Study on the reproducibility of human prolactin response to sulpiride, benserazide, insulin hypoglycaemia and arginine infusion.

In order to evaluate the spontaneous variability of prolactin (PRL) release in response to various stimuli applied repeatedly on different occasions, groups of 5 to 12 subjects each underwent consecutive identical tests with one of the following stimuli applied at 3-6 days' intervals: sulpiride (100 mg im), benserazide (50 mg po), insulin hypoglycaemia (0.1 U/kg b. w. iv) and arginine infusion (25 g iv in 30 min). When repeated in the same subjects, arginine and benserazide yielded superimposable results. In contrast to this, insulin hypoglycaemia yielded significantly lower PRL release, while the PRL response to the second sulpiride test was significantly higher than to the first one. When an interval of 10 days was left between two consecutive sulpiride tests, an identical PRL release was observed. These results indicate that arginine and benserazide are reproducible tests for PRL secretion and it is possible that the decreasing effect of insulin hypoglycaemia on PRL release is due to the stressful effect of the stimulus. Finally, sulpiride probably enhances both PRL release and synthesis thus making greater amounts of PRL available to a subsequent stimulus. Since some of the above stimuli are usual tools for the study of the neuroendocrine control of PRL secretion, our findings suggest that caution appears necessary in attributing to any (neuroactive) drug an effect which might be merely due to a lack of reproducibility of the stimulus employed.