THE STABILITY OF PLASMA GROWTH-HORMONE AND MHPG RESPONSES TO REPEATEDCLONIDINE CHALLENGE IN NORMAL MALES

Clonidine is a centrally acting alpha2-adrenergic agonist used in many psychiatric studies to assess adrenergic functioning. The short- and long-term stability of plasma growth hormone (GH) and plasma 3-methoxy -4-hydroxy phenylglycol (MHPG) responses to clonidine (2 mug/kg IV) ov er a 60-min period were assessed in subsets of 13 male normal controls on 2 consecutive days (Study A; n = 11) and on 2 days separated by se veral months (Study B; n = 11). In Study A, no significant differences between consecutive days were found in either baseline plasma GH or M HPG or their responses to clonidine. The 60 minute plasma GH responses between consecutive days were highly correlated (r = 0.75, n = 11, p < .001), while the 60 min plasma MHPG responses were not. In Study B. no significant differences in baseline plasma GH or MHPG, or their res ponses to clonidine challenge, were found between the 2 test days. How ever, neither the plasma GH responses nor the plasma MHPG responses to clonidine at 60 min correlated significantly between the 2 study days separated by several months. Both in Study A and in Study B, 8 of 11 subjects had a stable GH response to clonidine across both study days when defined dichotomously (blunted < 4 ng/ml; otherwise, not blunted) . These results suggest that the plasma GH response and plasma MHPG re sponse to clonidine are unaffected by repeat clonidine challenge separ ated by 24 h, and that the plasma GH response to clonidine may be more stable over time than the plasma MHPG response to clonidine.