Intramammary Pressure Research Articles

Mammary function and its control at the cessation of lactation in the goat.

1. The changes in mammary function following cessation of milking during declining lactation have been studied in conscious goats. 2. No significant changes in the rate of milk secretion, mammary blood flow or metabolism occurred in the first 24 h after cessation of milking. After then, secretory rate, mammary blood flow, oxygen consumption, glucose uptake and acetate uptake decreased markedly over the next 3 days. Up to the time of maximum udder distension on day 3, there were no major changes in milk composition. 3. It was found that the rate of milk secretion declined when the calculated pressure within the alveoli became positive. 4. After 3 days, mammary volume and intramammary pressure decreased, and the composition of milk changed slowly to resemble that of extracellular fluid, i.e. [Na+], [Cl-], [HCO3-] and pH increased while [K+], [lactose] and [citrate] decreased. During this time [lactose] and [K+] were positively correlated, and [lactose] and [Na+], and [lactose] and [Cl-] negatively correlated. 5. It is suggested that the changes in milk composition, the decreases in mammary volume and in intramammary pressure after day 3 are due to the loss of integrity of the mammary epithelium. 6. By about 7 weeks after the cessation of milking the udder volume was less than the empty udder volume before milking was stopped, indicating a loss of mammary tissue as well as the resorption of fluid. 7. When milking of an autotransplanted gland was stopped, while milking of the control gland in situ was continued, the rate of secretion in the transplant fell while that of the control did not change. 8. In goats milked normally but in which a volume of isosmotic lactose equal to the volume of milk removed at that milking was injected into the lumen of one gland at each milking, the rate of secretion of that gland, but not that of the other, decreased.

The role of central adrenergic receptors in the reflex release of oxytocin

The effects of the α-adrenergic receptor antagonists, phentolamine and phenoxybenzamine, and the β-antagonists, propranolol, oxprenolol and practolol on the milk-ejection reflex of the rat were studied. The mother rats, at day 8–11 or lactation, were anaesthetized with urethane (1.2–1.4 g/kg, i.p.) and litters of 9–11 hungry pups were placed on the nipples to suckle for 2–6 h. With 65% of the 425 rats studied, a regular pattern of milk ejection was observed, from the changes in pup behaviour and intramammary pressure, with milk ejection recurring at intervals of 3–15 min throughout the nursing period. Propranolol (1.0–1.5 mg/kg, i.v.) and the other β-antagonists failed to either facilitate or inhibit the suckling-induced reflex when given to animals that were already milk ejecting. When given to animals which were not milk ejecting or which had ceased to milk eject in response to the suckling stimulus, both propranolol and oxprenolol at doses as low as 30 μg/kg promoted a normal pattern of reflex milk ejection. Practolol (1.5 mg/kg, i.v.), a β-antagonist with limited access to the brain, did not facilitate the reflex when given to these refractory animals. Both the racemate and the l-isomer of propranolol were effective in promoting reflex milk ejection when given by either the intravenous or intraventricular route; the d-isomer was ineffective even at 1.5 mg/kg, i.v. The rats which had failed to milk eject when suckled had mammary glands that were as sensitive to oxytocin as other animals, and the abrupt onset of the milk-ejection reflex following propranolol was apparently not related to the lowered threshold of mammary gland sensitivity to oxytocin promoted by the β-antagonists. Slow infusions of adrenaline and isoprenaline (0.1–0.2 μg/min) failed to inhibit the milk-ejection reflex even at doses that reduced the gland sensitivity to oxytocin. The α-adrenergic antagonists had an opposite effect and produced a dose-dependent inhibition of the reflex when given to animals that were milk ejecting at regular intervals. Thus, 1 mg/kg phentolamine, i.v., increased the milk ejection interval 6-fold and 2 mg/kg increased the interval 13-fold. Both phentolamine and phenoxybenzamine caused a marked fall in blood pressure, but a similar fall in blood pressure was induced with hexamethonium (2 mg/kg, i.v.) without altering the pattern of reflex milk ejection. Two other centrally active drugs were also examined, picrotoxin (2–4 mg/kg, i.v.) and hyoscine (90 mg/kg, i.v.), but neither were found to influence the reflex. These results suggest that both α- and β-adrenergic receptors are involved in the central control of the milk-ejection reflex in the anaesthetized rat, and both are probably activated by neuronally released noradrenaline. The α-receptors would appear to form a component of the natural reflex and are excitatory. The β-receptors are external to the reflex and are inhibitory.

