MAMMARY OXYGEN TENSION AND THE MILK-EJECTION MECHANISM

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Abstract
SUMMARY: 1. Polarographic recording of pO2 from gold-plated needle electrodes inserted into lactating mammary glands of rabbits under light urethane anaesthesia gave resting values of 15–30 mm. Hg. Expelling the milk from distended glands raised the pO2 level. 2. Myoepithelial contraction (milk ejection) induced by i.v. injection of oxytocin lowered pO2 by an amount depending on the rise in intramammary pressure. 3. Surgical shock, spinal anaesthesia or i.v. injection of adrenaline depressed mammary pO2 and reduced the milk-ejection response to i.v. oxytocin. 4. When mammary pO2 was raised by O2 breathing, or lowered by N2 breathing, the milk-ejection response to i.v. oxytocin was not affected. 5. Electrical stimulation of the lateral and posterior hypothalamic areas reduced mammary pO2 and milk ejection in a way similar to i.v. adrenaline. Bilateral electrolytic lesions in these areas produced a sustained depression of mammary pO2. 6. It is concluded that the pO2 of mammary tissues gives a measure of capillary blood flow and that this, rather than the absolute pO2 level, determines the response of the myoepithelium to circulating oxytocin.

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