Re-evaluation of L-tryptophan-stimulated human growth hormone secretion: A dose-related study with a comparison with L-dopa and apomorphine tests

Summary

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 (p<0.01), after 8 g L-TP from 2.7±0.30 ng/ml to 6.7±0.7 ng/ml (p<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 loading.

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 (p<0.001). On the other hand, both L-dopa (p<0.05) and apomorphine (p<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 variable.

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.