Effects of propranolol on GH responsiveness to repeated GH-releasing hormone stimulations in normal subjects

The influence of β-adrenergic blockade by oral propranolol on the variability of GH responses to GHRH and on GH responsiveness to repeated GHRH administrations was investigated. Eight normal volunteers underwent three tests on three separate occasions. Each test consisted of two administrations of 80 μg GHRH at 2-h intervals without other medication (test 1) or combined with oral administration of 80 mg propranolol 90 min before the first (test 2) or the second GHRH injection (test 3). In test 1 GH levels increased significantly after the first, but not the second GHRH bolus (net incremental area under the curve [nAUC], mean ± SD: 1453 ± 974 and 178 ± 309 μg·1^-1·(120 min)^-1, respectively). In test 2 basal GH secretion was not influenced by propranolol administration, whereas the GH response to the first GHRH injection was significantly greater than in test 1 (2327 ± 1814 μg·1^-1·(120 min)^-1; p -1·(120 min)^-1), not significantly different from the GH response to the first bolus (1796 ± 1375 μg·1^-1·(120 min)^-1). However, only 4 subjects showed a marked restoration of the GH responsiveness to the second GHRH administration. In conclusion, oral administration of propranolol is able to increase GH responsiveness to GHRH without changing the great individual variability. The response to a repeated GHRH stimulation is only partially restored by propranolol.