PHYSIOLOGY OF PROLACTIN AND GROWTH HORMONE IN LACTATING COWS

THEODORE ALAN MOLLETT, Purdue University

Abstract

A new radioimmunoassay was developed and validated for the measurement of bovine growth hormone (GH). Plasma concentrations of prolactin (PRL) and GH were measured at 30-min intervals for three consecutive 24-hr periods in six lactating cows. Concentrations of PRL in milk were comparable with those in plasma. Concentrations of PRL in plasma followed a 24-hr periodicity with an additional 6-hr periodicity imposed upon the circadian rhythm. Plasma GH was stable and did not follow a circadian periodicity. Milk PRL concentrations were not significantly correlated with PRL concentrations in plasma. The total amount of milk PRL removed at each milking was correlated with plasma PRL averaged over the 12 hr before milking. The interaction between inhibition of PRL release by lergotrile and regular removal of prepartum mammary secretions (milk) was investigated. Eight cows were divided as follows: (a) placebo-treated (4 cows) and (b) two groups (2 cows/group) of lergotrile-treated (160 mg/day) designated lergotrile-I and lergotrile-II. Prepartum milk from one-half of the udder was removed by twice daily milking (removed half udder). The other half udder was not milked prepartum (retained half udder). The extent to which circulating PRL was suppressed by lergotrile was similar during most of the prepartum period in lergotrile-I and lergotrile-II cows. However, complete inhibition of the periparturient surge of PRL was observed only in the lergotrile-II cows. Plasma GH was unaffected by lergotrile. Concentrations of or the total amount of PRL transferred into the milk did not differ between placebo and lergotrile-I cows. Both of these parameters were reduced in the lergotrile-II cows. Prepartum milking in placebo and lergotrile-I cows, but not in lergotrile-II cows, increased the total cumulative amount of PRL transferred into the milk of the removed half udder compared to the retained half. It was clearly shown that PRL was accumulated into the alveolar lumen during the prepartum period at concentrations much higher than those observed in plasma. Since similar concentrations of milk PRL were observed in all three treatment groups in spite of different plasma PRL, it was concluded that the transfer of PRL into the intraluminal secretions was not correlated with levels of plasma PRL. It probably depended upon the hormonal environment of late pregnancy, lactogenic immaturity of the alveoli, and regular removal of milk to allow additional secretion. Both average daily yield of milk and total 60-day yield of fat-corrected milk were increased in the removed half udder of lergotrile-I cows as compared to their retained half udder, but this difference did not occur in placebo or lergotrile-II cows. We hypothesize that this increased production was the result of increased quantities of PRL transferred into the alveolar lumen as a result of prepartum milking and that this transferred PRL was stimulatory to lactogenesis and long term milk yield because plasma PRL was inadequate. Increased transfer of PRL into the removed half udder of placebo cows was of minimal consequence to lactogenesis because in the retained half udder circulating PRL was adequate to initiate lactogenesis and sustain the maximum milk synthesis of these cows. In lergotrile-II cows lactogenesis was delayed until near parturition at which time both halves of the udder were milked. Therefore, transfer of PRL was similar in both removed and retained halves and did not produce a differential effect on milk yield.

Degree

Ph.D.

Subject Area

Anatomy & physiology|Animals

COinS