Influence of torpor on milk protein composition and secretion in lactatingbats

In the pipistrelle bat (Pipistrellus pipistrellus), the metabolic load of l actation is not met to any significant extent by increased food intake or m obilization 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 m echanism for energy conservation appears to be the bat's ability to enter t orpor, but this may itself interrupt milk synthesis and secretion unless th e 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 0.2 0, 0.88, and 0.30 mL/d respectively. Protein synthesis measured by incorpor ation of radiolabeled amino acids was 44% lower (P < 0.001) in bat tissue e xplants cultured at ambient temperature (22 degrees C) compared with 37 deg rees 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 [S-35]methionine labelling showed the majority of proteins synthesis ed in culture to be milk proteins; it also demonstrated that the decrease i n protein synthesis at ambient temperature was a general phenomenon: synthe sis 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 var iation in the rate of milk secretion. J. Exp. Zool. 284:35-41, 1999. (C) 19 99 Wiley-Liss, Inc.