L-Dopa Restores Amplitude of Growth Hormone Pulses in Old Male Rats to That Observed in Young Male Rats

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Abstract
35 male Sprague-Dawley rats, 18 months old, were injected subcutaneously with the catecholamine precursor, L-dihydroxyphenylalanine (L-dopa) (100 mg/kg, twice daily), or vehicle alone, for 8 days in an attempt to increase the amplitude of growth hormone (GH) pulses in old male rats. 40% of young rats had at least one GH pulse greater than 600 ng/ml, over a 6.5 h sampling period, whereas only 22% of old vehicle-treated rats had pulses of similar amplitude. Treatment of old rats with L-dopa increased the number of animals showing elevated GH pulses to 53%. Young animals had mean GH concentrations of 136.5 ± 14.0 ng/ml over the 6.5 h sampling period, whereas vehicle-treated old animals had mean GH levels of 95.2 ± 7.7 ng/ml (p < 0.05). Treatment of old animals with L-dopa increased mean plasma GH concentrations to 132.9 ± 11.2 ng/ml (p < 0.05) which was not significantly different from GH values in young rats. Pituitary GH concentration was less in old than in young rats (301 ± 27.4 vs. 370 ± 17.4 µg/mg protein, p < 0.05). L-Dopa increased pituitary GH concentrations in old rats to 318 ± 32.2 µg/mg protein, which was not significantly different from that in either young or old vehicle-treated animals. Hypothalamic somatostatin content in both rostral and caudal areas was lower in old than in young rats (p < 0.01), and L-dopa had no significant effect on somatostatin content. These data indicate that L-dopa can increase the amplitude of GH pulses and elevate mean plasma GH in old male rats to levels present in young male rats, probably by increasing hypothalamic catecholamines.