MAMMARY GLAND AND LACTATION 



()19 



role in calcium metabolism, that the para- 

 thyroids were important in the maintenance 

 of secretion (see review by Folley, 1952a). 

 Indeed in the rat, we demonstrated that 

 the severe impairment of milk secretion pre- 

 viously observed in "thyroidectomized" rats 

 was due not to the removal of the thyroids, 

 but to the simultaneous ablation of the 

 l)arathyroids (Cowie and Folley, 1945). 

 This observation has since been confirmed 

 and extended by Munson and his colleagues 

 (Munson, 1955) who demonstrated an in- 

 fluence on the calcium-concentrating mech- 

 anism of the mammary glands. Within 24 

 hours of parathyroidectomy the concentra- 

 tion of calcium in the milk of the lactating 

 rat was increased markedly despite a 

 greatly depressed level of calcium in the 

 serum; there was also a decrease in water 

 content of the milk, but this did not entirely 

 account for the increase in calcium content 

 since the calcium content expressed as mg. 

 per gm. milk solids was significantly higher 

 after parathyroidectomy (Toverud and 

 Munson, 1956). Further studies in this field 

 are awaited with interest. 



F. INSULIN 



Early experiments (see review by Folley, 

 1952a) indicated that the endocrine pan- 

 creas might influence mammary function in 

 two ways; indirectly by way of the general 

 intermediary metabolism by which the sup- 

 ply of milk precursors may be regulated, 

 and directly through its role in the carbo- 

 hydrate metabolism of the mammary gland 

 itself. 



Most recent studies have been concerned 

 with the effect of insulin on mammary tis- 

 sue in vitro. Mammary gland slices from 

 lactating rats actively synthesize fat from 

 small molecules, glucose, and glucose plus 

 acetate, but not from acetate alone (Folley 

 and French, 1950). The addition of insulin 

 to the incubation medium very markedly 

 increases the R.Q. (see Table 10.2) and 

 glucose uptake of the tissue slices and ex- 

 periments with isotopes show that the rate 

 of fat synthesis is increased (Balmain, Fol- 

 ley and Glascock, 1952). Mammary gland 

 slices from lactating sheep, on the other 

 hand, can utilize acetate alone but not glu- 

 cose alone for fat synthesis (Folley and 

 French, 1950) and sheep tissue is not re- 



TABLE 10.2 



Effect of different substrates and of insulin on the 



respiratory quotient (R.Q.) of lactating mammary 



gland slices from various species 



(From S. J. Follev and M. L. McNaught, Brit. 



M. BulL, 14, 207-211, 1958.) 



sponsive to insulin in vitro. This clear-cut 

 species difference is interesting and under- 

 lines the need for further study. It is of 

 passing interest to note that the response 

 in vitro of rat mammary tissue to insulin 

 has been made the basis of a highly specific 

 in vitro bio-assay for insulin (Fig. 10.17) 

 (Balmain, Cox, Folley and McNaught, 

 1954; McNaught, 1958)! 



Further references and discussion on the 

 role of insulin in mammary function and 

 lipogenesis will be found in the reviews by 

 Folley (1956), and Folley and McNaught 

 (1958, 1960). 



IV. Removal of Milk from the 



Mammary Glands: Physiology 



of Suckling and Milking 



A. MILK-EJECTION REFLEX 



Since the second edition of this book, 

 there have been major advances in our 

 knowledge of the physiology of milk re- 

 moval. In the mammary gland the greater 



