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Role of Prolactin


Prolactin

Cows walking to the barn.

Prolactin is a primary component of the galactopoietic complex of hormones.

Considerable species variability exists in the importance of PRL in maintenance of lactation. For example, in hypophysectomized rabbits, exogenous PRL administration alone restores lactation to normal (Cowie et al., 1969, J. Endocrinol. 43:651) and PRL is galactopoietic in the intact lactating rabbit (Cowie, 1969, J. Endocrinol. 44:437).

In the hypox. rat, PRL alone does not quantitatively restore lactation to prehypox. levels (see Cowie, 1966, Anterior pituitary function in lactation. In The Pituitary Gland, Vol. 2, pp. 412-443, Eds. Harris and Donovan, Butterworths, London). Minimum requirement in hypox. rats to restore lactation to levels adequate to rear pups is administration of PRL plus glucocorticoid. Administration of GH plus PRL and glucocort.) improves lactational performance.

In the hypox. goat, administration of PRL, GH, glucocort., plus thyroid hormone are required to restore lactation to prehypox. levels (Cowie et al. 1961 J. Endocrinol. 23:79; Cowie et al. 1964 J. Endocrinol. 28:267)

Exogenous PRL is galactopoietic in intact lactating rabbits, while in intact lactating rats, administration of exogenous PRL stimulates milk secretion in early lactation (and reduces the time required for refilling of the mammary gland following suckling). Continuous infusion of PRL into lactating rats may prevent the expected decline in milk production in late lactation.

In many nonruminants, suppression of blood PRL concentrations by ergot alkaloids (such as bromocriptine) does reduce lactational performance. Administration of L-DOPA (which also inhibits PRL secretion from the pituitary) reduces milk yeild, although L-DOPA also inhibits milk ejection. Injection of antisera to PRL receptors can inhibit PRL binding in the rat mammary gland. This inhibits PRL-induced casein synthesis and milk yield is decreased.

So, PRL secretion rates from the pituitary may be limiting in rats and rabbits. The importance of PRL in maintaining lactation in nonruminants is well established, as the above examples illustrate. However, in most nonruminants and ruminants PRL is only one component of a complex of hormones which regulate lactation. Furthermore, the role of PRL in maintaining lactation is less clear in some other species, especially ruminants.

The role of PRL in galactopoietic in cattle is ambiguous. Suppression of PRL in cows and goats (and in guinea pigs) by ergot alkaloids (bromocriptine) has minimal effects on milk yield, especially compared with decreases of 50% or more when comparable experiments in rodents or near complete failure of lactation in rabbits. An exception to this seems to be the lactating ewe, where bromocriptine administration markedly reduces milk yield (Hooley, R.D., J.J. Campbell, and J.K. Findlay. 1978. The importance of prolactin for lactation in the ewe. J. Endocrinol. 79:301-310), and drugs which stimulate PRL release enhance milk yield (Bass, E., J. Shani, Y. Givant, R. Yagil, and F.G. Sulman. 1974. The effect of psychopharmacological prolactin releasers on lactation in sheep. Arch. Int. Pharmacodynami. Ther. 211:188-192).

So, blood concentrations of PRL in ruminants generally do not seem to be limiting for maximal milk yield (except perhaps in the ewe). However, some evidence does indicate that a 2 hr infusion of PRL into lactating goats starting 30 min after milking may result in a slight (2.6%) but statistically significant increase in milk yield (see Jacquemet and Prigge, 1991, J. Dairy Sci. 74:109).

For more on experimental models used to study lactogenesis and galactopoiesis see Wilde, C.J. and W.L. Hurley (1996. Animal models for the study of milk secretion. J. Mammary Gland Biol Neoplasia 1:123-134).


Mammary PRL receptors:

In the rat mammary gland, PRL receptors increase 4 fold within 2 days of parturition, then decline gradually over lactation. PRL receptors can also be affected by presence or absence of galactopoietic hormones, as indicated in the table below.

Treatment

decline in PRL receptors in rat mammary tissue

ovariectomy

59%

adrenalectomy

45%

hypophysectomy

35%

 

Suckling or milking induces a Prolactin surge in the blood

(see Tucker HA, 1994 above; also see Jakubowski and Terkel, 1986, Endocrinology 118:8)

There is a milking-induced or nursing-induced release of PRL (see graph below; adapted from Tucker 1994). This surge of PRL (green line in the graph) is small compared with the peripartum surge of PRL associated with lactogenesis; about a 3-fold increase over non-stimulated PRL concentrations (blue hatched line). However, the milking-induced PRL surge is a direct link between the act of nursing or milk removal and the galactopoietic hormones involved in maintaining lactation. The surge occurs over a period of about a half of an hour after milking or nursing. This compares with the oxytocin surge which only lasts about 5 to 10 minutes (red stippled box in the figure). Part of the galactopoietic response to nursing intensity (litter size) in pigs or rodents may involve the amount of PRL released at nursing.

Graph of blood prolactin concentrations after teat stimulation.

Suckling or milking probably works by decreasing prolactin inhibiting factor (PIF) from the hypothalamus, and therefore increasing PRL levels. It may also act to increase the response of the pituitary to prolactin releasing factors (Samson et al., 1989, Endocrinology 124:812). The effect of suckling on PRL declines with advancing lactation, even if nursing stimulus is kept equivalent throughout lactation.

Relationship of milk yield to blood prolactin

(see Koprowski and Tucker, 1973, Endocrinology 92:1480)

In dairy cattle, there is no correlation between milk yield and PRL levels 2-4 hr before milking or 1 hr after milking. However, there is a correlation between milk yield and PRL levels 5 min. post-milking. This may reflect the post-milking PRL response which only lasts for about an hour.


 
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