Dose-response profiles of plasma growth hormone and vasopressin after clonidine challenge in man

The objective of the study was to determine dose-response relationships of growth hormone, vasopressin, blood pressure, heart rate, and behavioral responses to clonidine. Ten healthy male volunteers were tested with each of three doses of clonidine (0.7, 1.4, and 2.1 micrograms/kg) with at least 1 week between tests. All doses gave a significant growth hormone response with a peak at 50 min. The high dose gave a significantly higher response than either the medium dose or the low dose, which did not differ from each other. Within individual subjects, there was a consistency of response to the different doses; thus, three of the volunteers had responses of 5 ng/ml or higher to the low dose. Those three subjects had higher growth hormone peaks after the highest dose than did six other subjects. Vasopressin showed a drop following clonidine after the two higher doses. Systolic blood pressure dropped following clonidine, showing a significantly greater drop for the medium and high doses than for the low dose. Diastolic blood pressure also showed a drop, but responsiveness did not differ between doses. There was significant dryness of mouth produced by clonidine, but no difference between doses. There was a significant sedative effect following clonidine which was greater for the high dose than for the medium or low dose. The finding that some subjects had a growth hormone peak of 5 ng/ml or greater after the low dose supports the hypothesis that use of a low dose strategy may be useful in confirming supersensitivity in conditions where increased responsiveness is suspected. The lack of difference between blood pressure responses to the medium and high doses of clonidine--doses that have different effects on growth hormone--supports the hypothesis that differences in responsiveness of presynaptic and postsynaptic alpha 2-adrenergic receptors exist.