bauer C, Binswanger U (1989) Erythropoiese und Serumerythropoietinkonzentration vor und nach Nierenallotransplantation. Nephron 51 [Suppl 1]:29-33 34. Koiso K, Kitagawa R, Takayasu H (1975) Plasma erythropoietin activity before and after renal homotransplantation in humans. Urol Res 2:167-173 35. Lamperi S, Carozzi S (1985) Erythroid progenitor growth in erythrocytosic transplanted patients. Artif Organs 9 : 200-204 36. Lin FK, Suggs S, Lin CH, Browne JK, Smalling R, Egrie JC, Chen KK, et al. (1985) Cloning and expression of the human erythropoietin gene. Proc Natl Acad Sci USA 82:7580-7584 37. McGonigle RJS, Husserl F, Wallin JD, Fisher JW (1984) Hemodialysis and continuous ambulatory peritoneal dialysis effects on erythropoiesis in renal failure. Kidney Int 25 : 430-436

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38. McGonigle RJS, Wallin JD, Shadduck RK, Fisher JW (1984) Erythropoietin deficiency and inhibition of erythropoiesis in renal insufficiency. Kidney Int 25:437-444 39. Mirand EA, Murphy GP (1969) Erythropoietin alterations in patients with uremia, renal allografts, or without kidneys. JAMA 209: 392-398 40. Nellans R, Otis P, Martin DC (1979) Polycythemia following renal transplantation. Urology 1:158-163 41. Nies BA, Cohn R, Schrier SL (1965) Erythremia after renal transplantation. New Engl J Med 273:785 788 42. Paganini EP, Braun WE, Latham D, Abdulhadi MH (1989) Renal transplantation: results in hemodialysis patients previously treated with recombinant human erythropoietin. ASAIO Transact 35 : 535-538 43. Pagel H, Jelkmann W, Weiss C (1989) Oz-supply to the kidneys and the production of erythropoietin. Respir Physiol 77:111-118 44. Powell JS, Adamson JW (1985) Hematopoiesis and the kidney. In: Seldin DW, Giebisch G (eds) The kidney: physiology and pathophysiology. Raven Press, New York, pp 85886O 45. Raghavachar A, Frickhofen N, Arnold R, Schmeiser T, Porzsold F, Heimpel H (1986) Hematopoietic colony formation after allogeneic bone marrow transplantation: enhancement by cyclosporin A and anti-(immune) interferon antiserum in vitro. Exp Hematol 14:621-625 46. Rejman ASM, Grimes AJ, Cotes PM, Mansell MA, Joekes AM (1985) Correction of anaemia following renal transplantation: serial changes in serum immunoreactive erythropoietin, absolute reticulocyte count and red-cell creatinine levels. Br J Haematol 61:421-431 47. Rosenmund A, Binswanger U, Straub PW (1975) Oxidative injury to erythrocytes, cell rigidity, and splenic hemolysis in hemodialyzed uremic patients. Ann Intern Med 82: 460-465 48. Schramek A, Adler O, Hashmonai M, Better OS, Tuma S, Barsilai A, Chaimowitz C (1975) Hypertensive crisis, erythrocytosis and uremia due to renal artery stenosis of kidney transplants. Lancet 1 : 70-71 49. Sj6gren U, Thysell H, Lindholm T (1981) Bone marrow morphology in patients in long-term treatment with azathioprine. Scand J Haematol 26:182-186 50. Stockenhuber F, Geissler K, Sunder-Plassmann G, Kurz RW, Steininger R, Muehlbacher F, Hinterberger W, Balcke P (1989) Erythrocytosis in renal graft recipients due to a direct effect of cyclosporine A. Transplant Proc 21:1560-1562 51. Summerfield GP, Gyde OHB, Forbes AMW, Goldsmith HJ, Bellingham AJ (1983) Haemoglobin concentration and serum erythropoietin in renal dialysis and