Signalling mechanisms coordinating nutritional status and lactation

Abstract

The pathways by which nutritional status is signalled to the mammary glands and the metabolic sites targeted by these pathways have not been identified. Understanding of these pathways is of particular importance in species such as rodents and ruminants in which mammary metabolism is extremely sensitive to food availability. The studies in this thesis investigated mechanisms by which nutritional state was signalled to the mammary glands using the lactating rat as an experimental model. An in vivo preparation for analysis of the effects of altered nutritional state on substrate supply to, and uptake by, the inguinal mammary glands was established, as this had not been accurately performed before in rats. This preparation was used to record a large fall in mammary glucose uptake with food deprivation and a rapid restoration of uptake after refeeding. Results demonstrated that mammary glucose uptake, and hence mammary metabolism, was not closely linked to glucose supply in lactating rats. Glucose supply is unlikely to be a key factor signalling nutritional state to the mammary glands. Experiments in which the cutaneous branch of the posterior division of the femoral nerve innervating the inguinal mammary glands was severed showed that these neural pathways did not contribute to the maintenance of mammary metabolic activity in the fed and refed states or to the suppressed activity in food deprived rats. Neural signalling is unlikely to have a direct role in controlling mammary metabolism in rats. An in vitro method for measuring the uptake of glucose by rat mammary acini was developed. Insulin administration increased glucose uptake in acini from both fed and food deprived rats. Treatment with a crude gut extract enhanced uptake of glucose in acini from food deprived, lactating rats, but not in acini from fed rats. It was concluded that insulin and/or a factor from the gut may be involved in signalling the mammary gland of the restoration of nutrient supply when food deprived rats are refed. Proteomic studies were performed to investigate the effect of food deprivation and insulin on the abundance of intracellular proteins in acini from fed and food deprived, lactating rats. Analysis of over 800 protein spots detected 7 that were regulated by food deprivation, 26 that were regulated by insulin, and 9 in which the regulation was different in acini from fed and food deprived rats. None were regulated by both food deprivation and insulin. This suggests that decreased blood insulin concentration during food deprivation is unlikely to be the only signal that results in decreased mammary metabolism. Identification of proteins affected by food deprivation and insulin has led to new insights into some of the intracellular mechanisms regulated by these factors.