Changes In Intramammary Pressure Research Articles

Intramammary pressure in the lactating sow in response to oxytocin and during natural milk ejections throughout lactation.

We developed a simple, non-invasive method to measure the intramammary pressure (IMP) response in sows during natural milk ejection and after i.v. injections of oxytocin. The teats of sows were cannulated and the amplitude and duration of IMP changes were measured via a pressure transducer linked to a computer. During natural milk ejections, the amplitude of the response was 5.5+/-1.8 kPa (mean +/- SD) and the duration of response at half the peak amplitude was 6.8+/-1.6 s. In response to oxytocin a threshold response occurred at 60 mIU oxytocin with no further change up to 200 mIU. The amplitude of the response to an i.v. dose of 60-200 mIU oxytocin (5.5+/-2.2 kPa) and duration (6.4+/-1.7 s) were similar to that during a natural milk ejection. The responses of two galactophorous ducts of a teat measured simultaneously were similar in peak amplitude and duration. When the responses of four glands of a sow were measured simultaneously, there was a lag time in the onset of the IMP peak from the anterior to the posterior glands, and an increase in the peak amplitude from anterior to posterior. Both during natural milk ejections and in response to oxytocin injections, there were significant differences in peak amplitude between sows. We also aimed to determine whether the pressure or duration of milk flow during natural milk ejections was limiting piglet growth rates. IMP of nine sows and the growth rates of their piglets were measured. Both the peak amplitude of IMP response and the growth rate of piglets increased over the course of lactation, but the relationship was not causal. We found that the growth rate of piglets did not depend on IMP amplitude or duration, but we suggest that it may depend on the frequency of suckling and/or the ability of piglets to withdraw milk while it is available.

Effects of exogenous relaxin on oxytocin and vasopressin release and the intramammary pressure response to central hyperosmotic challenge.

Experiments were done to study the effects of porcine relaxin on osmotically evoked changes in intramammary pressure and the release of oxytocin and vasopressin in anaesthetized rats. Injections (1 microliter) of hypertonic (0.75 M) NaCl into the left lateral cerebral ventricle were used to induce consistent rises in intramammary pressure and the release of oxytocin and vasopressin. Plasma hormone concentration was determined by radioimmunoassay. Relaxin (5 micrograms i.v.) significantly (P < 0.05) suppressed the intramammary pressure response to osmotic challenge 5 and 10 min after treatment. However, pretreatment with a specific vasopressin V1 receptor antagonist completely negated the effect of relaxin on intramammary pressure. Baseline levels of oxytocin and vasopressin in unstimulated rats were 41 +/- 1.6 and 36 +/- 1.1 pmol/l respectively. Osmotic challenge induced significant (P < 0.05) rises in plasma levels of both hormones (62.8 +/- 1.1 and 67.9 +/- 1.2 pmol/l respectively) which were further augmented by relaxin (81.3 +/- 1.8 and 117.1 +/- 2.4 pmol/l respectively; P < 0.05). The data confirm that central osmotic challenge provokes the release of oxytocin and vasopressin but the effects of oxytocin at the level of the mammary gland may be obscured by the action of vasopressin affecting blood flow to the gland.

Characteristics of milk ejection, associated intramammary pressure changes and oxytocin release in the mare.

The neuroendocrine reflex theory of milk ejection was investigated in the horse under natural suckling conditions. To this end 12 lactating mares were provided with acute jugular catheters and with intramammary pressure (IMP) recording catheters. The foal had free access to the contralateral mammary complex. Intramammary pressure could thus be recorded while blood was drawn simultaneously for oxytocin analysis from the undisturbed animal. Suckling periods associated with a characteristic increase in IMP lasted significantly longer than unsuccessful nursing attempts. Elements of successful sucklings involved physical stimulation of the mammary gland, a quiet phase and a sudden increase in IMP. Successful suckling took place at about 20-min intervals with a wide range from less than 5 min to greater than 100 min. Between 5 and 10 mU oxytocin i.v. were sufficient to evoke an increase in IMP identical in shape and duration to a naturally induced increase in IMP. Mean peak oxytocin levels reached 15.8 pmol/l plasma, with a maximal release of 39 pmol/l. In the majority of cases (greater than 80%) peak oxytocin release did not occur until after the increase in IMP; in some cases an oxytocin surge was not detectable at all, despite a milk ejection-associated increase in IMP. In three cases increase in IMP could be observed while the foals were away from the mother with no signs of any intention to suckle. The data indicate that in the horse some elements of the neuroendocrine reflex, such as tactile stimulation of the teat and a surge of oxytocin before an increase in IMP, are facultative and not essential for normal milk ejection.

