The Secretion Rate of Prolactin in the Rat during Suckling and Its Metabolic Clearance Rate after Increasing Intervals of Nonsuckling*

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Abstract
The amount of pituitary PRL depleted by suckling is known to increase in direct relationship to the length of the previous nonsuckling interval. Therefore, we examined the extent to which the nonsuckling interval influenced the plasma concentration of PRL attained during prolonged suckling, the metabolic clearance rate of continuously infused rat PRL, and the secretion rate of PRL into the circulation. Neither the plateau level of PRL attained in the plasma during suckling nor the clearance rate of infused PRL were altered by increasing the nonsuckling interval from 2 to 4 to 8 h. The secretion rates also were similar (554, 530, and 537 ng/min for the 2-, 4-, and 8-h nonsuckled groups, respectively). However, the number of minutes the steady secretion rate of PRL into the plasma could be maintained varied in direct relationship to the length of the nonsuckling interval. The steady secretion waned after 30 min of suckling in die 2-h nonsuckled group, after 60 min in the 4-h nonsuckled group, and did not subside at all during the 90-min suckling period in the 8-h group. The total amount of PRL secreted into the plasma approximated 17, 32, and 48 μg for the three groups, values which are quantitatively similar to amounts depleted from the pituitary during the first few minutes of suckling after similar nonsuckling periods. To reconcile the rapid and extensive depletion of PRL from the pituitary with the steady, potentially long term output of much smaller amounts of PRL into the plasma, we postulate that the first few minutes of suckling prepares PRL for release into the circulation (pituitary depletion stage); the amount so prepared depends upon the length of the previous nonsuckling interval. Once prepared, the PRL is secreted into the circulation in a steady minute by minute fashion, independent of the previous nonsuckling interval. This steady secretion can continue until the pool of prepared PRL is exhausted.