Does short-term fasting desensitize human somatotrophs to oral L-dopa stimulation?

To investigate whether short-term fasting influences hormone release from human somatotrophs and lactotrophs, two oral L-dopa tests, using 0.5 g L-dopa, were carried out in seven normal subjects. The first test was performed after an overnight fast (8 h), the second after an additional 48 h of total food withdrawal. Basal blood glucose declined by 21 +/- 2% (mean +/- SEM) between the two tests (P less than 0.001). Free T3 in serum (FT3) fell by 32 +/- 4% (P less than 0.001). GH responsiveness to L-dopa--as reflected by the GH incremental area (ng/ml X min)--decreased from 588 +/- 162 to 24 +/- 240 (P less than 0.02). The latter response area did not even differ significantly from zero. Food deprivation failed to affect the lactotrophs, inasmuch as similar PRL decremental areas (ng/ml X min) were obtained in response to L-dopa before (704 +/- 165) and after (686 +/- 128) the prolonged fast. In order to prevent blood glucose from falling significantly during the period of food deprivation eight additional normal subjects were given 9 small oral doses of glucose (each dose 0.5 g/kg) at regular intervals between the two L-dopa tests. This resulted in similar FT3 concentrations before and after the prolonged fast. Furthermore, glucose supplementation restored normal GH responsiveness to L-dopa, as evidenced by similar GH incremental areas before and after the glucose fast. These results imply that short-term fasting desensitizes human somatotrophs to exogenous L-dopa. The finding that the decreased GH responsiveness returns to normal if a decline in FT3 is prevented by oral glucose supplementation, suggests that changes in FT3 might contribute to the abolished GH responsiveness to L-dopa in fasting.