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 less than 0.05). Treatment of old animals with L-dopa increased mean plasma GH concentrations to 132.9 +/- 11.2 ng/ml (p less than 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 micrograms/mg protein, p less than 0.05). L-Dopa increased pituitary GH concentrations in old rats to 318 +/- 32.2 micrograms/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 less than 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.
Age Factors Animals