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1 Lactation and Mammary Gland Biology Group, Department of Animal Science, University of Vermont, Burlington 05405
2 Department of Animal Sciences, University of Illinois, Urbana 61801
3 Animal and Veterinary Science Department, Clemson University, SC 29634
Corresponding author: Thomas B. McFadden; e-mail: tmcfadde{at}zoo.uvm.edu.
Exposure to short day photoperiod (SD; 8 h light:16 h dark) during the dry period increased milk yield of cows in the subsequent lactation. We hypothesized that this effect is due to increased growth of mammary cells in response to enhanced prolactin signaling to influence the insulin-like growth factor (IGF) axis. Multiparous Holstein cows were dried off 60 d before parturition and assigned to long day photoperiod (LD; 16 h light:8 h dark) or SD during the dry period. Mammary biopsies were obtained at approximately –40, –20, –10 and +10 d relative to expected calving. Expression of IGF-I, IGF-II, and IGF binding protein-5 mRNA was assessed by real time reverse transcription-polymerase chain reaction. In SD cows, incorporation of [3H]-thymidine in vitro increased from –40 d to –20 d and was greater at –20 d than in LD cows. A later increase in proliferation was observed at –10 d in LD cows. For both groups, cell proliferation decreased during lactation. Analysis by terminal deoxynucleotidyl transferase dUTP nick end labeling revealed that the apoptotic index in mammary epithelial cells was less in SD cows than in LD cows. Expression of IGF-II mRNA increased during the dry period and into lactation and was greater in SD cows. Expression of IGF binding protein-5 mRNA increased during lactation, but was unaffected by day length. Expression of IGF-I did not differ over time or between treatments. We concluded that exposure to SD during the dry period enhanced mammary growth relative to LD, and this may be related to increased expression of IGF-II. Treatment differences in the temporal pattern of proliferation indicated the existence of a critical period wherein photoperiod affects mammary gland development during the dry period.
Key Words: photoperiod • mammary growth • insulin-like growth factor-II • dry period
Abbreviation key: DABS-E = buffer containing 0.5 M dibasic NaPO4, 0.5 M monobasic NaPO4, 2 M NaCl, and 0.1 M EDTA, dpm = disintegrations per min, IGFBP-5 = IGF binding protein-5, LD = long day (16 h light:8 h dark) photoperiod, PRL = prolactin, PRL-R = prolactin receptor, SD = short day (8 h light:16 h dark) photoperiod, TdT = terminal deoxynucleotidyl transferase, TUNEL = terminal deoxynucleotidyl transferase dUTP nick end labeling.
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