Milk ejection solicitations and non-nutritive nursings: an honest signaling system of need in domestic pigs?

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.

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CATECHOLAMINE ANTAGONISM TO OXYTOCIN-INDUCED MILK-EJECTION

Acta Endocrinologica ◽

10.1530/acta.0.067s005 ◽

1971 ◽

Vol 68 (1_Suppla) ◽

pp. S5-S38 ◽

Cited By ~ 14

Author(s):

Helmuth Vorherr

Keyword(s):

Receptor Blockade ◽

Milk Ejection ◽

Beta Adrenergic ◽

Beta Receptor ◽

Beta Receptors ◽

Alpha Receptors ◽

Adrenergic Blockers ◽

Beta Receptor Blockade ◽

Second Pain ◽

Stimulation Of

ABSTRACT In lactating rats and rabbits the mode of antagonism of sympathomimetics, angiotensin or pain toward oxytocin-induced milk-ejection was investigated. In rats intra-arterial (intrafemoral) doses of 0.01–0.02 μg or intravenous (iv) doses of 0.1–0.5 μg of either epinephrine, isoproterenol, norepinephrine, angiotensin or 10 μg of phenylephrine injected simultaneously with, or 30 seconds before an oxytocin dose (10 μU intrafemoral, 300 μU iv) greatly inhibited or suppressed the oxytocin response. A 15 second pain stimulus caused moderate inhibition. With alpha-receptor blockade pain, epinephrine, isoproterenol, norepinephrine, phenylephrine and angiotensin inhibition were, respectively, 70%, 75%, 100%, 40%, 0% and 100%. Under beta-receptor blockade the corresponding values were 14%, 40%, 0%, 70%, 100% and 100%; with simultaneous intrafemoral injections neither catecholamine was inhibitory toward oxytocin. In corresponding rabbit experiments approximately 10-fold higher iv drug dosages were applied and similar results were observed. In both species, combined alpha and beta-receptor blockade nearly eliminated the antagonistic actions of sympathomimetics toward oxytocin, whereas angiotensin inhibition persisted unchanged. The results indicate: 1) Mammary myoepithelial cells contain beta-adrenergic receptors but no alpha-receptors; 2) Inhibition of oxytocin-induced milk-ejection by isoproterenol and phenylephrine is meditated through stimulation of myoepithelial beta-receptors (myoepithelial relaxation) and vascular alpha-receptors (vasoconstriction), respectively; 3) Epinephrine and norepinephrine inhibition of milk-ejection is due to stimulation of vascular alpha-receptors and myoepithelial beta-receptors; 4) Angiotensin effects are unrelated to adrenergic receptor mechanisms; 5) Administration of both alpha and beta-adrenergic blockers is desirable for stabilizing the sensitivity of the oxytocin milk-ejection assay preparation against interference from endogenous or exogenous catecholamines; 6) Other than using adrenergic blockers, pharmacologic doses of oxytocin can correct nursing difficulties in animals and man with hyperfunction of the adrenal-sympathetic system.

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