Inhibition of suckling-induced milk ejections in the lactating rat by delta 9-tetrahydrocannabinol.

The effect of delta 9-tetrahydrocannabinol (THC) on suckling-induced oxytocin release was investigated by recording intramammary pressure changes in suckled rats treated iv with THC (0.5 mg/kg BW) or vehicle. Latency to the first posttreatment milk ejection and posttreatment milk ejection intervals and pressure wave amplitudes were compared between THC- and vehicle-treated rats. Before treatment, intervals between milk ejections averaged 6.5 +/- 1.3 (+/- SE) and 7.0 +/- 0.7 min for vehicle- and THC-treated groups, respectively. Vehicle injections did not alter the frequency of milk ejections, which continued at an overall mean interval of 7.6 +/- 0.7 min after treatment. In contrast, THC treatment was followed by a transient suspension of milk ejections, with a latency of 59.3 +/- 7.4 min before the first posttreatment milk ejection was recorded (P less than 0.001). Intervals between subsequent ejections averaged 15.3 +/- 2.0 to 16.1 +/- 1.3 min and were lengthened relative to corresponding intervals in vehicle-treated animals (P less than 0.05). The amplitudes of pressure waves were not significantly affected by treatment. Oxytocin (0.5 mU) injections 10 or 30 min after THC treatment evoked abrupt increases in intramammary pressure, indicating continued responsiveness of the mammary gland to oxytocin stimulation. These data suggest that THC interferes with the release of oxytocin in response to suckling. To our knowledge, this provides the first evidence that THC inhibits posterior pituitary function.

The subfornical organ and relaxin-induced inhibition of reflex milk ejection in lactating rats.

Experiments were carried out on anaesthetized, lactating rats to investigate the possible role of the subfornical organ in mediating relaxin-induced inhibition of reflex milk ejection. Milk ejection was judged by the behavioural response of the sucklings and by transient rises in intramammary pressure. Radiofrequency lesions of the subfornical organ, or control lesions in adjacent areas of the cerebrum, did not affect the pattern or the magnitude of intramammary pressure changes at reflex milk ejection. Purified porcine relaxin given by either i.v. (5 micrograms) or intracerebroventricular (50 ng) injection suppressed reflex milk ejection in intact, sham-lesioned and control-lesioned rats, but had no effect on either the pattern or magnitude of reflex milk ejection in rats with lesions of the subfornical organ. The subfornical organ, which is situated at the interface between the blood, brain and the cerebrospinal fluid appears to mediate, at least in part, the relaxin-induced inhibition of reflex milk ejection in the rat.

Effects of porcine relaxin on oxytocin release from the neurohypophysis in the anaesthetized lactating rat.

The effect of relaxin on electrically evoked release of oxytocin from the posterior pituitary was examined by monitoring changes in intramammary pressure in the anaesthetized lactating rat. The amount of oxytocin released by electrical stimulation of the neurohypophysis in vivo was dramatically reduced following i.v. injection of highly purified porcine relaxin (2.5-10 micrograms/rat). Relaxin inhibited oxytocin release in a dose-dependent manner and the onset of inhibition occurred within 6-10 min and lasted for 10-60 min. No effect on the sensitivity of the mammary gland to exogenous oxytocin was observed after relaxin treatment. During the period of inhibition, i.v. injection of the opioid antagonist naloxone chloride (1 mg/kg) completely and immediately restored electrically evoked oxytocin release. The neurohypophysis is known to contain endogenous opioid peptides, therefore the effect of relaxin on electrically stimulated release of oxytocin from the rat isolated neural lobe in vitro was examined. Relaxin (500-2000 ng/ml) failed to inhibit oxytocin release in vitro. The results suggest that relaxin can inhibit the release of oxytocin from terminals in the neurohypophysis, but by an indirect mechanism. This action appears to be mediated through endogenous opioid peptides whose source is not clear. They are unlikely to be of neurohypophysial origin and may probably come from the adrenal medulla, since acute adrenalectomy negated the inhibitory effect of relaxin on oxytocin release.

