270 ADVENTURES IN RADIOISOTOPE RESEARCH 



As seen in Table 6, the labelled phosphatide content of all the organs 

 but that of the liver can be interpreted as being present in the inter- 

 spaces though this must not actually be the case. The liver contained, 

 after the lapse of 4 hours, about ten times more phosphatides as can 

 be explained by an uptake of the liver interspaces. This result suggests 

 the explanation that not only the capillary wall but also the membrane 

 of the liver cells is very easily permeable to phosphatides. The capillary 

 wall of the small intestine, brain, and muscles is but fairly permeable, 

 its permeability decreasing in the above sequence. The uptake of 

 labelled phosphatides by 1 gm muscle makes out only about 1/170 

 part of the labelled phosphatides taken up by 1 gm liver. The cor- 

 responding figure for the small intestine mucosa is about 1/20. 



FORMATION AND EXCHANGE OF PHOSPHATIDES IN THE LIVER 



It is of interest to compare the amount of phosphatides synthesized 

 in the liver with the amount which reaches the liver through an exchange 

 process from the plasma. In the first case, we investigate the formation 

 of labelled phosphatide molecules, in the second case no new labelled 

 molecules were formed but all the labelled phosphatide molecules pre- 

 sent were taken up by the liver from the plasma. This uptake is presum- 

 ably followed by the release of a similar amount of phosphatide mole- 

 cules previously present in the liver. An alternative explanation would 

 be that the uptake of phosphatide molecules from the plasma by the 

 liver is followed by a destruction of these molecules in the liver, the 

 phosphatides lost by the plasma being replaced by phosphatides synthe- 

 sised in other organs and liberated into the circulation. 



As found by us, in the course of 4 hours 150 mgm liver phosphatides 

 were newly formed, while during the same time 52 mgm phosphatides 

 are carried from the plasma into the liver; if this amount is not replaced, 

 at least to a large extent, by an equal amount of phosphatides migrating 

 in the opposite direction, then it must be supplied by another source 

 to the plasma. The organ responsible for such a supply must be one in 

 which phosphatide molecules are formed at an appreciable rate. This 

 is primarily the case— besides the liver— in the small intestine. We have, 

 therefore, to ask it the amount of phosphatides supplied during 4 hours 

 by the intestine into the circulation suffices to compensate the uptake 

 of phosphatide molecules by the liver from the plasma. Sullmanx 

 and WiLBRANDT (1934) determined the amount of phosphatides carried 

 into the circulation by the intestinal lymph of the rabbit. They found 

 that up to 1/2 mgm phosphatide P can be carried by the lymph stream 

 in the course of 4 hours, thus appreciably less than given off by the 

 plasma to the liver during the same time. As the amount of phospha- 



