Influence of torpor on milk protein composition and secretion in lactating bats

In the pipistrelle bat (Pipistrellus pipistrellus), the metabolic load of lactation is not met to any significant extent by increased food intake or mobilization of body reserves, and aerial foraging accounts for most of the animal's energy expenditure even during lactation. Energy conservation must, therefore, play a critical role in maintaining lactation. The principal mechanism for energy conservation appears to be the bat's ability to enter torpor, but this may itself interrupt milk synthesis and secretion unless the pipistrelle mammary gland is adapted to counteract its effect. The effect of torpor on mammary tissue function was studied in mammary tissue explant cultures prepared in weeks 1-3 of lactation, when milk water yield was 0.20, 0.88, and 0.30 mL/d respectively. Protein synthesis measured by incorporation of radiolabeled amino acids was 44% lower (P < 0.001) in bat tissue explants cultured at ambient temperature (22 degrees C) compared with 37 degrees C. The reduction was similar to that observed in mouse mammary tissue (57%) and was unaffected by stage of lactation. Analysis of explant protein after [35S]methionine labelling showed the majority of proteins synthesised in culture to be milk proteins; it also demonstrated that the decrease in protein synthesis at ambient temperature was a general phenomenon: synthesis of both secretory and intracellular mammary proteins was reduced at the lower culture temperature. The results suggest that bat mammary tissue has no mechanism to counteract the effect of reduced body temperature and that periods of lactational torpor are likely to cause a pronounced diurnal variation in the rate of milk secretion.