Lack of plasma prolactin response to intravenously injected vasoactive intestinal polypeptide in patients with prolactin-secreting adenoma.

The effect of an iv bolus injection of 1 microgram/kg body weight of vasoactive intestinal polypeptide (VIP) on plasma prolactin (Prl) levels was tested in 13 normal volunteers and 15 patients with hyperprolactinaemia of various aetiology: 9 with Prl-producing pituitary tumours (6 prolactinoma, 3 mixed pituitary adenoma, secreting Prl and growth hormone (GH)), 6 with hyperprolactinaemia secondary to a hypothalamic lesion (4 craniopharyngioma, 1 hypothalamic germinoma, 1 meningoencephalitis). In the normal subjects, an iv injection of VIP caused a prompt increase in plasma Prl with peaks 2- to 3-fold greater than the basal values. On the other hand, none of the 9 patients with a Prl producing pituitary tumour showed any obvious Prl rise after VIP irrespective of a marked difference in their basal Prl levels. Lack of a Prl response to VIP was also found in the 2 patients with hypothalamic lesions (1 craniopharyngioma, 1 hypothalamic germinoma) whose basal Prl concentration was higher than 100 ng/ml. However, in the remaining 4 patients with hypothalamic lesions whose basal Prl concentration was less than 100 ng/ml, VIP injection resulted in a stimulation of the Prl secretion with a maximal net increment of 11.3 +/- 3.8 ng/ml, which is not different statistically form that (16.3 +/- 3.3 ng/ml) in the normal subjects, but significantly higher than that (-2.3 +/- 2.7 ng/ml) in the 4 patients with Prl-secreting adenoma and a basal Prl concentration of less than 100 ng/ml. These results indicate that the VIP test may be a useful diagnostic tool for discriminating a Prl-producing tumour from a hypothalamic lesion in patients with mild hyperprolactinaemia.