Plasma concentrations of prolactin and thyrotrophin during suckling in urethane-anaesthetized rats.

Plasma levels of prolactin and TSH were determined by radioimmunoassay in urethaneanaesthetized lactating rats during suckling. Oxytocin release was monitored by recording intramammary pressure. Application of ten pups, 3 h after administration of urethane (1-1 g/kg, i.p.), evoked a parallel rise in prolactin and TSH concentrations which reached a maximum during the 3rd hour of suckling and then declined. Peak hormone concentrations represented a 25-fold increase in prolactin and a ten-fold increase in TSH. Suckling also elicited a pulsatile (every 5-10 min) release of 0-5--1-0 mu. oxytocin. The gradual rise in prolactin and TSH occurred between the 1st and 20th oxytocin pulses. Intravenous injection of thyrotrophin-releasing hormone (TRH) into unsuckled, anaesthetized lactating rats resulted in a 7- to 30-fold increase in TSH concentration, whereas prolactin levels showed no substantial change. These results indicate that suckling releases TSH as well as prolactin in the urethaneanaesthetized rat. However, the absence of prolactin release after injections of TRH makes it unlikely that both endocrine responses are regulated solely by the actions of this one releasing hormone.

Excitation of phasically firing hypothalamic supraoptic neurones by carotid occlusion in rats.

1. The activity of supraoptic neurones has been recorded extracellularly during bilateral occlusion of the common carotid arteries in anaesthetized rats. 2. Experiments in lactating rats showed that occlusion liberated sufficient amounts of neurohypophysial hormones to cause a rise in intramammary pressure 15-25 s after the onset of occlusion. 3. Ninety-one percent of the phasic neurones (defined as those showing bursts of activity alternating with periods of silence) were activated by carotid occlusion less than 10 s after the onset of occlusion. Most randomly firing neurones were inhibited or were unaffected. 4. The activation of phasic neurones is unlikely to be just a nonspecific effect, because in the same animals, phasic neurones were excited whilst random neurones were not. 5. Moreover, in phasic neurones, statistical analysis shows (a) that the intervals during which an occlusion was performed were significantly shorter than the intervals between spontaneously occurring bursts, and (b) that this activation was followed by a period of reduced firing probability. 6. The results are discussed with reference to the correlation of supraoptic neuronal activity with hormone release. The possibility is considered of relating the tendency of some supraoptic neurones to fire in bursts with the secretion of vasopressin.

Isolation of kallikrein from mammary gland of cows.

Small amounts of kallikrein were isolated from bovine mammary tissue. Immediately after slaughter, the udder was removed, perfused with ice-cold xylocaine-Rheomacrodex solution, and homogenized in ice-cold acetic acid solution. Kallikrein-like substances were adsorbed on DEAE-cellulose, eluted with ammonium formate and fractionated by acetone precipitation. The 40-70% acetone fraction was separated on a Shephadex-G 75 column and aliquots of each fraction were incubated for 1 or 2 hr with kininogen. After incubation, extracts were tested for kinin activity on the isolated rat uterus, the rat duodenum, blood flow through the femoral artery of dogs and by intramammary pressure assay in sheep. Release of kinins was demonstrated by all these techniques, highest kallikrein activity being detected in the molecular weight range 50,000 to 60,000. Direct injection of the enzyme into the udder artery of sheep, without prior incubation with kinninogen, resulted in vasodilation and a rise in intramammary pressure. The kallikrein activity was strongly inhibited by aprotinin, but not by soybean trypsin inhibitor or ovomucoid. Kallikrein activity was always associated with esterase activity.

