Blindness impairs plasma growth hormone response to L-dopa but not to arginine

To investigate how blindness influences GH secretion, we studied the GH response to L-dopa and arginine in 8 blind adult males and 10 normal age-matched control males. Arginine and L-dopa tests were performed in random order at least 1 week apart at 0800 h, and plasma GH was measured by RIA. The blind subjects showed GH responses to arginine similar to those in normal subjects [peak, 22.1 +/- 1.3 vs. 20.5 +/- 1.3 micrograms/L (+/- SE)], but their GH response to L-dopa was significantly reduced [peak, 5.4 +/- 1.3 vs. 20.3 +/- 2.4 micrograms/L (+/- SE); P less than 0.01]. Because L-dopa is believed to release GH by stimulating endogenous GHRH, whereas arginine may act by suppressing endogenous somatostatin secretion, we propose that blindness may impair GH release by inhibiting GHRH secretion.