360 J. Endocrinol. Invest. 22: 360-368, 1999 ABSTRACT. Hexarelin (HEX) is a synthetic hexapep- tide with strong GH-stimulating activity. We eval- uated GH response (expressed as maximum value after stimulus [C max ] and as area under the curve [AUC]) to HEX at the doses of 1 μg/kg iv (HEX 1) and 2 μg/kg iv (HEX 2), in comparison with the re- sponses to GHRH (1 μg/kg iv) + pyridostigmine (PD, 60 mg po) and to arginine (ARG, 0.5 mg/kg iv) + ethinylestradiol (EE, 1 mg/day po for 3 days be- fore the stimulation), in 5 subjects with familial short stature (FSS), 11 with constitutional growth delay (CGD), 6 with GH neurosecretory dysfunc- tion (NSD), and 5 with isolated growth hormone deficiency (GHD). C max and AUC after HEX 1 were 26.8±10.5 ng/ml and 1448±514 ng/min x ml in FSS, 23.6±14.4 ng/ml and 1146±750 ng/min x ml in CGD, 36.9±21.5 ng/ml and 2048±1288 ng/min x ml in NSD, 9.4±5.8 ng/ml and 498±200 ng/min x ml in GHD (C max and AUC in FSS and CGD, p<0.05 vs GHD). C max and AUC after HEX 2 were 37.7±16 ng/ml and 1979±888 ng/min x ml in FSS, 32.5±16.2 ng/ml and 1613±237 ng/min x ml in CGD, 39.7±20.7 ng/ml and 2366±1569 ng/min xml in NSD, 13.4±4.2 ng/ml and 645±293 ng/min x ml in GHD (C max in FSS, CGD and NSD p<0.01 vs GHD; AUC in NSD, p<05 vs GHD). C max and AUC after GHRH±PD were 46.6±8.8 ng/ml and 3294±1031 ng/min x ml in FSS, 25.9±11.2 ng/ml and 1464±735 ng/min x ml in CGD, 38.8±21.7 ng/ml and 2428±1399 ng/min x ml in NSD, 8.4±6.2 ng/ml and 685±572 ng/min x ml in GHD (C max and AUC in FSS, p<0.001 vs CGD and GHD; C max in CGD and NSD, p<0.001 vs GHD). C max and AUC after ARG+EE were 21.3±4.2 ng/ml and 1432±514 ng/min x ml in FSS, 14.8±10 ng/ml and 805±489 ng/min x ml in CGD, 22.2±12.8 ng/ml and 1199±309 ng/min x ml in NSD, 4.6±2.5 ng/ml and 247±191 ng/min x ml in GHD (C max and AUC in FSS, CGD and NSD, p<0.01 vs GHD). Specificity was 62% for HEX 1 and 75% for HEX 2, GHRH+PD and ARG+EE. From a diagnostic point of view, HEX 1 + HEX 2 was the association with the largest percentage of false positives (20% in FSS, 27% in CGD and 33% in NSD), HEX 1+GHRH+PD resulted in 9% in CGD, while the combined use of HEX 1 or HEX 2 with GHRH+PD or ARG+EE and of GHRH+PD with ARG+EE did not show false posi- tive responses. In conclusion: I) the most effective dose of HEX was 2 μg/kg iv; 2) HEX did not show more specificity than GHRH+PD and ARG+EE; 3) the association of GHRH+PD with ARG+EE could yield the best results at lower costs, confirming these tests as first-line tools in evaluating GH se- cretion. (J. Endocrinol. Invest. 22: 360-368, 1999) © 1999, Editrice Kurtis INTRODUCTION Hexarelin (HEX) is a hexapeptide belonging to a wide family of molecules endowed with strong GH-stimu- lating activity (1, 2), similar to GHRP-6 but more sta- ble and powerful and involving few side effects (3-8). Receptors for these peptides have been identified, distinct from those of GHRH, both in the pituitary and hypothalamus (9-16). It was suggested that this group of substances could counteract, at least in part, the somatostatinergic activity (1, 17-23), though there is evidence of a synergistic action with GHRH (10, 12, 18, 19); moreover, the existence of a natural endoge- nous ligand was recently suggested (23, 24). HEX administered iv has proved a potent stimulus for exploring GH secretion both in children and in adults, eliciting a greater response than that of GHRH (25-27) which increases during the pubertal development, more in females than in males (27). Key-words: Hexarelin, GHRH, pyridostigmine, arginine, estrogen prim- ing, short stature. Correspondence: Dr. Gabriele Guzzaloni, Divisione di Auxologia, Ospedale San Giuseppe IRCCS, Istituto Auxologico Italiano, Casella Postale 1, 28044 Verbania Intra, Italy. Accepted January 26, 1999 Hexarelin-induced growth hormone response in short stature. Comparison with growth hormone-releasing hormone plus pyridostigmine and arginine plus estrogen G. Guzzaloni, G. Grugni and F. Morabito Divisione di Auxologia, Ospedale San Giuseppe IRCCS, Istituto Auxologico Italiano, Verbania Intra, Italy