Factors governing the periodic activation of supraoptic and paraventricular neurosecretory cells during suckling in the rat.

1. One hundred and thirty-two antidromically identified paraventricular (PV) and supraoptic (SO) neurones were studied during milk ejection evoked by the suckling of the young, in thirty-four lactating rats anaesthetized with urethane (1.1 g/kg I.P.). The spike activity of these neurones was analysed to determine some of the factors governing their orthodromic activation. Changes in spike activity were compared with the amount of oxytocin released from the neurohypophysis, as measured by the rise in intramammary pressure at milk ejection. 2. Sixty-five units, from both the PV and SO nuclei, displayed a characteristic burst of accelerated activity 12-18 sec before milk ejection. With a suckling stimulus of nine or ten pups, these neurosecretory responses were both regular in their occurrence (recurring evey 4-8 min) and uniform in character, i.e. all responsive cells accelerated to 30-80 spikes/sec for 1-4 sec. Two units deviated from this pattern and both were recorded in animals which failed to display a detectable milk ejection...

Electrophysiological evidence for the activation of supraoptic neurones during the release of oxytocin.

1. Antidromically identified supraoptic (SO) units were recorded in lactating rats anaesthetized with urethane (1.1 g/kg), and their activity was studied during milk ejection evoked by the suckling of the young. Fifty-eight SO units were recorded through 174 milk ejections. Each milk ejection was the result of a neurohypophysial release of an oxytocin pulse of 0.5-1.5 m-u.2. Fifty-five of the SO units displayed background activity and three were silent. The firing rates ranged from 0 to 15.4 spikes/sec, the distribution was exponential with 26% of the units firing at < 1 spike/sec. Sixteen (28%) of the SO units displayed a phasic pattern of activity characterized by periods of activity (6-132 sec) and silence (4-71 sec).3. Twenty-five of the non-phasic units displayed a large and stereotyped acceleration in spike activity some 10.5-17.4 sec before the rise in intramammary pressure at milk ejection. Units accelerated to rates between 9-66 spikes/sec, an increase of about thirtyfold (median) on background activity. The response was brief (0.9-4.7 sec) and was followed by a period of after-inhibition.4. It was concluded from studies of double recordings and observations of multi-unit activity that all the responsive units were synchronously activated. The mean latency of 13.3 sec between the onset of the neurosecretory response and milk ejection was similar to that observed following the experimental release of endogenous oxytocin by electrical stimulation of the neurohypophysis (50 pulses/sec for 2-4 sec).5. Four of the phasically active units were correlated with the oxytocin release for milk ejection. Three of these units displayed a burst of activity superimposed on the terminal portion of an active phase, some 10.2-14.7 sec before milk ejection. The fourth unit, recorded in conjunction with a responsive non-phasic unit, consistently switched from silence to activity coincident with the onset of the SO activation.6. The remaining SO units and a further ten units that were not antidromically activated by neurohypophysial stimulation displayed no change in activity during either the period of neurosecretory activation or the period of after-inhibition.7. This activation of the SO neurones, in the formulation of oxytocin release and milk ejection, is the same as that we have previously observed in recordings from the paraventricular (PV) region, and the proportion of neurones displaying the response is similar: 48% in the SO nuclei, 58% in the PV nuclei. We conclude, since the SO nuclei contain 80% of the neurosecretory cells that project to the neurohypophysis, that the SO nuclei are as important, if not more so, than the PV nuclei in the control of oxytocin release.

Measurement of intramammary pressure in rats by a pressure sensitive radio-pill

A simple method for the adaptation of a pressure sensitive radio-pill for the measurement of intramammary pressure in rats is described. Intramammary pressure was recorded by this telemetric system in both anaesthetized and conscious animals.

