G08 



PHYSIOLOGY OF GONADS 



increased respiratory activity of the tissue 

 about the time of parturition in the rat 

 mammary gland, Moore and Nelson (1952) 

 reported increases in the content of certain 

 respiratory enzymes, succinic oxidase and 

 cytochrome oxidase, in the guinea pig mam- 

 mary gland at about this time. Greenbaum 

 and Slater (1957b) made similar observa- 

 tions about mammary gland succinic oxi- 

 dase in the rat. Recent work is beginning to 

 throw light on the metabolic pathways in- 

 volved in this increase in respiratory ac- 

 tivity. Thus McLean (1958a) has adduced 

 evidence indicating an increase in the ac- 

 tivity of the pentose phosphate pathway 

 in the rat mammary gland at about the time 

 of parturition. Mammary gland slices taken 

 from rats at various stages of the lactation 

 cycle were incubated in media containing 

 either glucose 1-C^^ or glucose 6-C^-^, and 

 the amount of radioactivity appearing in 

 the respiratory CO2 was determined. The 

 results given in Figure 10.9 show that al- 

 though the recovery of C^^'Oo from C-6 was 

 relatively unaffected by the initiation of lac- 

 tation, the C^^Oo originating from C-1 be- 

 gan a striking increase at the time of 

 parturition (see also Glock, McLean and 

 Whitehead, 1956, and Glock and McLean, 

 1958, from which Figure 10.9 was taken). 



pregnancy 



in\'oliition 



Imc;. 1().<», The relative amounts of C'Oi; formed 

 fioin iiiilucosc 1-C'^ and glucose 6-C" by rat niani- 



maiy gland slices. O O, C'^Oi formed from 



glucose 1-C^'. • • . C^'Oi! formed from glucose 



6-C". (From G. E. Glock and P. McLean, Proc. 

 Roy. Soc, London, ser. B, 149, 354-362, 1958.) 



Despite the well known pitfalls which sur- 

 round the interpretation of C-1: C-6 quo- 

 tients in experiments such as these, it seems 

 clear that lactation is associated with an 

 increase in the metabolism of glucose by 

 the pentose phosphate cycle, whereas the 

 proportion going by the Embden-Meyerhof 

 jmthway would appear to be relatively un- 

 affected. These conclusions are supported by 

 the fact that the levels in rat mammary 

 tissue of two enzymes concerned in this 

 pathway of glucose breakdown, glucose 

 6-phosphate dehydrogenase and 6-phospho- 

 gluconate dehydrogenase, show very strik- 

 ing increases at the time of parturition 

 (Glock and McLean, 1954; McLean, 1958a). 

 Other enzymes concerned in glucose break- 

 down whose activities in mammary tissue 

 begin to increase at parturition are hexo- 

 kinase and phosphoglucose isomerase (]\Ic- 

 Lean, 1958a). In connection with the glu- 

 cose metabolism of rat mammary tissue it 

 may be noted that addition of insulin to 

 the incubation medium markedly increases 

 the — QOo and R.Q. of rat mammary slices 

 metabolizing glucose or glucose plus acetate 

 (see page 619), and that this tissue only 

 becomes sensitive to insulin just after par- 

 turition (Balmain and Folley, 1951). It 

 is interesting to speculate which of the two 

 above-mentioned pathways of glucose 

 breakdown in mammary tissue resjjonds to 

 the action of insulin. According to Abraham, 

 Cady and Chaikoff (1957) addition of in- 

 sulin in vitro increased the production by 

 lactating rat mammary slices of C^'^Oo from 

 glucose l-C^'*, but not from glucose 6-C^'*, 

 which might indicate that insulin stimulates 

 preferentially the pentose phosphate path- 

 way. Against this, insulin increased the in- 

 corporation of both these carbon atoms 

 (and also the 3:4 carbon atoms of glucose) 

 into fatty acids of the slices to about the 

 same extent. McLean (1959) believes that 

 the stimulatory effect of insulin on the 

 pentose jihosphate pathway in the lactating 

 rat mammary gland is secondary to its 

 stimulating effect on lipogenesis. The latter 

 l)rocess generates the oxidized form of tri- 

 l)hosphopyridine nucleotide (TPN) which is 

 needed for the first two steps of the pentose 

 phosphate cycle. 



The inci-casc in the R.Q. of mammary 



