SUMMARY

The mammary glands of rats either maintained continuously in the animal room on day 14 post partum or isolated in a room without other rats on day 21 post partum, refilled only partially within 8 h after the pups had withdrawn the milk from the glands by suckling with the aid of oxytocin injections to the mother. The partial refilling occurred regardless of whether or not the release of prolactin at the time of suckling was blocked by an injection of an extract of stalk median eminence (SME). The mammary glands of rats not injected with SME continued to refill and reached near capacity within 16 h whereas those of the SME-injected rats failed to refill any further. The glands of the SME-injected rats could be stimulated to refill more completely within 8 h after the initial emptying provided prolactin either was injected or was released endogenously by exposing the mothers to exteroceptive signals from their pups midway during the 8-h period of non-suckling which preceded the initial emptying. More complete mammary refilling also occurred within 8 h if a 4-h rather than an 8-h period of non-suckling preceded the initial emptying of the glands. The results of these investigations indicate that the prolactin released by suckling at the time of the initial emptying of the glands influenced only that milk which reaccumulated in the emptied glands during the 8th—16th h following suckling. The milk which reaccumulated during the first 8 h appeared to be influenced by prolactin released several hours before suckling.

The results suggest, therefore, that a time delay of several hours exists from the time of release of prolactin by suckling until the full stimulatory effect of the hormone upon milk secretion is manifest. Thus the milk secreted after emptying of the glands at a given suckling would be the result of prolactin released earlier either at previous sucklings or in response to exteroceptive stimulation.

SUMMARY

The results of experiments in which the prolactin in the primiparous rat pituitary was bioassayed suggested that the failure of suckling to release prolactin after 8 h of non-suckling on day 21 post-partum was due to the fact that prolactin had been discharged from the pituitary during the 8-h non-suckling period, presumably by exteroceptive signals emanating from the general environment of the animal room. This was substantiated in other experiments in which prolactin release was assessed indirectly through its stimulatory effects upon milk secretion. In these experiments, the mammary glands of rats maintained continuously in the animal room filled faster on day 21 after complete emptying of the glands by exogenous oxytocin, than did either rats on day 14 post-partum maintained continuously in the animal room or rats isolated in a room without other rats on day 21 post-partum. The glands of the latter two groups of rats could be stimulated to fill faster provided prolactin was injected 4 h before the initial emptying of the glands. The exteroceptive stimuli in the animal room environment that stimulated the release of prolactin in the 21-day post-partum rat apparently emanated at least in part from other lactating rats and/or their litters, since faster mammary gland refilling occurred in isolated 21 day post-partum rats when they were exposed to the presence of lactating rats with their litters for 30 min halfway through the 8-h non-suckling period which preceded the initial emptying of the gland. Exposure to male rats, on the other hand, was totally ineffective. A release of prolactin occurred in response to animal room environmental stimuli in the day 14 primiparous rat provided 13–14 day old foster pups were inserted in place of the mother's own pups on day 7. Thus, the rapidly changing characteristics of the pups from 14 to 21 days of age in some manner is involved in the increasing responsiveness of the exteroceptive mechanism for prolactin release which occurs from day 14 to day 21 post-partum.

Lactating rats were fed at a level equivalent to 75% of the daily food consumed by controls, which had food available ad libitum, from days 4–14 or from days 4–12 post partum. The underfed mothers lost about 10% of their initial weight whereas control mothers gained weight slightly during this time. The litters (six pups) of underfed mothers gained 20% less weight than those of controls but remained healthy. The milk obtained by the pups from six glands (the remaining six glands had been ligated on day 3) during a timed suckling, plus that subsequently obtained after oxytocin injections to the mother, totalled 7·4± 0·80 g in underfed and 15± 0·62 g in control mothers on day 14, and 6 ± 0·41 g in underfed compared with 8 ±0·50 g in control mothers on day 12. The pups of underfed mothers obtained a significantly greater percentage of the total milk after 10 (30 v. 6%), 15 (84 v. 32%) and 45 (100 v. 57%) min of suckling than did those of control mothers. In two trials for the milk ejection test, the litters of control and underfed mothers were exchanged. The pups of control mothers suckled by underfed mothers in both trials obtained a significantly greater percentage of the total milk (64%) in 15 min than did the pups of underfed mothers suckled by control mothers (25%).