Release of an oxytocic substance following genital stimulation in bulls.

Summary. The oxytocic activity of the jugular blood plasma of bulls before and after copulation, ejaculation into an artificial vagina, massage of the seminal vesicles and ampullae and stimulation with an electroejaculator increased from below 0\m=.\01\g=m\U/mlto a peak level varying from 2 to 1000 \g=m\U/ml 4 to 6 min after ejaculation or 6 to 9 min after the start of electrical or manual stimulation. Although males store oxytocin in the posterior pituitary in amounts similar to those found in females, the mechanism of oxytocin release and its function in the male are not clearly understood. In an experiment using cross circulation between a ram and a ewe, Debackere, Peeters & Tuyttens (1961) observed a sharp increase in the intramammary pressure in the ewe after massage of the seminal vesicles and ampullae in the ram, indicating a release of oxytocin in the ram. Oxytocic activity (OA) has been found in the blood of the bull (Bereznev, 1963a; Sharma & Hays, 1968) and the ram (Sharma, Fitzpatrick & Ward, 1972). The present investigation was undertaken to study the release of an oxytocic substance into the blood of bulls following varying types of stimulation of the genital organs. Five normal, 2-year-old, sexually experienced bulls with good libido (two Brown Swiss, one Guernsey and two Holstein) were used in the experiment. An indwelling polyethylene cannula was inserted into the external jugular vein of each bull 2 to 3 days before the start of the experiments. The genital organs were stimulated by: allowing the bull to copulate, serving an artificial vagina, manually massaging the seminal vesicles and ampullae per rectum for 5 min, or stimulation with an electroejaculator intermittently for 5 min. Control samples of blood from each bull were collected before removing the bull from the pen and before exposure to stimulation. Blood samples were collected, 1, 2, 3, 4, 5, 10, 15, 20 and 25 min after ejaculation or stimulation. Blood (10 ml) was collected using a polyethylene syringe containing 0-1 ml heparin solution (1 mg heparin sodium/ml isotonic saline). The time required to collect each blood sample was 5 to 10 sec and the samples were immediately cooled by placing in chopped ice. Within 2 hr of collection, the samples were centrifuged, the plasma

Intramammary Pressure and Mammary Blood Flow in Lactating Goats

Blood flow to the udders of four conscious, lactating goats was recorded from a surgically implanted electromagnetic flow transducer while intramammary pressure was artificially altered in the range 5 to 100mm mercury. Elevation of intramammary pressure consistently reduced mammary blood flow. The response was linear with the regression equation: % of Control Flow = 99.5−.83×Intramammary Pressure. Mammary blood flow responded slowly to abrupt changes in intramammary pressure, and reactive hyperemia was never observed following pressure release. Both air and milk were utilized to alter intramammary pressure, and no difference in response was significant. When the contralateral side was insufflated, no alteration in the blood flow occurred. Lidocaine in a supramammary line block partially diminished the response to elevated intramammary pressure.

Effect of locally implanted oestrogen on the response of the rat's lactating mammary gland to oxytocin.

ABSTRACT On the day following delivery (day 1 of lactation) one abdominal mammary gland was implanted with oestrogen and the contralateral gland received an empty needle. At 2, 5 or 10 days of lactation the rats were anaesthetized with pentobarbital and the nipples of both abdominal glands were cannulated and their pressures recorded by means of transducers coupled to an amplifier and recording system. The normal mammary glands of 5-day lactating rats responded to very low doses of oxytocin (Syntocinon®, Sandoz) (5× 10−8 mU) with a rhythmic elevation in pressure. However, saline infusion also evoked a small rise in intra-mammary pressure. Earlier (2 days) and later (10 days) in lactation the responses were smaller. Oestrogen decreases significantly the milk ejection response to oxytocin, and the effect was maximal at day 10 of lactation. Histological observations confirmed the diminished reaction of the gland to oxytocin, since the milk was retained in the alveoli of rats bearing a mammary-oestrogen implant. A paradoxical rise in pressure was detected in normal as well as in oestrogen-implanted glands when the lowest dose of oxytocin was injected in lactating rats which had previously received a high dose of oxytocin (50 mU or 500 mU). These results reinforce the hypothesis that oestrogen alters the milk ejection response to oxytocin and that the mechanism is probably related to changes in the contractility of the myoepithelial cells.

