Metoclopramide challenge: a measure of human lactotroph activity

Prolactin response to an intravenous injection of 5 mg of metoclopramide was monitored in 1) normal subjects during the follicular and luteal phases of the menstrual cycle; 2) subjects with known prolactinomas, two of whom were breast-feeding; 3) subjects with suspected prolactinomas; 4) normal lactating women; and 5) normal postmenopausal women. The magnitude of prolactin released was greater in the luteal than in the follicular phase controls (P less than .001), the estrogen status being important, as further seen in the postmenopausal group. Comparison of responses in the prolactinoma and control groups is a useful diagnostic tool when assessing the possibility of a prolactinoma. Comparison of physiologically hyperprolactinemic subjects (lactating women) with pathological ones (prolactinomas) suggests that the main mode of metoclopramide's prolactin-releasing action is mediated via dopamine antagonism, though a small direct action on the lactotroph is possible.