The effect of nursing on neurohypophyseal hormone and prolactin secretion in human subjects.

The effect of nursing on plasma levels of oxytocin (OT), arginine vasopressin (AVP), and PRL was studied in six normal women 2-3 days post partum. Maternal blood samples were obtained for measurement of OT, AVP, PRL, sodium, and osmolality 3 and 0 min before suckling, at 3-min intervals for 15 min during suckling, and 5 min after completion of suckling. Plasma OT rose during suckling from a mean (+/- SEM) baseline value of 1.1 +/- 0.2 to 3.6 +/- 0.6 microU/ml by 3 min (P < 0.001), reached a peak level of 6.4 +/- 1.5 microU/ml by 6 min (P < 0.005), and remained elevated for the entire 15-min period of suckling. Serial measurements of plasma OT during suckling failed to show a pattern consistent with episodic secretion. The baseline plasma AVP concentration was 0.4 +/- 0.1 microU/ml and was not significantly altered by suckling. Plasma sodium and osmolality remained unchanged during the suckling period. The baseline serum PRL level was 268 +/- 24 ng/ml and rose to 362 +/- 31 ng/ml after 15 min of suckling (P < 0.05). The data suggest that suckling is a specific stimulus for OT and PRL secretion but has no effect on AVP release.