CHANGES IN THE INTESTINAL WALL 443 



vided better growth and longer survival when the carotene was given in a 

 single dose intravenously than when the same amount of the provitamin 

 was administered orally. The intestinal wall is not required for the trans- 

 formation of intravenously-injected carotene in Tween 40 into vitamin A, 

 as is indicated by the fact that the plasma vitamin A of vitamin A-deficient 

 rats was markedly increased four to six hours after the administration of the 

 provitamin, even when the rats had previously been subjected to the sur- 

 gical removal of the small intestine. ^^"-^^^ Kirschman and Mac Vicar ^^^ 

 later obtained a similar result with ileectomized sheep. Other positive 

 reports of increased vitamin A in the plasma after parenterally-introduced 

 carotene include those of Eaton and co-workers^^* and of Warner and May- 

 nard ^^'* with calves, and of Kowalewski et al.'^^^ with dogs. Although Eaton 

 et al.^^^ could detect no concomitant increase in liver vitamin A, Kowalewski 

 and associates'*''^ did note an increase of liver vitamin A in dogs two hours 

 after the provitamin had been injected. Kon'®^ also confirmed the fact that 

 parenterally-introduced /3-carotene can be converted to vitamin A. On the 

 other hand, Church and co-workers'" were unable to demonstrate any in- 

 crease in the vitamin A content of the plasma or liver of Hereford calves 

 which had received aqueous suspensions of carotene. However, in the 

 case of sheep, these workers reported that an increase in plasma vitamin A 

 followed intravenous injection of carotene. The vitamin A level in the 

 blood rose from a control value of 21.4 jug. % to 34 /xg. % one hour later, and 

 to 47.9 /ig. % two hours after the carotene injection. One can only con- 

 clude from these data that extra-intestinal sites exist for the conversion of 

 carotene into vitamin A, not only in the species whose blood and tissues 

 normally contain appreciable amounts of carotene, but also in those ani- 

 mals which do not ordinarily have appreciable quantities of the provitamin 

 A in these sites. 



e. Factors Affecting the Conversion of Carotene into Vitamin A. (a) 

 The Effect of the Thyroid Gland. The effect of thyroxine and of thyroid 

 preparations in stimulating the absorption of carotene, as determined by a 



1" J. G. Bieri and C. J. Pollard, Biochem. J., 55, xxxi-xxxii (1953). 



'62 J. C. Kirschman and R. Mac Vicar, Federation Proc, 14, 439 (1955). 



163 H. D. Eaton, L. D. Matterson, L. Decker, C. F. Helmboldt, and E. L. Jungherr, 

 /. Dairy Sci., 34, 1073-1080 (1951). 



i«* R. G. Warner and L. A. Maynard, ./. Animal Sci., 11, 780 (1952). 



"* K. Kowalewski, E. Henrotin, and J. van Geertruyder, Acta gastro-enterol. belg., 14, 

 607-616 (1951); Nutrition Revs., 13, 124 (1955). 



"6 S. K. Kon, 1954. Private communication; cited by T. W. Goodwin, The Chemi- 

 cal Pathology of Carotenoids, in R. T. Williams, The Chemical Pathology of Animal Pig- 

 ments, Biochem. Soc. Symposia, No. 12, Cambridge Univ. Press, 71-84 (1954), p. 72. 



'" D. C. Church, R. Mac Vicar, J. G. Bieri, F. H. Baker, and L. S. Pope, /. Animal 

 Sci., IS, 677-683 (1954). 



