Mammary sensitivity to protein and energy intake during lactation in rats
Goodwill, Mark George.
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The importance of dietary protein and energy intake to the development and activity of the mammary gland was investigated in lactating rats. Four rat trials were undertaken. The first examined the influence of protein undernutrition and realimentation on mammary gland size and secretory cell activity. The results showed that rats offered a protein restricted diet during lactation suffer mammary underdevelopment, but this may be rapidly reversed by re-alimentation with a diet of high protein concentration. However, lactating rats offered a lower proteinconcentration diet significantly reduce voluntary food intake, which poses a dilemma of interpretation as it is not possible to definitively ascribe the mammary underdevelopment to a protein or energy deficiency.The reduction in food intake shown by these rats also led to a significant loss of body weight due to mobilisation of body tissue. It has previously been proposed that mobilisation of muscle carnosine and haemoglobin, both of which contain relatively high levels of histidine, resulted in elevated levels of plasma histidine. This is converted in the brain to the neurotransmitter histamine which acts on the hypothalamus suppressing food intakeThe second trial investigated the role of the histamine receptor antagonist, cyproheptadine on the voluntary food intake of low protein concentration diets offered to both lactating or young, growing rats. This work indicated that lactating rats offered a protein deficient diet increased their voluntary food intake and also lactational ability until day 8 of lactation, when injected intraperitoneally with cyproheptadine, after which intake dropped. However, the drug had no significant effect on the variables measured in the growing animals.Since cyproheptadine was only effective in lactating rats over the short term, trial 3 was designed to investigate whether the mammary gland’s response to protein re-alimentation, seen in trial 1 was due to an increase in the intake of dietary protein, energy or both. This experiment aimed to differentiate the effects of dietary proteinv and energy intake on mammary development and milk composition by rationing dietary protein and energy allowances separately. In addition, the ability of the maternal body reserves, both protein and lipid, to supplement dietary inadequacies was investigated. The results concluded that increasing the supply of dietary energy, but not protein induced mammary cell proliferation and increased secretory cell activity, while increased intakes of both energy and crude protein improved lactational performance. However, milk composition was largely unaffected by dietary treatment. Maternal lipid reserves were significantly reduced between day 1 and 10 of lactation in all dietary treatment groups. However crude protein reserves were only catabolised following severe protein restrictionThe rapid increases in mammary cell mass observed during lactation in trials 1 and 3 must have occurred through either an increase in cell division, a decrease in cell death or a combination of both. Trial 4 aimed to develop a method suitable for quantifying rates of mammary cell division in vitro using [3H]-thymidine incorporation in explants removed from the mammary glands of rats. A method was developed and it was concluded that the rapid increase in secretory cell mass which occured on realimentation with a diet of high protein to energy ratio was a result of a high rate of cell proliferation and a decreased rate of cell death.