Plasma concentrations of oxytocin and intramammary pressure in goats during manual stimulation of the udder and hand-milking.

The concentration of oxytocin in the jugular venous plasma of goats was studied by radioimmunoassay and the intramammary pressure measured upon manual stimulation of the udder and hand-milking. Before stimulation the concentrations of oxytocin were close to the limit of detection, about 3 pg/ml. Manual stimulation of the udder resulted in a short-lasting increase in plasma oxytocin in 11 out of 18 experiments carried out on nine goats. There were large variations between and within individuals in the magnitude of the oxytocin increase. Hand-milking was in general as efficient stimulus for oxytocin release. In experiments in which manual stimulation had led to a large increase in plasma oxytocin, the ensuing hand-milking did not lead to a further increase. In three experiments neither manual stimulation of the udder not hand-milking resulted in any significant increase in plasma oxytocin. The intramammary pressure increased upon manual stimulation of the udder and/or hand-milking in four out of six experiments. There was, in general, a close time-relationship between changes in intramammary pressure and plasma oxytocin.

Bromocriptine and alpha-ergocryptine do not inhibit oxytocin secretion in the lactating rat.

Two experiments were performed to study the effects of bromocriptine and alpha-ergocryptine on oxytocin secretion in lactating rats. In both experiments, after overnight separation from their litters, rats were injected with either vehicle alone or ergot alkaloid plus vehicle; 4 h later the litters were returned. In the first experiment the mothers were conscious. Treatment did not affect suckling behaviour, number of stretch reactions or little weight gain in the first 30 min. Oxytocin injection before the second 30 min period of suckling caused no extra milk to be obtained. In the second experiment the mothers were anaesthetized with ethyl carbamate (1.1 g/kg body weight) at the time of the ergot alkaloid or vehicle injection. Changes in intramammary pressure were recorded during suckling. Ergot alkaloids altered neither the number of milk ejections caused by suckling, nor the proportion of milk ejections equivalent to 0.2 millimicron or more oxytocin. In both experiments treatment with ergot alkaloids suppressed secretion of prolactin. It is concluded that (a) in suppressing lactation, bromocriptine and alpha-ergocryptine do not inhibit oxytocin secretion as well as prolactin secretion, and that (b) prolactin secretion is not a necessary concomitant of oxytocin secretion.

Electrochemical deposition of iron onto rat oxytocinergic neurones does not result in milk-ejection.

Exposure of the preoptic-tuberoinfundibular system to iron ions causes ovulation in the pro-oestrous rat. This electrochemical treatment has been used frequently to study the way that the hypothalamus triggers secretion of gonadotrophic hormone and most investigators have assumed that the effect is mediated by the direct excitation of those neurones exposed to the cation. The present paper reports experiments designed to confirm that iron directly excites hypothalamic neurones to the firing frequencies essential for ovulation. Because of the difficulty of measuring input-output relationships in the diffuse preoptic-tuberoinfundibular system the experiments were performed on the anatomically distinct oxytocinergic neurones of the paraventricular-hypophysial tract in lactating rats. These neurones were simulated electrically (from 10 to 50 Hz for between 1 and 300 s) and subjected to the electrochemical treatment by depositing iron ions from the tip of the stimulating electrode (up to 250 muA anodal current for between 60 and 180 s). Changes in intramammary pressure were used to indicate oxytocin release and mammary glands were calibrated for sensitivity by intravenous injection of oxytocin. Electrical stimulation in excess of about 15 Hz invariably caused release of sufficient oxytocin to cause a rise in intramammary pressure. In contrast, no changes in pressure were observed during or after the electrochemical deposition of iron. The results suggest that the response of hypothalamic neurones to electrochemical treatment is not the same as their response to electrical stimulation.

