Frequent milking of dairy cows during early lactation
results in a persistent increase in milk yield; however,
the mechanism underlying this effect is unknown. We
hypothesized that increased exposure of the mammary
gland to prolactin (PRL) mediates the milk yield response.
Fifteen multiparous Holstein cows were assigned
to 3 treatments for the first 3 wk of lactation:
twice daily milking with (2× + PRL) or without (2×)
supplemental exogenous PRL, or 4 times daily milking
(4×). Mammary biopsies were obtained at 7 DIM, and
rates of [3
H]thymidine incorporation into DNA in vitro
were determined. Mammary expression of suppressors
of cytokine signaling (SOCS)-1, -2, and -3; the long form
of PRL-receptor; and α-lactalbumin mRNA was measured
by real-time reverse-transcription PCR. Incorporation
of [3
H]thymidine into DNA was not affected by
frequent milking or PRL treatment; however, analysis
of autoradiograms revealed that stromal cell proliferation
was greater in 4× cows. Mammary expression of
SOCS-1 was not affected by milking frequency or PRL
treatment. Expression of SOCS-2 mRNA was increased
with frequent milking or PRL treatment, whereas expression
of SOCS-3 mRNA was reduced by frequent
milking or exogenous PRL. Abundance of PRL-receptor
mRNA was reduced, whereas α-lactalbumin mRNA
was increased with PRL treatment. These results demonstrate
that the bovine mammary gland is responsive
to exogenous PRL during early lactation. In addition,
differences in the response to frequent milking or exogenous
PRL during early lactation indicate distinct effects
of PRL and milk removal on the mammary function of
dairy cows.