THE THYROID AND ABSORPTION OF VITAMIN B₁₂IN RATS¹

Abstract

The absorption of vitamin Bi 2 and the concentrations of vitamin Bi 2 in the plasma and liver were measured in normal rats and in rats in which the thyroid activity was altered. It was found that the absorption of an oral dose of radioactive vitamin Bi 2 is markedly reduced after thyroidectomy and that the thyroid hormone treatment readily normalizes absorption in thyroidectomized rats. However, the administration of intrinsic factor (IF) prepared from rat stomachs did not improve absorption in such animals. The measurement of IF activity in the stomach demonstrated that the amount of IF in the stomach is not reduced in thyroidectomized rats. It seems, therefore, that the thyroidal influence on the absorption of vitamin B-2 is not mediated through the production of IF. A possible hormonal regulation acting directly on the intestinal wall for the absorption of vitamin Bi 2 has been discussed. S TUDIES on the mechanism of absorption of vitamin B i2 have centered primarily on the role of intrinsic factor (IF), little on the regulatory mechanism for intestinal absorption of this vitamin under various abnormal conditions. In our previous report, it was demonstrated that levels of vitamin B i2 in plasma of human cretins are low and can be elevated after thyroid therapy (1). These data suggest the possibility that thyroid activity has a regulatory effect on the absorption of vitamin Bi 2 . We, therefore, studied the influence of thyroid activity on the absorption of vitamin B 12 and its concentration in tissues and plasma of animals which were made athyroid or fed thyroid hormone in the diet. This communication reports findings of such studies which demonstrated a hormonal regulation of intestinal absorption of vitamin B i2 . Efforts were also made to elucidate the control mechanism, particularly with reference to the effect of thyroid activity on the production intrinsic factor which is known to aid absorption of vitamin Bi 2 . MATERIALS AND METHODSPreparation of animals, (a) Thyroidectomized rats. In studies in which absorption of radioactive vitamin Bi 2 was measured by fecal excretion test, thyroidectomized and control rats were obtained from one source. 2 In other studies, these two groups of rats

Vitamin‐B12 Absorption and Excretion in the Rat

“…This difference in dietary intake would seem the most likely explanation of the significantly higher urinary excretion of radioactivity and microbiological activity which is observed in rats on the semi-synthetic diet. Howevcr, the Lecal excretion of radioactivity is highest in rats on the cake diet, hence if the increase in urinary radioactivity is explaincd by a higher intake and higher total body content of the vitamin, this flushing effect is not manifest in the biliary excretion which largely accounts for reexcretion into the gastro-intestinal tract (Okuda et al, 1958).…”

Intestinal Mucosa and Vitamin B₁₂ Absorption

“…The limited capacity of the receptor and a humoral control have been shown by further experiments in which mucosal adsorption of the complex was found to be markedly reduced in hypothyroid rats [65]. Such animals exhibit severe B12 malabsorption [68]. It is of interest to note that when sufficient time wasallowed for the absorption to proceed in the closed loop, the hypothyroid rats did absorb B12 to the same extent as in the control.…”

Biochemistry of hyperthyroidism and hypothyroidism

“…Pyridoxal-5-phosphate content was low because of defective phosphorylation; conversely, it was high after thyroidectomy (Labouesse, Chatagner & Jolles-Bergeret, 1960). Vitamin B12 content was low in the tissues of thyrotoxic rats and hypothyroid female rats (Gershoff et al, 1958;Kasbekar et al, 1959); hormone administration raised renal B12 to normal in hypothyroid rats, and above normal in euthyroid rats (Okuda & Chow, 1961). Ascorbic acid content was low in the blood and tissues of thyrotoxic subjects.…”