Different responses in little and bigbig prolactin to metoclopramide in subjects with hyperprolactinemia due to 150-170 kD (bigbig) prolactin.

We have studied in vivo induction of serum prolactin (PRL) levels in four females and one male, and regulation of PRL in the menstrual cycle in three females all with hyperprolactinemia with large amounts (72-92%) of bigbig PRL (MW 150-170 kD). Metoclopramide (MTC) iv induced a 4-29-fold increase in little PRL (25 kD PRL) at 30 min, while the increase in 150-170 kD PRL was 1.1-2.2-fold. The maximal response in 150-170 kD PRL was seen after 2-6 h, and the decrease after the maximal PRL values for 150-170 kD PRL was delayed compared to the decrease in 25 kD PRL. The different kinetics for 25 kD PRL and 150-170 kD PRL was responsible for the prolonged increase in total PRL seen in the subjects with large amounts of 150-170 kD PRL compared to the controls. The percentage of 150-170 kD PRL decreased to 29-60% at 30 min and returned to unstimulated values after 6-24 h. In contrast, prolonged stimulation of PRL secretion, as in the luteal phase, did not change the percentage of 150-170 kD PRL. In a male subject secreting large amounts of 150-170 kD PRL the increase in PRL after MTC was less, while the temporal changes in the 25 kD PRL levels were almost the same as in the females.