Increase in activity of glucose 6-phosphate dehydrogenase in mouse mammary tissue cultured in insulin

Alterations in the activities of rat tissue hexose monophosphate dehydrogenases in response to premature weaning and dietary restriction at mid-lactation

The activities of the hexose monophosphate dehydrogenases increased in adipose tissue, remained unchanged in liver and decreased in mammary gland following the weaning of rats at mid-lactation (day 14). When dietary intake was restricted at mid-lactation, the activities of the hexose monophosphate dehydrogenases increased in adipose tissue, decreased in liver, but were unaltered in mammary gland. Premature weaning on day 14 postpartum resulted in maternal increases in both plasma insulin and glucose, which peaked at day 16. The plasma insulin levels decreased from day 14 to day 18 postpartum in the normal lactating rat, and a similar trend was observed for animals on a restricted dietary intake. Daily food consumption in the lactating rat decreased from 50 g to 20 g after premature weaning. The live weight of pups raised on dams given a restricted food intake from day 14 had decreased by day 17 postpartum, whereas an increase in daily live weight gain was recorded for the litters from the lactating controls. The results demonstrate that the activities of the hexose monophosphate dehydrogenases are regulated differentially between tissues of the lactating rat.

Regulation of enzyme turnover during tissue differentiation. Studies on 6-phosphogluconate dehydrogenase in rabbit mammary gland in organ culture

(1) Explants of mammary gland from mid-pregnant rabbits were cultured in Medium 199 in the presence or absence of insulin, prolactin and cortisol. (2) Antiserum to 6-phosphogluconate dehydrogenase was raised in sheep and used to titrate the amount of enzyme activity present in explant extracts. Changes in enzyme activity were found to be due to corresponding changes in amount of the enzyme. The greatest increases in the amount of the enzyme were only brought about by culture of explants in the presence of hormones (insulin, prolactin and cortisol) in Medium 199 which contained glucose. (3) The increases in the amount of the enzyme were similar in explants cultured with hormones in Medium 199 which contained 1.39 mM, 5.55 mM or 55.5 mM glucose. (4) When explants were cultured with hormones in Medium 199 which contained glucose (5.55 mM) for 24 h and then cultured with hormones in Medium 199 which contained glycerol (10.9 mM), a decrease in the amount of the enzyme occurred. In contrast, the culture of explants with hormones in Medium 199 which contained glycerol (10.9 mM) for 24 h followed by transfer of the explants to medium which contained glucose (5.55 mM) resulted in an increase in the amount of the enzyme to reach values which were not different from those found in explants cultured throughout with hormones in Medium 199 which contained glucose.

A soluble super-active form of insulin

Insulin-like material can be extracted from insulin-Sepharose. This soluble material elicits super-insulin responses from mammary epithelial cells. It stimulates the accumulation of alpha-aminoisobutyric acid by virgin mouse mammary cells, which are unresponsive to insulin itself. It exerts a greater stimulatory effect on mammary cells from pregnant mice than insulin. It also is more efficacious than insulin as a stimulant of DNA synthesis and the combined activities of glucose-6-phosphate dehydrogenase (EC 1.1.1.49) and phosphogluconate dehydrogenase (EC 1.1.1.43). Its specific biological activity on these cells is more than five times greater than that of insulin.

Development of insulin-sensitivity by mouse mammary gland in vitro

Explants from mammary gland of pregnant mice are sensitive to insulin, but corresponding explants from mature, virgin mice are insensitive to insulin in certain respects. Isolated virgin tissue can acquire insulin-sensitivity independent of exogenous insulin. Some endocrinological and developmental implications are discussed.