Aspects of the Regulation of Lipid Metabolism in the Mammary Gland and Adipose Tissue of the Rat

The effects of insulin, glucagon and other agents on rates of lipogenesis and activity of acetyl-CoA carboxylase were measured in rat mammary glands during lactation in vitro. As a comparison to this, the effect of glucagon on rates of lipolysis in rat adipose tissue in vitro was investigated at different stages of the reproductive cycle. The rate of lipogenesis in acini isolated from mammary glands of mid-lactating rats was studied by measuring the rate of incorporation of 3H from 3H2O into total lipid and fatty acids, with glucose as a substrate. Glucagon did not affect the rate of lipogenesis in rat mammary glands in vitro in the presence or absence of agents which modulate its effects on adipocytes. Glucagon did not antagonize the maximal stimulatory effect of insulin, nor did it alter the insulin dose-response curve. Theophylline, at concentrations up to 20mM, was a potent inhibitor of lipogenesis in acini. Glucagon did not augment the degree of inhibition of lipogenesis induced by 5mM-theophylline. The results suggest that the mammary gland acini do not respond to glucagon in vitro. In agreement with these observations, only a minimal degree of specific binding of 125I-labelled glucagon could be detected in acini which bound insulin normally. This difference in responsiveness of mammary and adipose cell preparations in vitro to glucagon suggests that the two tissues may be differentially responsive to changes in the circulating insulin/glucagon concentration ratio in vivo. Other agents, adenosine and adenosine deaminase were found to have no effect on lipogenesis in mammary gland acini. Relationships between the cyclic AMP content and the rate of lipogenesis in acini prepared from lactating rat mammary tissue were investigated by exposing them to theophylline which increases their cyclic AMP content in the presence or absence of insulin. The dose-dependent inhibition of lipogenesis by theophylline in acini isolated from fed rats was highly correlated with the induced increases in acinar cyclic AMP content. Cyclic AMP of acini from 24h-starved lactating rats was more sensitive in its response to theophylline than cyclic AMP in acini from fed animals. The activity of acetyl-CoA carboxylase in mammary acini was studied under the same conditions as were used for the lipogenesis experiments. Insulin and theophylline were found to have no effect on the activity of acetyl-CoA carboxylase which was in contrast to their effects on rates of lipogenesis. Glucagon, adenosine and adenosine deaminase had no effect, which supported earlier findings. The dose-response curves for the effect of insulin on lipogenesis in rat mammary gland acini were obtained for preparations from lactating animals in different physiological and pathological conditions. A much closer correlation between changes in acinar rates of lipogenesis in vitro and mammary lipogenesis in vivo induced by different physiological states was observed than in previous studies. Acini from diabetic and starved rats had the lowest rates of lipogenesis. Cafeteria-feeding did not affect either the basal or maximally insulin-stimulated rates. Acini from streptozotocin-diabetic rats were most sensitive and responsive to insulin, whereas those from early-lactating rats were least responsive. In addition, lipogenesis in acini from early lactating rats and cafeteria-fed rats were least sensitive to insulin. Starvation (24h) did not affect insulin sensitivity but, after 2h refeeding, insulin sensitivity was significantly decreased compared to that in acini from normal, fed mid-lactating rats. The results indicated that in conditions with low-circulating insulin and low rates of lipogenesis, relative to fed animals, the response to insulin was greater and the concentration of insulin required for the half-maximal response was smaller i. e. increased sensitivity. A study of rates of lipolysis in adipocytes from rats during pregnancy and lactation was undertaken. Incubations were performed in the presence or absence of sub-maximally effective concentrations of adenosine deaminase and in the presence of several concentrations of glucagon (10-11 to 10-5M). Both the maximal response to glucagon and the concentration of the hormone required to elicit half-maximal response (EC50) were markedly altered during pregnancy and lactation compared to those cells from weight-matched virgin rats. The effects of 24h-starvation on these parameters was also altered in pregnant and lactating animals. In the fed state, the highest glucagon-stimulated rates of lipolysis were obtained in adipocytes from mid-lactating rats whereas in the starved state the highest rates were obtained in cells from pregnant and mid-lactating rats. Adipocytes from fed or starved late-pregnant animals had EC50, values for glucagon that were several-fold lower than those from animals in any of the other physiological states studied. By contrast, adipocytes from early lactating rats were least responsive and least sensitive to glucagon. These changes in sensitivity and response of lipolytic rate to glucagon in isolated adipocytes correlated with the extent of triacylglycerol mobilization from adipose tissue in vivo as evidenced from changes in mean cell volume of adipocytes obtained from animals in different stages of the reproductive cycle