012 



PHYSIOLOGY OF GONADS 



although it has been suggested that a com- 

 plex of hormones rather than prolactin alone 

 is released (Folley, 1947). Williams (1945) 

 showed that prolactin could in fact main- 

 tain the integrity of the mammary gland in 

 the unsuckled mouse thus mimicking the 

 effects of the suckhng stimulus; other sup- 

 porting evidence has been reviewed by 

 Folley (1952a). Recent studies in goats, 

 however, have shown that milk secretion 

 may continue more or less at the normal 

 level after complete denervation of the ud- 

 der (Tverskoi, 1958; Denamur and Mar- 

 tinet, 1959a, b, 1960) and it may be that in 

 some species the suckling or milking stimu- 

 lus is loss important in the maintenance of 

 milk secretion. 



Milk secretion is essentially a continuous 

 process whereas the suckling or milking 

 stimulus is intermittent ; indeed the milking 

 stimulus may be of remarkably brief dura- 

 tion (in the cow about 10 minutes in all per 

 24 hours) and it is therefore likely that the 

 stimulus triggers off the release of sufficient 

 galactopoietic complex to maintain mam- 

 mary function for some hours. Grosvenor 

 and Turner (1957b) reported that suckling 

 causes a rapid drop in the prolactin content 

 of the pituitary in the rat, and that the 

 prenursing level of prolactin in the pituitary 

 is not fully regained some 9 hours later. 

 It is difficult, however, to relate pituitary 

 levels of prolactin to the rate of its secre- 

 tion into the circulation and, although these 

 observations are interesting, further ad- 

 vances are unlikely until a method of assay 

 for blood prolactin becomes available and 

 the "half-life" of prolactin in circulation is 

 known. 



The experiments of Gregoire (1947) on 

 the maintenance of involution of the thymus 

 during nursing suggests that the suckling 

 stimulus releases ACTH which, as we have 

 seen, is galactopoietic in the rat; thus, so far 

 as the rat is concerned, there would appear 

 to be good evidence that the suckling stim- 

 ulus releases at least two known important 

 components of the galactopoietic complex. 



The milking and suckling stimulus is also 

 responsible for eliciting the milk-ejection 

 reflex and the relation between the two re- 

 flexes will be discussed later in this chapter 

 (sec ])age 619 1. 



B. HORMONES OF THE ADRENAL CORTEX 



Adrenalectomy results in a marked in- 

 hibition of milk secretion and the early ex- 

 periments in this field were reviewed by 

 Turner in 1939. Since then, however, puri- 

 fied adrenal steroids have become available 

 enabling further analysis to be made of the 

 role of the adrenal cortex in lactation. 



Gaunt, Eversole and Kendall (1942) con- 

 sidered that in the rat the defect in milk 

 secretion after adrenalectomy could be re- 

 paired by the administration of the adrenal 

 steroids most closely concerned with carbo- 

 hydrate metabolism, whereas we came to 

 the somewhat opposing view that the defect 

 was best remedied by those hormones 

 primarily concerned with electrolyte metab- 

 olism (Folley and Cowie, 1944; Cowie and 

 Folley, 1947b, c). The reasons for these 

 differing observations are not yet entirely 

 clear. Virtually complete restoration of 

 milk secretion was subsequently obtained 

 in our strain of rat by the combined ad- 

 ministration of desoxycorticosterone acetate 

 (DCA) and cortisone, or with the halogen- 

 ated steroids, 9a-chlorocortisol and 9a- 

 fluorocortisol (Cowie, 1952; Cowie and 

 Tindal, 1955; Cowie and Tindal, unpub- 

 lished; see also Table 10.1). It would there- 

 fore seem that both glucocorticoid and 

 mineralocorticoid activity was necessary to 

 maintain the intensity of milk secretion at 

 its normal level. The interesting observation 

 was made by Flux (1955» and later con- 

 firmed by Cowie and Tindal (unpublished) 

 that the ovaries contribute to the mainte- 

 nance of lactation after adrenalectomy, a 

 contribution which could be simulated in the 

 adrenalectomized-ovariectomized rat by the 

 administration of 3 mg. progesterone daily. 

 The differences in the size of the ovarian 

 contribution may partly accoimt for the ap- 

 parent differences in various strains of rat of 

 the relative importance of mineralo- and 

 glucocorticoids in sustaining milk secretion 

 after adrenalectomy. The only other species 

 in which the maintenance of lactation after 

 adrenalectomy has been studied is the goat 

 in which, as in the rat, lactation can be 

 maintained with cortisone and desoxycorti- 

 costerone, the latter being apparently the 

 more critical steroid (Cowie and Tindal. 

 1958; Figs. 10.12a, b). 