Oxytocin-induced intramammary pressure (IMP) responses recorded under urethane anaesthesia achieved a greater amplitude in underfed than in control mothers in response to each dose (0·4, 0·8 and 1·6 mu.) of oxytocin administered intravenously. The average slope of the dose–response curve was 1·25 cm H₂O/0·1 mu. oxytocin compared with 0·75cm H₂O/0·1 mu. for fed mothers. The amplitude of the IMP in response to a 1·6 mu. dose of oxytocin also was greater in underfed mothers over a range of volumes of milk added incrementally to empty glands. The difference was most striking at low volumes of milk (0·2–0·6 ml) within the gland. The data from these experiments suggest the more rapid transfer of milk from mother to pups in underfed rats is due at least in part to adaptations within the mammary gland, possibly involving reductions in the sympathetic-mediated tone of mammary ducts.

SUMMARY

The mechanism by which suckling affects the hypothalamo-pituitary-ovarian system was studied in lactating Wistar rats. Unilateral ovariectomy was carried out on day 4 post partum and the subsequent ovarian compensatory hypertrophy (OCH) was assessed on day 14 post partum. The weight of the corpora lutea (CL) of lactation, the CL of pregnancy and of the remainder of the ovary, here termed interstitial tissue, was determined for both ovaries. A clear OCH, mainly due to an increase in interstitial tissue, occurred in rats whose pups were removed on day 4 post partum. Suckling by two pups inhibited OCH, whereas suckling by six or ten pups produced significant ovarian atrophy, these effects being due to a decrease in weight of both interstitial tissue and CL of pregnancy which overcame an increase in weight of the CL of lactation. Inhibition of OCH occurred in spinal-cord transected rats suckling six pups. Suckling was suspended in unilaterally ovariectomized rats nursing ten pups on day 14 post partum, and the weight of the remaining ovary was determined 16, 32, 64 and 96 h later in four groups of rats. The weight of the remaining ovary returned to that of the control ovary 64 h after withdrawal of suckling. This effect was due to an increase in weight of interstitial tissue and of the CL of lactation. Maintenance of suckling for 64 h in rats with the galactophores ligated to prevent milk removal, inhibited OCH. By contrast, both oxytocin injections and exteroceptive stimulation provided by litters failed to block ovarian growth after suckling withdrawal. The results suggest that suckling itself is an important influence in the inhibition of OCH, but that other factors may contribute to the ovarian changes observed during lactation in the rat.

ABSTRACT

Adenohypophysial prolactin of lactating rats was pulse-labelled by [³H]leucine injected i.v. at the time of removal of the pups. The [³H]prolactin concentration in the pituitary gland, analysed by polyacrylamide-gel electrophoresis, progressively fell as the time from labelling to removal of the pituitary gland increased from 8 to 24 h, which suggests that there was a loss of hormone as it aged within the gland. Suckling effectively provoked the depletion–transformation of total and [³H]prolactin (extracted at pH 7·2) when applied after 8 h but not when applied after either 16 or 24 h after removing the pups.

In rats whose pups were removed for 8 h, suckling also depleted–transformed [³H]prolactin labelled 4 h, but not that labelled 1 h before suckling. The pituitary glands of other lactating rats were labelled with [³H]leucine injected i.v. at various times before removing the glands and incubating them in medium 199. The secretion into the medium of [³H]prolactin labelled either 4, 8, 16 or 24 h beforehand was maximal during the first 30 min then declined from 30 to 240 min of incubation. However, secretion of prolactin labelled 1 h and 10 min beforehand reached a maximum after 0·5–1 h and 2 h of incubation respectively, then remained constant during the remainder of the 4-h incubation period. The total 4-h secretion of [³H]prolactin was greatest (65% of preincubation concentration) from those glands labelled 4 h before in contrast to those labelled 10 min (15%) or 1 (38%), 8 (34%), 16 (18%) or 24 h (26%) before incubation. Taken together, these data suggest that prolactin synthesized 4 h earlier is more likely to be released in response to physiological stimuli than is more recently formed prolactin or prolactin which has remained in the pituitary gland for 16 h or longer.

J. Endocr. (1984) 101, 27–32

SUMMARY

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 2–3 days, then rebounded to the expected level over the next 1–2 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 β-adrenergic blocking 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.