Influence of Large Doses of Oxytocin on Milk Ejection and Metabolic Rate of Rat Mammary Gland

Milk ejection activity and milk synthesis were studied in primiparous lactating Wistar rats (9 to 12 days postpartum) 24h after a single intraperitoneal injection of 10 IU of oxytocin or 1ml saline. In a first series of experiments litters were allowed to suckle during this period. Gain in litter weight was blocked completely in the group treated with oxytocin with a decrease in oxygen consumption of mammary gland as compared with saline controls. However, lactose, glucose, and total sugar content did not differ between groups. In a second series of experiments pups were taken away immediately after injections. Weaning 24h decreased oxygen consumption of mammary gland in saline controls. Weaning together with injection of oxytocin caused an even more pronounced decrease. Lactose content of the weaned control group was lower than in the suckled control group and decreased after injection of oxytocin. In vitro milk ejection of small pieces of mammary gland also was completely inhibited in the oxytocin group. Prolactin administration to the oxytocin group increased both lactose and total sugar content of the gland. A pronounced increase in in vitro milk ejection activity in the oxytocin group followed prolactin treatment. Milk ejection in vivo, as measured by intra-mammary pressure, also was drastically inhibited by injection of oxytocin. In contrast to the in vitro experiments prolactin injections did not remove this inhibition, though lactose concentrations of mammary gland increased.

Reflex release of milk-ejection activity by stimulation of organs in the pelvic region in sheep as studied by cross-circulation experiments.

SUMMARY Cross-circulation experiments were performed on sheep by joining the jugular veins of two animals by anastomoses. One of the two animals (B) was a lactating ewe, and the release of hormones from the neurohypophysis in the other animal A (ewe, ram or wether) was investigated by recording the intramammary pressure in B. The following types of stimulation were carried out on A: (1) distension of vagina or anus; (2) massage of seminal vesicles (SV) or cervix via the rectum; (3) deep massage of the rectal walls, SV or cervix excluded; (4) direct massage of cervix, SV or uterine horns through the abdomen; (5) manipulations in the deep pelvic cavity; (6) distension of the urinary bladder in rams and wethers and pressure upon the filled bladder. Direct massage of cervix and uterine horns through the abdomen was ineffective. Stimulation of the urinary bladder by distension and pressure induced milk-ejection responses in some animals. Massage of the SV through the abdomen was hardly effective in rams but was effective in two out of seven wethers. The most efficient stimuli, classified according to their ability to induce milk ejection in B, were: distension of the vagina, stimulation of receptors in the retroperitoneal part of the rectum, manipulation in the deep pelvic cavity. The hypothesis is advanced that reflex release of neurohormones is induced by stimulation of receptors of the afferent pelvic nerves wherever they are situated in the pelvic region. The release of neurohypophysial hormones, provoked by massage of the cervix or SV via the rectum in sheep, is to be attributed primarily to stimulation of receptors in the rectal wall. Reflex hormone release can be elicited just as well in wethers as in rams.

The milk-ejection reflex of the rat: a 20- to 40-fold acceleration in the firing of paraventricular neurones during oxytocin release.

