Since Banti postulated causal relationship between splenomegaly and anemia in 1882, the hypersplenism attracted interest of many clinicians and investigators. However, the exact mechanism of hypersplenism remains unclear.
Hypersplenism is characterized by splenomegaly associated with diminution of one or more of formed elements in peripheral blood and hyperplasia of their precursors in bone marrow, Such conditions are seen in a variety of clinical conditions, and
if splenomegaly is the result of other known disease processes, it is called as secondary hypersplenism, while when the cause is unknown as primary. In either condition, splenectomy usually brings temporary or permanent improvement of hematologic abnormality, even if it does not alternate the basic disease process. The exact role of spleen in these circumstances, however, is not clearly elucidated, and only different hypotheses were elaborated.
Animal experiments have also been conducted by many investigators, inducing splenomegaly by several different means, such as ligation of splenic and gastric coronary veins, administration of hydroxylamine, anti-marrow serum, and
macromolecular inert substances, such as polyvinyl alcohol and methyl cellulose. Among all of these methods, injection of marcromolecular inert substances produced most constant and marked degree of splenomegaly.
Hematologic alterations following intraperitoneal injection of methyl cellulose varied according to different investigators and the explanation of the mechanism of anemia in this condition by previous investigators showed discrepancy.
Therefore, the present investigation is undertaken to clarify the mechanism of splenomegaly and anemia induced by intraperitoneal injection of methyl cellulose in rat.
Materials and Methods
Rats weighing from 200 to 300 gm, regardless of sex, were divided into three major groups; normal control, splenectomized and methyl cellulose treated groups.
Methyl cellulose group wss divided into two groups; injection after splenectomy and intact rats, 2.0m. of 2.5% aqueous solution, twice a week intraperitoneally for 12 weeks. Following 12 week injection of methyl cellulose, non-splenectomized rats
were divided into two subgroups; splencetomized and non-splenectomized.
Hematologic examinations included hematocrit by microhematocrit method, hemoglobin by cyanmethemoglobinometry, and enumeration of white blood cells, red blood cells, reticulocytes and platelets by ordinary laboratory methods.
Red cell survival time was determined by method of Thompson el al, using Cr**51 tagged red cells.
Total circulating blood volume was measured by diffusion method using Cr**51 labelled red cells.
Tissue contents of radioactivity to investigate sequestration and destruction of Cr**51 tagged red cells were measured on spleen, liver, kidney and heart.
All measurement of radioactivity was counted out with well-type scintillation counter.
Hemoglobin turnover and production rate were calculated with total hemoglobin, mean red cell life and blood volume ratio to normal.
Histological examination was performed on spleen, liver, kidney, bone marrow, heart, adrenal and thymus.
1. Repeated intraperitoneal injection of methyl cellulose in rats induced splenomegaly constantly, and the degree of enlargement was paralleled to the number of injection, up to 10 times of normal. The weight of liver, kidney and heart also increased, but in slight degree.
2. Histological examination of spleen in methyl cellulose treated group, revealed numerous macrophages containing methyl cellulose and occasionally red blood cells, and frequent granuloma formation. Liver showed many macrophages in hepatic lobules and portal areas, but much lesser degree than spleen. Kidney revealed macrophages in most of glomeruli and frequently in stroma, resulting enlargement of glomeruli. Bone marrow in anemic animals with splenomegaly showed marked hyperplasia of
predominantly erythroid series.
3. Hematocrit and hemoglobin level dropped markedly following methyl cellulose administration and the decrease reached maximum in 3 to 4 weeks, and remained at this level as long as the administration of methylo cellulose continued. After the cessation of methyl cellulose injection, hematocrit and hemoglobin increased slightly, but remained much lower than normal. However, when spleen was removed after 12 weeks administration of methyl cellulose, hematocrit and hemoglobin rose rapidly to the value of normal splenectomized rats.
Reticulocyte count showed a marked increase depending on the degree of anemia, but changes of white blood cells and platelet counts were rather insignificant.
Rats receiving methyl cellulose after initial splenectomy showed only a mild degree of anemia during 12 weeks of administration, but no increase of reticulocytes.
4. Non-specific agglutination of red cells developed following methyl cellulose administration, but the agglutinability disappeared at 7-10 days after the cessation of administration. Serum from the rats receiving methyl cellulose twice a week aggregated red cells as same degree of normal rat serum added methyl cellulose in concentration of 50 to 100mg percent.
5. Total circulating blood volume increased following methyl cellulose administration ; namely, 6.44% of body weight while methyl cellulose is being administered, and 6.06% at 3 months after the cessation of methyl cellulose in comparison to 5.26% in normal rats. Normal splenectomized rats showed 4.83%, and 6.06% in rats treated with methyl cellulose after initial splenectomy. This increase in obviously due to plasma expanding effect of methyl cellulose, but a part of increase is likely due to enlarged spleen.
6. Red cell survival time among normal control, normal splenectomized and methyl cellulose treated after splenectomy groups showed no significant difference(T-1/2 Cr**51 : 17.3, 19.7 and 16.4 days, respectively). However, it was markedly shortened in the group treated with methyl cellulose without splenectomy, and even at 3 months after the cessation of methyl cellulose administration(T-1/2 Cr**51:2.8 and 2.7 days respectively). These indicate that the shortening of red cell survival time is mostly due to enlarged spleen, and direct effect of methyl cellulose on red cells played little role.
7. Tissue accumulation of radioactivity following transfusion of Cr**51 tagged red blood cells in normal control group and in rats at 3 months after the cessation of methyl cellulose administration for 12 weeks, showed the highest count in spleen, more in the latter group, indicating more sequestration and destruction of red blood cells in spleen of this group than normal. Splenic radioactivity during the administration of methyl cellulose showed lesser degree of accumulation than above two groups. This is perhaps due to increased excretion of Cr**51 through kidney in this group.
8. Markedly increased hemoglobin production was noted in the rats with enlarged spleen. This apparently indicated that there is no depressive action of enlarged spleen on erythropoiesis in the bone marrow.
Repeated intraperitoneal adminstration of methyl cellulose in rats constantly induced splenomegaly, accompanied by marked anemia with reticulocytosis. Histologically, the enlarged spleen showed infiltration of numerous storage cell clusters. Red cell survival time in splenomegalic rats was markedly shortened, and
accumulation of tissue radioactivity in spleen was the highest. Hemoglobin production rate was markedly increased in the rats with enlarged spleen.
Results obtained from present investigation indicate that the anemia induced by methyl cellulose administration in rats is mostly due to increased destruction by enlarged spleen, and the direct action of methyl cellulose played only minor part. There was no evidence of depressive action on erythropoiesis in bone marrow by the enlarged spleen.