Lack of enhanced nocturnal growth hormone release in tethered cynomolgus monkeys

To determine spontaneous 24-h patterns of growth hormone (GH) plasma levels in unsedated and unrestrained nonhuman primates, a jacket and tethering system were used to study six cynomolgus monkeys. Hourly blood samples were collected, and body temperatures were recorded over 24-h periods. Measurements of GH were made on all samples. In one 24-h study cortisol levels were also measured as well to document a normal circadian rhythm. GH was released at mean intervals of 4.5 +/- 0.47 h (mean +/- SE) over the 24-h studies. There were no day-to-night differences in either the mean interval of GH release (day, 4.6 +/- 0.66 h; night, 4.4 +/- 0.51) or the mean GH values (day, 9.8 +/- 1.7 mU/l; night, 7.9 +/- 0.8). An apparent midday peak in GH in the 24-h studies followed feeding. As expected, body temperature was higher during the day than night, documenting a normal circadian rhythm. Plasma cortisol also showed a normal circadian variation with a low point midday and a progressive rise during the night in the one 24-h cycle in which it was measured. GH in unsedated, unrestrained cynomolgus monkeys was released in 4- to 5-h cycles both day and night without increased nighttime release. This contrasts sharply with the known nocturnal sleep release of GH seen in humans.