Method for Determining Oxytocin Concentrations in Unextracted Sera; Characterization in Lactating Cattle

SummaryA highly sensitive, specific and reproducible double antibody RIA for measuring oxytocin in unextracted serum of cattle was developed. Serum concentrations of oxytocin were significantly (P < 0.05) elevated from 8.0 ± .11 μU/ml to 25 ± 7.6 μU/ml (mean ± SE) 2 min after commencement of milking. The range of peak oxytocin concentrations seen 2 min after milking in six animals was 11.2-65 μU/ml. The T1/2 for oxytocin, after milk ejection, was 3.6 min. Simultaneous determinations of oxytocin activity in blood sera and intramammary pressure were monitored in two cows before, during and after stimulation of the milk ejection reflex. Oxytocin activity in blood closely paralled changes in intramammary pressure.

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.

Changes in intramammary pressure and distribution of milk in the udder due to withdrawal at varying intervals after establishment of the milk ejection response

Abstract Changes in distribution of milk in the cow's udder modify the shape of the milk withdrawal curve during machine milking. The influence of different periods of delay between pre-milking stimulus and the start of milking, and the effect on the pressure-volume relationship in the milk cistern, are discussed. A simple technique for estimating duration of the milk ejection response is described. Ruakura Agricultural Research Centre, Department of Agriculture, Private Bag, Hamilton.

Bovine Intramammary Pressure Changes during Mechanical Milking

Summary Pressure changes within the intact bovine udder were measured during mechanical milking. Simultaneous pressure recordings were obtained from the teat sinus, gland sinus, pulsator line, and milk line. Indwelling polyvinyl catheters were used to obtain pressure measurements in the teat and gland sinuses. Data from 24 recordings on 12 different quarters of five cows were evaluated. Intramammary pressure increased with milk letdown and was followed by a gradual decline during milk flow. At the end of milk flow a pulsating vacuum developed in the teat sinus, and a residual teat sinus vacuum persisted after inflation collapse. A slight residual gland sinus vacuum also developed after the end of milk flow. After milk removal and teat canal ligation, residual vacuum did not develop in the teat sinus during normal milking machine operation, although a pulsating vacuum was noted. Pressure change in the teat sinus apparently resulted from inflation action upon the teat wall and extension of milk line vacuum through the teat canal.

666. Intramammary pressure changes in the lactating ewe: I. The nature of the pressure change under standard conditions

The effect of oxytocin on the intramammary pressure of the lactating ewe has been described. The pressure curve produced by the ewe is shown to be different from both sow and cow.An improved pressure recorder is described.

593. Intramammary pressure changes in the lactating cow: I. Changes during the milking process

Using an electrical pressure-recording system simultaneous pressure records have been made in both the milked and unmilked quarters of a dairy cow. The unmilked quarters showed a very slow fall in pressure, while, as would be expected, the milked quarter showed a steady pressure fall. Re-stimulation some time after the beginning of the experiment resulted in a rise and fall in pressure within the milked quarters, but no change when the quarter was not milked.

548. Intramammary pressure changes in the lactating sow: II. The effects of vasopressin and acetylcholine

Measurements of intramammary pressure changes in the sow following intravenous injection have shown that the milk-ejecting action of vasopressin and acetylcholine is similar to that of oxytocin. Doses up to 0·4 mg. of histamine produced no measurable effects within the mammary gland.

549. Intramammary pressure changes in the lactating sow: III. The effects of level of dose of oxytocin and the influence of rate of injection

The injection of small amounts of oxytocin into the lactating sow has shown that the minimum quantity required to evoke a response in a normal adult animal is of the order of 0·05 i.u. If the injection is prolonged, the dose must be increased to produce a response. Large doses (0·8–1·0 i.u.) given at widely different rates produce multiple ejection pressure graphs with two or more peaks.