Growth hormone and insulin response to intravenous arginine injection in the lamb

Abstract

Crossbred lambs were injected with L-arginine hydrochloride (arg) to determine the effects of single of multiple arg challenges on growth hormone (GH) secretion. Indwelling jugular catheters were inserted. At the beginning of each of 3 trials, lambs were injected with saline and blood samples were collected for 90 min to establish baseline GH. Blood sampling continued at 5-min intervals until 1 h after the final injection, then at 10-min intervals for one additional hour. In trial 1 arg (.5g/kg) was injected into 6 lambs while the other 6 received a second saline injection. Trial 2 consisted of 3 arg injections given at 1-h intervals. Trial 3 utilized 4 arg injections given at 15-min intervals. Trials 2 and 3 were replicated using a switchback design. Serum GH and insulin were measured by double antibody RIA. Mean GH for treated lambs in trial 1 was 3.89 ng/ml versus 1.74 ng/ml in controls (P<.01). GH peaked 30 min after injection (9.47 ng/ml), declined to 5-times baseline and remained near that level throughout the sampling period. Serum insulin was not different between treatments. In trial 2 arg treated lambs had higher mean GH (2.57 ng/ml) than controls (.86 ng/ml; P<.01). Peaks of GH were observed 20 min after the first injection (7.59 ng/ml) and 1 h after the second injection (5.6 ng/ml). No increase in GH was observed after the third injection. Insulin tended to follow the same pattern, but was not significantly elevated in treated lambs. Differences in trial 3 mean GH between arg-treated lambs (5.25 ng/ml) and controls (1.16 ng/ml) were significant (P<.01). GH peaked (13.8 ng/ml) at 25 min after the first injection, surged again 100 min later (7.4 ng/ml), declined to levels 3-times baseline and remained elevated. Trial 3 insulin levels were significantly higher in treated lambs (.64 ng/ml) compared to control lambs (.15 ng/ml; P<.01). Control lambs showed no significant GH or insulin increases at any time. GH secretion patterns were significantly altered in lambs injected with arg. One arg injection caused GH to peak within 30 min. Further challenges resulted in smaller, delayed rise and persistence of elevated GH levels. Insulin levels tended to increase with arg stimulus, but were not significantly elevated except at extremely high doses of arg. The data reflects a consistent relationship between arg stimulus and serum GH.