The role of growth hormone, prolactin and insulin-like growth factors in the regulation of rat mammary gland and adipose tissue metabolism during lactation

Abstract

Inhibition of prolactin secretion with bromocriptine and neutralization of GH action with a specific antiserum to rat GH (rGH) were used to explore the modes of action of GH and prolactin in maintaining lactation in the rat. Treatment of dams with anti-rGH caused a small reduction in litter weight gain whilst bromocriptine reduced litter weight gain by 50%. When both treatments were combined, however, milk yield ceased completely and this was accompanied by a wide variety of effects on mammary lipid metabolism including decreases in the mRNA concentrations of acetyl CoA carboxylase, fatty acid synthase, malic enzyme and lipoprotein lipase. Activities of acetyl CoA carboxylase and lipoprotein lipase were also significantly reduced. Reciprocal changes were evident in adipose tissue with increases in acetyl CoA carboxylase and lipoprotein lipase activities. In conjunction with a decreased lipolytic response to noradrenaline in adipose tissue of animals given the combined treatment of bromocriptine and anti-rGH, this represented a co-ordinated series of changes to reduce lipid synthesis in the mammary gland and enhance lipogenesis and triglyceride storage in adipose tissue as milk production ceased. All of these effects could be prevented in part by concurrent treatment with GH, but insulin-like growth factor-I (IGF-I) and IGF-II failed to affect any of the parameters measured.(ABSTRACT TRUNCATED AT 250 WORDS)

GH but not IGF-I or insulin increases lipoprotein lipase activity in muscle tissues of hypophysectomised rats

“…We did not observe any significant effect of GH or IGF-I on LPL mRNA levels in adipose tissue, although IGF-I treatment tended to decrease LPL mRNA levels. Other studies in the rat have indicated that GH decreases LPL activity in adipose tissue (Murase et al 1981, Barber et al 1992. The discrepant results may be explained by different experimental designs and differences in the duration of treatment.…”

“…The lack of effect of GH treatment on LPL activity in parametrial adipose tissue was surprising in view of previous findings in the rat on this activity (Murase et al 1981, Barber et al 1992. Although the experimental designs differ in several ways, one obvious difference is the mode of administration of GH.…”

“…Although the experimental designs differ in several ways, one obvious difference is the mode of administration of GH. It is possible that two daily injections, as previously used by Barber et al (1992), could result in another response, which could be dependent on the time of killing. Hypophysectomised rats were therefore given two daily injections or a continuous infusion of GH for 7 days.…”

“…In adipose tissue, GH has been shown to decrease LPL activity in humans (Ottosson et al 1995, Oscarsson et al 1996 and rats (Murase et al 1981, Barber et al 1992. Comparatively less attention has been paid to the hormonal regulation of muscle LPL activity.…”

Limits to sustained energy intake VII. Milk energy output in laboratory mice at thermoneutrality

“…The suckling stimulus induces the release of oxytocin and prolactin from the pituitary gland via a neuroendocrine reflex. Oxytocin is responsible for milk let-down and ejection, whereas prolactin, which is the most important lactogenic hormone, activates the transcription of RNAs for milk proteins and enzymes involved in the synthesis of milk fats and sugars (Russel, 1980;Barber et al, 1992;Flint and Knight, 1997). Consequently, frequent suckling (or milking) increases milk production, whilst forced weaning, or a gradual cessation of milk removal that occurs during natural weaning, initiates the process of mammary gland involution (e.g.…”