SUMMARY

The metabolic clearance of rat prolactin, assessed by the continuous infusion to equilibrium method, increased from 0·77 to 2·15 ml/min in lactating and from 0·65 to 1·75 ml/min in non-lactating rats as the infusion rate was increased from 100 to 472 ng/min. The clearance rate of rat prolactin did not increase when large doses of ovine prolactin were infused simutaneously with rat prolactin. The clearance rates computed from equilibrium levels of immunoreactive ¹³¹I-labelled rat prolactin in plasma were highly correlated with, but were considerably greater than, those computed from unlabelled rat prolactin which was infused simultaneously. In lactating and non-lactating rats, the clearance of each prolactin tested (RP-1, B-1, Nicoll's secreted and ¹³¹I-labelled) increased and had stabilized within the 30–35 min of infusion of each new higher dose of the hormone. The ability of the rat to clear quickly greater amounts of prolactin from the circulation as the infusion rate increases implies that plasma prolactin concentrations do not necessarily mirror the rate of prolactin secretion from the pituitary.

SUMMARY

Intramammary pressure responses to intravenous injections of oxytocin were compared in the urethane-anaesthetized rat before and after various denervation procedures and before and after administration of various adrenergic blocking drugs. Intravenous administration of either phenoxybenzamine, propranolol or pentolinium after a period of time resulted in a more quickly ascending, higher amplitude pressure response but which usually returned more slowly — often oscillating — to the pre-existing basal pressure. Unilateral severance of the dorsal roots supplying the six abdomino-inguinal glands resulted in immediate though lesser increases in both the slope and amplitude of the intramammary pressure response. The responses were increased to a similar extent in contralateral as well as ipsilateral glands caudal to the point of root section whereas the responses of glands rostral to the section were unaffected. The effect of dorsal root section persisted unchanged in rats pretreated with either phenoxybenzamine or propranolol. The intramammary pressures also increased immediately when the segmental nerves were cut. The intramammary pressure was decreased in all glands tested after transecting the spinal cord between T-10 and T-11, then increased dramatically in slope and amplitude over the next 1 to 2 h in glands caudal to the level of section; the responses of glands rostral to the level of cord section were unchanged. The resulting high amplitude responses obtained after cord section usually oscillated and were slow to return to pre-existing basal pressures. The intramammary pressure profile changed in a manner similar to that following spinal cord section when 25% KCl was placed bilaterally on the cerebral cortices. The effect of KCl persisted in rats pretreated with phenoxybenzamine and pentolinium. The results obtained from these experiments suggest that a diffuse motor control of ductal and vascular tone exists in the rat mammary gland which appears to be inhibitory in function and to be sensitive to central as well as to peripheral influences.

Milk yields were measured at 8-h intervals in rabbits during early (days 11–14) and late (days 31–34) lactation. A single injection of 1 mg bromocriptine given to rabbits 30 min before suckling on days 11 or 31 caused a significant reduction in milk yield after approximately 8 h. The depressant effect of the drug was then maintained over the next 24–36 h. Recovery of milk yield occurred in bromocriptine-treated rabbits during both early and late lactation 8-16 h after a single injection of 3 mg prolactin. The recovery accelerated more in the rabbits in the early lactating group. Attainment of the maximal stimulatory effect occurred by 24 h after prolactin injection during both early and late lactation although the improvement in milk yield lasted for a shorter period (8 h compared with 24 h) during late lactation compared with early lactation. These differences in response of the rabbit to prolactin during late lactation may contribute substantially to the declining milk yields characteristic of late lactation in this species.

SUMMARY

The reaccumulation after suckling of prolactin by the pituitary of spayed lactating rats was significantly increased after a single s.c. injection of acidic extract of rat hypothalamus (stalk-median eminence region) given at the start of the reaccumulation period. Injection of ammonium acetate, an extract of ovine hypothalamus rich in luteinizing hormone-releasing factor (LH-RF), extract of rat occipital cortex or oxytocin was ineffective. An extract of ovine hypothalamus rich in prolactin-inhibiting factor (PIF) (0·15 ml./rat) blocked the fall in pituitary prolactin concentration induced by suckling, but in the same or higher (0·3 ml.) doses neither increased nor depressed the reaccumulation of prolactin after suckling. A single s.c. injection after suckling of 2–8 mg. ovine prolactin in each of four instances significantly increased the amount of prolactin reaccumulated by the pituitary. The level attained was equal to or greater than that resulting from injecting rat hypothalamic extract. These results suggest that the increased blood level of prolactin after suckling stimulates the reaccumulation of prolactin in the pituitary possibly indirectly via a factor or factors in the hypothalamus.