MAMMARY GLAXD AND LACTATION 



009 



tissue beginning at parturition observed 

 by Folley and French (1949) was inter- 

 i:)reted as indicating that this tissue assumes 

 the power of effecting net fatty acid syn- 

 thesis from ghicose at this time. Much sub- 

 sequent evidence confirming this idea has 

 been reviewed by Folley (1956). It only 

 rt'mains to add that Ringler, Becker and 

 Nelson (1954), Lauryssens, Peelers and 

 Donck (1956), and Read and Moore (1958) 

 ha^-e shown that the amount of coenzyme 

 A in mammary tissue undergoes an increase 

 at parturition. Moreover, the recent findings 

 of McLean (1958b), who showed that the 

 levels of pyridine nucleotides in the mam- 

 mary gland of the rat begin to increase 

 at parturition, reaching a high level by the 

 end of lactation, may be significant in this 

 connection. McLean found that although 

 the increase in the tissue levels of diphos- 

 l^hopyridine nucleotide was almost entirely 

 due to an increase in the oxidized form 

 (DPN), in the case of TPN it was the re- 

 duced form (TPNH) which increased. The 

 latter might well be used for reductive syn- 

 theses such as lipogenesis. 



The rate of synthesis of milk constituents 

 other than fat must also begin to increase at 

 parturition, and Greenbaum and Greenwood 

 (1954) showed that an increase in the levels 

 of glutamic aspartic transaminase and of 

 glutamic dehydrogenase in rat mammary 

 tissue occurs at this time. The authors be- 

 lieve these enzymes are concerned in the 

 provision of substrates for the synthesis of 

 milk protein. It is significant in connection 

 with milk protein synthesis that the mam- 

 mary gland ribonucleic acid (RNA) in the 

 rat undergoes a marked rise at parturition 

 (Greenbaum and Slater, 1957a). 



The above - mentioned biochemical 

 changes in mammary tissue which occur at 

 al)out the time of parturition are almost 

 certainly closely related to the effect on this 

 tissue of members of the anterior pituitary 

 lactogenic complex, and particularly pro- 

 lactin. Attempts have been made to elicit 

 the characteristic respiratory changes, de- 

 scribed above, in mammary slices in vitro 

 by addition of prolactin and adrenal gluco- 

 corticoids to the incubation medium (see 

 Folley, 1956). So far, however, definitive re- 

 sults luive not been obtained and it is doubt- 



ful whether any biochemical changes in 

 lactating mammary gland slices in vitro 

 have been demonstrated which could with 

 certainty be ascribed to the action of pro- 

 lactin (in this connection see also Bradley 

 and Mitchell. 1957). 



2. Maintenance of Milk Secretion — Galac- 

 topoiesis 



It is well known that the removal of the 

 pituitary of a lactating animal will end 

 milk secretion (for references see Folley, 

 1952a). The cessation of milk secretion has 

 been generally ascribed to the loss of the 

 anterior lobe, but when the importance of 

 the neurohypophysis in milk ejection be- 

 came established (see page 621), it was 

 clear that in the hypophysectomized animal 

 it was necessary to distinguish between a 

 failure in milk secretion and a failure in 

 milk ejection, since either would lead to 

 failure of lactation. It has now been shown 

 in the rat that adequate oxytocin therapy 

 ensuring the occurrence of milk ejection 

 after hypophysectomy will not restore lac- 

 tation (Cowie, 1957) and it may thus be 

 concluded that the integrity of the anterior 

 lobe is essential for the maintenance of 

 milk secretion. The effect of hypophysec- 

 tomy on milk secretion is dramatic, be- 

 cause in the rat, milk secretion virtually 

 ceases within a day of the operation and 

 biochemical changes in the metabolic ac- 

 tivity of the mammary tissue can be de- 

 tected within 4 to 8 hours (Bradley and 

 Cowie, 1956). It is of interest to note that 

 these metabolic changes are similar to those 

 observed during mammary involution (see 

 page 598). 



Since the second edition of this book, 

 there have been surprisingly few studies on 

 replacement therapy in hypophysectomized 

 lactating animals. In such studies we would 

 stress the need for rigorous methods of 

 assessing the efficacy of treatment. In the 

 past the presence of milk in the gland as 

 revealed by macroscopic or microscopic 

 examination has been regarded as an indi- 

 cation of successful replacement. This, 

 however, gives no measure of the degree of 

 maintenance of lactation and some measure 

 of the daily milk yield of such animals 

 should be obtained (see also Cowie, 1957). 



