Prolactin in human milk: correlation with lactose, total protein, and alpha-lactalbumin levels

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Abstract
To determine whether prolactin (PRL) could be conclusively demonstrated in human milk, samples from 9 puerperal women were examined by radioimmunoassay techniques. Recovery of 78 .+-. 5% (mean .+-. SE) PRL added to 8 milk samples and mean recoveries of 83, 92 and 92% of PRL after 1, 2 and 3 h incubation at 21.degree. C indicated that quantitative recovery was possible. PRL immunoreactivity was not lost by dialysis for 96 h. Serial dilution of milk samples before and after dialysis gave inhibition curves parallel to PRL standards. Milk PRL concentrations were high for the first 3 days after birth, with a peak mean value of 157 .+-. 18 ng/ml (SE) on the 3rd day. Milk PRL levels then fell sharply to a mean of 24 ng/ml 13 days after delivery. Milk PRL concentrations showed a significant negative correlation (P < 0.001) with milk lactose (r [correlation coefficient] = -0.59) and positive correlations (P < 0.01) with total milk protein (r = 0.49) and .alpha.-lactalbumin (r = 0.33) estimations. The results indicate that PRL is a normal constituent of human milk; high concentrations of PRL are present in human milk for the first 3 days postpartum but subsequently fall rapidly, and milk PRL levels correlate significantly with milk lactose, total protein, and .alpha.-lactalbumin values.