Ultradian rhythm of growth hormone secretion in unrestrained, conscious male rabbits: role of endogenous somatostatintdagger.

Abstract The profiles of growth hormone (GH) secretion were examined by obtaining serial blood samples every 15 min for a 5 to 24 h observation period from freely-moving, conscious male rabbits chronically implanted with a right atrial cannula. The effects of restraint or surgical stress on GH secretion were also investigated in these animals. Four days after cannulation of the right atrium, plasma GH levels remained low without oscillation, during a 5 h observation period (1100 to 1600 h) with the mean (+/- SEM) value of 1.6+/-0.2 ng/ml. Individual rabbits exhibited a spontaneous, pulsatile GH secretion 7 days after the surgery. Mean 6 h GH levels were 5.6 +/- 0.8 ng/ml at 7 days after the surgery, 6.3 +/- 0.6 ng/ml at 14 days and 7.0 +/- 1.2 ng/ml at 28 days. Therefore, the animals, 7 to 14 days after cannulation, were used to analyse the pulsatile pattern of GH secretion throughout 6 to 24 h. Two episodes of 45 min immobilization stress, separated by 75 min, caused a complete suppression of the spontaneous GH secretion (mean 6 h GH levels, 2.2 +/- 0.1 ng/ml vs control, 5.0 +/- 0.5 ng/ml, P<0.01). No surges appeared after the first restraint stress. In 14 non-treated rabbits, plasma GH levels fluctuated in an episodic manner throughout the study with the peaks of 14.2 + 0.7 ng/ml, the nadirs of 2.6 +/- 0.2 ng/ml and the peak to peak intervals of 2.20 +/- 0.17 h. The iv administration of normal goat lambda-globulin (NGG) affected neither GH secretory patterns nor baseline levels of plasma GH. In contrast, the iv administration of anti-sornatostatin goat lambda-globulin (ASG) caused a significant increase in the amplitude of plasma GH peaks (38.8+/-1.9 vs NGG-treated, 13.7 +/- 0.8 ng/ml, P<0.001) as well as the trough level (13.5 +/- 0.6 vs NGG, 2.9 +/- 0.1 ng/ml, P<0.001) during a 24 h observation period. Also, ASG treatment increased numbers of plasma GH peaks per day (18.8+/-2.7 vs NGG, 12.2 +/- 0.8, P < 0.05) with concomitant shortening of the peak to peak interval (1.25 +/- 0.10 vs NGG, 2.03+/-0.12h, P<0.01). These findings suggest: 1) that GH is episodically secreted throughout the day in conscious male rabbits, 2) that surgical and restraint stresses suppress the spontaneous GH secretion, and 3) that endogenous somatostatin might rather play a tonic inhibitory role in GH release in conscious male rabbits, since ASG treatment resulted in sustained marked increases of plasma GH levels irrespective of the stage in GH pulsatile rhythm.