The effect of hypophysectomy and subsequent replacement therapy with sheep prolactin or bovine growth hormone on the lactose synthetase activity of rabbit mammary gland

Abstract

1. The effects of hypophysectomy and replacement therapy with sheep prolactin and bovine growth hormone on the lactose synthetase activity of the mammary glands of lactating rabbits were studied. 2. There was an approximately fourfold decline in the lactose synthetase activity of homogenates calculated on a DNA basis within 6-7 days of hypophysectomy. Prolactin reversed this decline but growth hormone had no effect. 3. Changes in the properties of a particulate fraction isolated from the glands indicated that a decline in the effective concentration of alpha-lactalbumin was one factor contributing to the decreased lactose synthetase activity after hypophysectomy. 4. As the changes in lactose output produced by hypophysectomy and prolactin therapy are much greater than the changes in total lactose synthetase activity it is concluded that the activity of this enzyme is not the main factor controlling lactose output under these conditions.

Properties of a prolactin receptor from the rabbit mammary gland

“…We have previously demonstrated that two hormones other than prolactin, namely human growth hormone and human placental lactogen, which are equipotent with prolactin in the rabbit in studies in vivo and in vitro, are the only hormones that are capable of competing with 125I-labelled prolactin for the receptor site (Shiu et al, 1973). On the other hand, growth hormones from other species, which cannot compete for the prolactin receptors in our membrane fractions, are also not lactogenic in the rabbit (Cowie & Tindal, 1971;Forsyth et al, 1965;Forsyth & Folley, 1970;Forsyth, 1972;Frantz et al, 1972a,b;Barnawell, 1965;Jones & Cowie, 1972). The finding that '25I-labelled ovine prolactin is accumulated by the rabbit mammary gland in vivo (Birkinshaw & Falconer, 1972) further suggests that the binding of prolactin to membranes in vitro is not an artifact.…”

Mammary Growth During Pregnancy in Hypophysectomized or Bromocriptine-Treated Goats

“…A portion of the homogenate was centrifuged for 30 min at 57 000 g and the supernatant fraction was collected. Lactose synthetase activity was assayed in the homogenate following the procedure of Jones «fe Cowie (1972). Before performing the assay a portion of the homogenate was mixed with an equal volume of 1-5% digitonin in 0-1 M-Tris-HCl, pH 7-4, and kept at 0°C for 30 min to disrupt particulate structures and to allow the full enzyme activity to be measured (Jones, 1972).…”

“…Biochemical assays A 1-5 g sample of lobulo-alveolar tissue was minced, washed and homogenized in medium containing 0-28 M-mannitol and 1-1 mM-EDTA, pH 7-4, following the procedure previously used for rabbit mammary gland (Jones «fe Cowie, 1972). A portion of the homogenate was centrifuged for 30 min at 57 000 g and the supernatant fraction was collected.…”

Lactose biosynthesis

“…Digitoxin proved an effective agent for this purpose. Nevertheless, it is difficult to explain why the levels of c~-lactalbumin estimated by JONES (1972) in lactating glands were about 2-6 gmolar as compared with concentrations of ~-lactalbumin in milk of 0.1-1.0 millimolar. MCKENZlE et al (1971) have also pointed out that there are species differences between the rat and mouse (Figs.…”