transplant patients. Scand J Haematol 30:389-400 52. Sun CH, Ward H J, Paul WL, Koyle MA, Yanagawa N, Lee DBN (1989) Serum erythropoietin levels after renal transplantation. N Engl J Med 321:151-157 53. Swales JD, Evans DB (1969) Erythremia in renal transplantation. Br Med J 2:80-83 54. Thevenod F, Radtke HW, Grfitzmacher P, Vincent E, Koch KM, Schoeppe W, Fassbinder W (1983) Deficient feedback regulation of erythropoiesis in kidney transplant patients with polycythemia, Kidney lnt 24:227-232 55. Urabe A, Saito T, Fukamachi H, Kubota M, Takaku F (1987) Serum erythropoietin titers in the anemia of chronic

renal failure and other hematological states. Int J Cell Clon 5 : 202-208 56. Varkarakis MJ, Sampson D, Gerbasi JR, Bender MA, Mirand EA, Murphy GP (1971) Polycythemia following renal transplantation unrelated to the allograft. J Surg Oncol 3:157-161 57. Wahlberg J, Jacobsson J, Odlind B, Tufveson G, Wikstr6m B (1988) Haemodilution in renal transplantation in patients on erythropoietin. Lancet 2:1418 58. Walle AJ, Wong GY, Clemons GK, Garcia JF, Niedermayer W (1987) Erythropoietin - hematoerit feedback circuit in the anemia of end-stage renal disease. Kidney Int 31 : 1205-1209 59. Webb DB, Price KA, Hutton RD, Newcombe RG, Salzman JR, Orchard J (1987) Polycythaemia following renal transplantation: an association with azathioprine dosage? Am J Nephrol 7:221-225 60. Westerman MP, Jenkins GL, Dekker A, Kreutner A, Fisher B (1967) Significance of erythrocytosis and increased erythropoietin secretion after renal transplantation. Lancet 2:755-757 61. Wickre CG, Norman DJ, Bennison A, Barry JM, Bennett WM (1983) Postrenal transplant erythrocytosis: a review of 53 patients. Kidney Int 23:731-737 62. Wider6e TE, Sanengen T, Halvorsen S (1983) Erythropoietin and uremic toxicity during continuous ambulatory peritoneal dialysis. Kidney Int 24 [Suppl 16]:208-217 63. Wikstr6m B, Goch J, Danielson BG, Backman U, Fellstr6m B, Fr6din L (1989) Serum erythropoietin in renal transplant patients. Transpl Proc 21 : 2043-2045 64. Williams D, Wickramasinghe SN, Hulme B (1978) Effects of azathioprine therapy on the MCV of patients with renal grafts: evidence for alterations in the kinetics of erythropoiesis over a prolonged period. Scand J Haematol 20 : 258-246 65. Winearls CG, Oliver DO, Pippard MJ, Reid C, Downing MR, Cotes PM (1986) Effect of human erythropoietin derived from recombinant DNA on the anaemia of patients maintained by chronic haemodialysis. Lancet 2:1 175-1178 66. Wu KK, Gibson TP, Freeman RM, Bonney WW, Fried W, DeGowin RL (1973) Erythrocytosis after renal transplantation. Its occurrence in two recipients of kidneys from the same cadaver donor. Arch Intern Med 132:898-972 67. Yoshimura N, Oka T, Ohmori Y, Aikawa I (1989) Effects of recombinant human erythropoietin on the anemia of renal transplant recipients with chronic rejection. Transplantation 48 : 527-529 68. Zappacosta AR, Caro J, Erslev A (1982) Normalization of hematocrit in patients with end-stage renal disease on continuous ambulatory peritoneal dialysis. The role of erythropoietin. Am J Med 72:53-57 Received: July 6, 1990 Returned for revision: October 22, 1990 Accepted: November 5, 1990 Dr. Martin Wolff Physiologisches Institut I Universitfit Bonn Nussallee 11 W-5300 Bonn 1, FRG