SUMMARY Antidromically-identified paraventricular (PV) neurones were studied in the anaesthetized rat during milk ejection (ME) evoked by the natural stimulus provided by the suckling young. An intermittent pattern of ME was observed, though the ME interval was slightly longer (10–20 min) in the doe during unit recording than in unoperated or conscious animals (5–15 min). PV neurones displayed a steady background of spike discharge in both the non-lactating rat and lactating rat during suckling. The firing rates ranged from 0 to 9·4 spikes/s and were exponentially distributed with 32% of the units firing at &lt;1 spike/s. Phasic patterns of discharge were seen in 18% of the units. Fifty-eight per cent of the PV units displayed a stereotyped and explosive acceleration in spike activity some 15–20 s before the rise in intramammary pressure at ME. The peak firing rate during this brief (2–4 s) response was in the range of 24–84 spikes/s. A prominent after-inhibition was then displayed by these responsive units, though this second component of the response was variable in duration (7–56 s). The electrical activity of the remaining units, including all the cells with phasic discharge, was not correlated with ME. There was no apparent change in the intensity of the suckling stimulus at the time of PV activation, i.e. 15–20 s before ME. Secondly, the activity of the PV neurones was not influenced by the rise in the intramammary pressure and the increased sucking of the pups at the time of ME. Simulation of an individual ME was obtained with both 1 mu. exogenous oxytocin, i.v., and electrical stimulation of the neurohypophysis (50 pulses/s for 4 s). The latencies to ME ranged from 7–14 s and 11–23 s, respectively. In conclusion, an explosive increase in the electrical activity of PV neurones precipitates the release of oxytocin at ME.

The milk-ejection reflex of the rat: an intermittent function not abolished by surgical levels of anaesthesia.

SUMMARY Milk ejection (ME) was studied in the lactating rat by the measurement of intramammary pressure, milk yield and behaviour. An intermittent pattern of ME was observed, both in conscious rats and in rats anaesthetized with tribromoethanol, pentobarbitone sodium and ethyl carbamate. Milk ejection was never seen as a reflex response with the onset of the suckling stimulus; the latency to the first ME varied from 10 min in the conscious rat to &gt; 60 min during deep anaesthesia. Thereafter, ME occurred at regular intervals ranging from 3 to 5 min under optimal conditions to &gt; 15 min during less favourable circumstances, though the young were content to suckle continuously for several hours irrespective of whether they obtained milk. Each ME was associated with an abrupt and uniform rise in intramammary pressure and simulation of the individual ME was obtained by a rapid i.v. injection of 1 mu. oxytocin. The pups reacted immediately to the rise in pressure with a synchronized extensor reaction and a vigorous increase in suckling which lasted for about 10 s. There was, however, no apparent change in the suckling stimulus to account for the release of endogenous oxytocin some 10–20 s prior to these events. Milk ejection was a reflex phenomenon, nonetheless, for the removal of the pups abolished the intermittent release of oxytocin. The rise in intramammary pressure was associated directly with the ejection of the milk and the pups abruptly increased in weight. Yields of 10–12 g were obtained from both conscious and anaesthetized rats after 18 h of separation from their young. Most of this milk was expressed in the first ten milk ejections. Mammary distension and the size of the litter were important parameters determining the pattern of ME. The incidence of ME was appreciably lower in unseparated rats whether anaesthetized or not and in separated rats after prolonged suckling or artificial removal of the milk. A similar reduction was encountered when the number of suckling pups was changed from ten to five. The physiological principles of the ME reflex in the rat are discussed and a theoretical model is presented.

CENTRAL AND PERIPHERAL NEURAL INFLUENCES ON THE OXYTOCIN-INDUCED PRESSURE RESPONSE OF THE MAMMARY GLAND OF THE ANAESTHETIZED LACTATING RAT

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.

The susceptibility of milked and unmilked udder quarters to intra-mammary infection.

