The effects of oxytocin and milk removal on milk secretion in the goat

Abstract

SUMMARY1. When goats were milked each hour after being given a dose of synthetic oxytocin within the range thought to be released by the pituitary, there was a progressive rise in milk yield becoming statistically significant by 5 hr. The effect was reduced if the milk was not removed from the gland each hour.2. Milking transplanted glands each hour without injecting oxytocin also increased milk yield. The yield of the unmilked glands on the same animals was not affected. Massaging the transplanted glands had no effect on the milk yield. 5. In some goats very small doses of oxytocin caused changes in milk composition and in one such animal these changes were mimicked by the close arterial infusion of bradykinin.6. Reasons are given for believing that the changes in composition are incidental to the main action of oxytocin in expelling milk and could be caused by a small number of leaks between the tight junctions connecting secretary cells.7. The increase in the rate of milk secretion following milk removal is probably of greater physiological significance than the small changes in milk composition and supports Levy's idea of a local negative feed-back via a chemical component of milk.

Effects of immunization against an autocrine inhibitor of milk secretion in lactating goats.

“…Such a mechanism has been observed in all species that have been studied, including man (see Wilde, Peaker & Knight, 1995b). Recent studies have provided evidence compatible with the hypothesis of Linzell & Peaker (1971) that intramammary control is effected by the presence in milk of a chemical inhibitor of milk secretion. An inhibitory glycoprotein (relative molecular mass (Mr)$ 7600) has been isolated from goat's milk.…”

“…The change in the rate of milk secretion during the declining phase of lactation was calculated by linear regression analysis of the total daily milk yield over successive periods of 4 weeks for each animal and expressed as the mean change (± g day-) over that period. Possible unilateral effects on milk secretion brought about by an ipsilateral change in milking frequency were detected by calculation of the relative milk yields quotient (RMYQ) (Linzell & Peaker, 1971):…”

“…For example, in the goat, raising the milking frequency in one gland from twice daily to hourly (Linzell & Peaker, 1971) or thrice daily (Henderson, Blatchford & Peaker, 1983;Henderson & Peaker, 1984) increases the rate of milk secretion in that gland only. By contrast, decreasing the frequency to once daily decreases the rate (Wilde & Knight, 1990).…”

Mechanism of milk secretion: milk composition in relation to potential difference across the mammary epithelium

“…5) of each animal were milked. By comparing the volume of milk obtained from the treated and untreated glands and applying the RMQ procedure for detecting unilateral effects on milk yield (Linzell & Peaker, 1971d) the amount of milk that would have been expected from the treated gland was calculated. The volume obtained at the end of the experiment was considerably in excess of the volume expected indicating that water had entered milk during the experiment.…”

Influence du nombre de corps jaunes sur la libération d'ocytocine lutéale, le transfert du lait alvéolaire dans la citerne et la production laitière chez la brebis

“…En effet, ce nonapeptide peut aussi être synthétisé, stocké et libéré par le corps jaune (Wathes et Swann, 1982 ;Flint et Sheldrick, 1982 ;Watkins, 1983 ;Rodgers et al, 1983 ;Swann et al, 1984) et c'est ainsi que les augmentations de pression intramammaires observées dans cette espèce, après injection intrajugulaire de PGF2a (Labussière ef al, 1983), résul-tent probablement de l'action de cette hormone lutéale puisque les réponses sont abolies après castration (Labussière et al, 1986), lutectomie (Labussière et al, 1990) ou interruption de la circulation sanguine dans la vascularisation ovarienne (Labussière et Combaud, 1989 (Lakshmanan et al, 1958 ;Elliot, 1959 ;Linzell et Peaker, 1971 ;Labussière et al, 1978). (Nissenson et al, 1978 ;Soloff et al, 1983), il faut malgré tout signaler qu'un pré-traitement à l'acé-tate de fluorogestone suivi d'un traitement progestéronique peut conduire à des résul-tats inverses (Vallet et al, 1990) et il faut aussi savoir que les récepteurs à l'ocytocine de l'utérus et de la mamelle ne semblent pas être soumis au même type de régulation (Soloff et al, 1979 ;Soloff, 1982 Il apparaît donc souhaitable de poursuivre ces travaux sur l'effet galactopoïéti-que des corps jaunes multiples en cherchant à dissocier l'importance relative de l'ocytocinémie (qui facilite le transfert du lait alvéolaire dans la citerne) et de la progestéronémie qui pourrait intervenir non seulement sur le métabolisme des cellules sécrétrices mammaires mais aussi sur la réceptivité à l'ocytocine des cellules myoé-pithéliales.…”