Effect of Acute Reduction in Milk Removal From Some Glands Upon the Ability of a Single Rabbit Mammary Gland to Secrete Milk

Abstract

Milk yield fell in a single rabbit mammary gland within 24 h after the litter had been reduced from six to eight pups to one pup. The diminished secretion lasted 2\p=n-\3 days, then rebounded to the expected level over the next 1\p=n-\2 days and thereafter maintained this level. During early to mid-lactation the decrease in milk secretion after reduction of the litter to one pup was prevented either by s.c. injections of prolactin or by s.c. injections of the \g=b\-adrenergicblocking drug, propranolol. These results suggest that milk secretion was reduced in the single gland as the result of sympathetic activation, triggered possibly by the turgid state which developed in the remaining glands after the litter was reduced. The results suggest also that the depression of milk secretion by the sympathetic system was due to a reduction in the effective amount of prolactin and probably of other adenohypophysial hormones reaching the mammary secretory cells rather than to an impairment of their release from the anterior pituitary.The magnitude of the depression and rebound in milk secretion of a single rabbit mammary gland, and the effectiveness of the preventive action of either propranolol or prolactin, was considerably less during late as compared with early to mid-lactation. We postulate from these results that the mammary gland of the rabbit has a functional lability, i.e. responsiveness, which diminishes as lactation progresses and which dictates the extent to which local, neural and humoral factors may alter the milk secretory function of the mammary gland. Since suckling in the rabbit does not change from a once-a-day frequency throughout lactation, a diminishing lability may be an important factor in the decline and cessation of lactation in this species.

The effects of prolactin and oxytocin on milk secretion and on the permeability of the mammary epithelium in the rabbit.

“…But he offered no statistical treatment of the results which makes interpretation difficult. Mena, Reyes, Aguayo & Grosvenor (1974) in a complex series of experiments also suggested that tissue sensitivity to circulating prolactin diminishes as lactation progresses. At present no firm data exist to indicate whether prolactin secretion decreases, whether tissue sensitivity declines, or whether both processes occur and this problem will probably not be solved until a method becomes available for the determination of plasma prolactin concentrations in the rabbit.…”

“…It has previously been argued that the passage of ['4C]sucrose from blood to milk is a measure of paracellular movements across the mammary epithelium c, 1974. Therefore the passage of [F4C]sucrose, 24Na and 36C0 from blood to milk was studied in four prolactin-treated rabbits, 12 hr after the last injection.…”

The possible involvement of protein kinase C(s) and inositol phosphate metabolism in the basal but not in the prolactin stimulated casein release by the lactating rabbit mammary epithelial cell

“…In addition, release of PRL by suckling in rabbits accounts for high milk yields during early lactation and for maintenance of lactation (Mena et al, 1974;Taylor and Peaker, 1975). Moreover, PRL exerts a direct effect (secretagogue effect) on the release of caseins by epithelial mammary cells (Ollivier-Bousquet, 1978).…”

Lactation in the Rabbit: Mammary Blood Flow and Cardiac Output

“…prevent the changes in composition and yield [Gachev, 1963a[Gachev, , 1965[Gachev, , 1971aCowie, 1969;Linzell, Peaker and Taylor, 1975], it has been suggested that either the release of prolactin is less in late lactation [Cowie, 1969] or the sensitivity of the mammary tissue to prolactin is reduced [Gachev, 1971a;Mena, Reyes, Aguayo and Grosvenor, 1974]. Recently, McNeilly andFriesen [1977, 1978] have developed a radioimmunoassay for rabbit prolactin and have found that the amount appearing in plasma in response to suckling decreased as lactation advanced.…”