SummaryThe teats of quarters which were milked, and those in which milking had ceased, of 16 cows were exposed to bacteria twice daily for 2 weeks by dipping them in a suspension of streptococci and staphylococci.Of 32 quarters in which milking had ceased 22 became infected in this period. Eighteen infections were with Streptococcus dysgalactiae, 2 with Streptococcus uberis, 1 with Staphylococcus aureus and 2 with organisms not used in the culture. Only 3 of 32 milked quarters became infected in the experimental period: 2 infections were with Str. aureus and 1 with an organism not used in the culture. Animals differed in their susceptibility to infection.Quarters which had been unmilked for 2 weeks were as susceptible to new infectionas those in which milking had just ceased.In this experiment, factors such as intramammary pressure, leakage of milk, concentration of bacterial inhibitors, composition of secretion and cell counts of the secretion did not appear to influence the rate of infection. Yield at drying-off, milking rates and teat patency were not related to the differences in susceptibility between animals. It is suggested that the main reason for the lower new infection rate in the milked quarters is that pathogens entering the streak canal or teat sinus are usually flushed out with the secretion during milking and do not cause infection.

Effects of volume and distribution of milk on the oxytocin-induced contraction of the lactating rat mammary gland in vivo.

SUMMARY Milk was added intraductally in 0·2 ml increments to previously emptied mammary glands in rats. Direct microscopic observations of the glands showed that each increment of milk did not distribute evenly either within a single lobule or among lobules. Increase of the intramammary pressure either by i.v. injection of oxytocin or by compression of the gland between thumb and forefinger, distributed the milk more evenly within the gland. These observations led to the routine injection of 1·6–3·2 mu. oxytocin in order to distribute each increment of milk evenly throughout the gland. The intramammary pressure response of the gland to different doses of oxytocin was then determined. The rises in intramammary pressure above equilibrium pressure in response to 0·8 and 1·6 mu. oxytocin increased fourfold on average with increasing gland volume over the compliant portion of the static pressure-volume curve. The slopes of the dose oxytocin-intramammary pressure response lines became progressively steeper and shifted to the left as the volume of the mammary gland increased, indicating a progressive reduction in the dose of oxytocin required to elicit a given response. No differences were noted between rats lactating for 9–13 days and those lactating for 20–24 days. In a related study the duct system of the rat gland was found to contain little fluid even after 8 h of non-suckling, though it became filled swiftly after a single i.v. injection of oxytocin. Non-suckling for 16–24 h was required to overcome passively the resistance to entry of milk from the alveoli.

Oxytocic activity in posterior pituitary glands of male rats measured by two bioassays.

SummaryOxytocic activity in posterior pituitary glands of male rats at 20, 40, 80, and 200 days of age was measured using two different bioassays. The first was an in vitro assay based on the log time response of a 2 mm3 mammary cube in exuding milk to the log dose of oxytocin between 10-1 and 10-9 IU. The second was an in vivo assay based on the ability of the guinea pig mammary gland to increase intramammary pressure rectilinearly in the range of 0.25 to 1.0 mU of oxytocin. Oxytocic activities of posterior pituitaries by the in vitro assay were 9.1, 78.6, 150.2, and 147.9 mU at 20, 40, 80, and 200 days of age, respectively; while the in vivo assay gave values of 18.2, 96.7, 195.3, and 484.5 mU in the same order. Only the values obtained in 200-day-old rats were significantly different between the two assays (p<.05).

Intermittent release of oxytocin during suckling in the rat.

MILK ejection in mammals is brought about by a contraction of the myoepithelial cells surrounding the alveoli of the mammary gland. This process is normally induced by release of oxytocin from the neurohypophysis as a reflex response to stimulation of the nipples by the suckling young. In most species there is an elevation in plasma oxytocin associated with the onset of the suckling stimulus, though hormone levels often vary considerably on a minute to minute basis1–3. Furthermore, secondary releases of oxytocin during suckling are not related to any particular change in the afferent stimulus. From this kind of evidence, Folley and Knaggs1 suggested that there is an intermittent or “surge-like” release of oxytocin. We report a new line of evidence in support of this concept based on recordings of intramammary pressure during suckling in the rat.