To find optimum dose for L-tryptophan (L-TP) to be used in growth hormone (GH) stimulation tests, a dose-related study on the effects of L-TP on human GH secretion was performed. 2, 5 and 8 g L-TP was given perorally in random order to 8 healthy male volunteers, and the GH response compared with values obtained after placebo. Furthermore, the L-TP test (performed with the 5 g dose) was compared with placebo, L-dopa and apomorphine tests. Each of these groups consisted of 24 male volunteers.
In the dose-response part of the study, all doses of L-TP induced statistically significant stimulation of GH release: the mean peak GH level rose after 2 g L-TP from the mean basal level of 2.0±0.28 ng/ml to 8.2±1.8 ng/ ml (p<0.01), after="" 5="" g="" l-tp="" from="" the="" mean="" basal="" level="" of="" 2.4±0.35="" ng/ml="" to="" the="" mean="" peak="" level="" of="" 9.2±1.8="" ng/ml=""><0.01), after="" 8="" g="" l-tp="" from="" 2.7±0.30="" ng/ml="" to="" 6.7±0.7="" ng/ml=""><0.05) and="" after="" placebo="" from="" the="" mean="" basal="" of="" 1.8±0.28="" ng/ml="" to="" 3.2±0.7="" ng/ml="" (ns.).="" although="" the="" gh="" responses="" to="" these="" three="" doses="" of="" l-tp="" did="" not="" statistically="" differ,="" there="" was="" a="" clear="" tendency="" to="" attenuated="" gh="" response="" to="" 8="" g="" l-tp="">
In the material consisting all L-TP tests performed in our laboratory with 5 g dose of L-TP, 5 subjects out of 24 (20.8 %) failed to respond to L-TP (serum GH level <5.0 ng/ml).="" however,="" when="" comparing="" l-tp="" test="" with="" placebo,="" significant="" stimulation="" of="" gh="" release="" was="" observed=""><0.001). on="" the="" other="" hand,="" both="" l-dopa=""><0.05) and="" apomorphine=""><0.01) proved="" to="" be="" more="" potent="" stimulators="" of="" gh="" secretion="" in="" man.="" the="" peak="" gh="" levels="" after="" l-dopa="" and="" apomorphine="" occurred="" constantly,="" while="" the="" timing="" of="" the="" peak="" gh="" response="" to="" l-tp="" was="" more="">
The dose-response study demonstrates that the maximal responsiveness of GH secretion to L-TP loading is limited, and by no means comparable with the dopamine-controlled GH secretion. Because the avident decline in GH response to high-dose L-TP loading, a dose of 5 g seems most suitable to be used in human GH studies.