Effect of rise in plasma osmotic pressure on the milk-ejection reflex in the rat.

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The effect of an increase in plasma osmolality on the milk-ejection reflex of rats was studied. The lactating rats, at day 8-12 of lactation, were anesthetized with urethane (1.1 g/kg, i. p.) and 8-11 pups which had been separated from their mother 16-18 h were put to the nipples to suckle. In 21 of 47 rats studied an intermittent pattern of milk ejection was recorded with a latency of 15-74 min (group-I rats). The mean interval between recurring milk ejections was 8.3±0.6 (S.E.M.) min and the mean amount of oxytocin released at each milk ejection estimated in 11 of them was 0.26±0.04 mU. The remaining 26 rats showed no milk ejection throughout the nursing period of more than 90 min (group-II rats).<BR>The intraperitoneal injection of 1.5 M NaCl (1 ml) had no effect on the interval between milk ejections but increased the amount of oxytocin released at each milk ejection by 2.4 times in group-I rats, as the plasma osmolality changed from 297.9±2.7 mOsm/kg to 310.4±3.6 mOsm/kg. On the other hand, the 1.5 M NaCl injection induced recurring reflex milk-ejections in all of the group-II rats, while the plasma osmolality increased from 307±2.8 mOsm/kg to 320±3.1 mOsm/kg. The mean interval was not significantly different from that observed in the group-I rats. The sensitivity of the mammary gland to oxytocin was not altered by the 1.5 M NaCl injection. The injection of isotonic NaCl had no effect on the milk-ejection reflex.<BR>These results indicate that an increase in plasma osmolality facilitates the milk ejection reflex in rats by increasing the amplitude but not by changing the rhythmicity.
社団法人日本内分泌学会の論文

Effect of rise in plasma osmotic pressure on the milk-ejection reflex in the rat.

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