Provocative stimulation of growth hormone: a monozygotic twin study discordant for spinal cord injury

Abstract

Background/objective: A blunted growth hormone (GH) response to provocative testing and/or low levels of plasma insulin-like growth factor-I (IGF-I) have been reported in persons with spinal cord injury (SCI). A reduction in activity of the GH-IGF-I axis may have deleterious effects on body composition and function. Provocative testing for GH stimulation was performed to determine the response in monozygotic twins that were discordant for SCI.

Methods: GH stimulation testing was performed by the administration of intravenous arginine.

Results: Nine SCI twins with paraplegia, a mean age of 39 +/- 9 years, and duration of injury of 14 +/- 9 years were studied. The twins with SCI had a significantly lower body mass index than non-SCI twins (22.5 +/- 4.0 vs 25.1 +/- 4.2 kg/m2; P < 0.05); percent fat mass was greater in the twins with SCI (30 +/- 11% vs 22 +/- 10%; P < 0.05). Baseline serum GH was correlated with percent fat only in the SCI twins. The response to GH provocative stimulation was less in the twins with SCI: peak GH response was 5.8 +/- 6.6 vs 13.0 +/- 7.3 ng/mL (P < 0.05), and sum GH response was 15.7 +/- 15.6 vs 30.2 +/- 17.3 ng/mL (P = 0.06). Although baseline serum GH was correlated with stimulated response in the SCI twins, this relationship was not found in the non-SCI twins. Adiposity was positively related to the provocative serum GH response in twins with SCI rather than negatively related, as noted in the non-SCI twins.

Conclusions: This study confirms and extends prior work that reported a reduction in stimulated GH release in persons with SCI, which was related to baseline values.

Figure 1. The GH response to intravenous arginine. Solid squares, subjects with SCI; gray circles, non-SCI subjects. Each point is expressed ±SE.

Figure 2. The peak and sum GH responses to intravenous arginine. Symbols above bars display significant differences between the SCI and non-SCI groups: *P < 0.05, ^†P = 0.06. Solid bars, SCI twins; gray bars, non-SCI twins. Values are ±SE.

Figure 3. The relationship between percent fat and stimulated GH responses in SCI and non-SCI twins. (A and B) SCI twins: percent fat with sum and peak serum GH values (r=0.58, P=0.10 and r=0.58 and P=0.11, respectively). (C and D) Non-SCI twins: percent fat with sum and peak serum GH values (r=−0.64, P=0.06 and r=−0.64, P=0.06, respectively).