Insulin-like growth factor 1 prevents neuronal cell death and paraplegia in the rabbit model of spinal cord ischemia.

OBJECTIVE: Insulin-like growth factor 1 has been shown to be cytoprotective against ischemia-reperfusion injury in various organs. However, spinal cord protection by insulin-like growth factor 1 has not been tested. We have therefore examined the effect of insulin-like growth factor 1 on neuronal cell death and motor function after spinal cord ischemia.
METHODS: Japanese white rabbits were subjected to spinal cord ischemia by clamping the abdominal aorta for 15 minutes. Insulin-like growth factor 1 (0.3 mg/kg) at a dose equipotent to insulin (0.3 IU/kg) in lowering blood glucose level or the control (phosphate-buffered saline solution as a vehicle) was administered intravenously 30 minutes before the aortic clamp.
RESULTS: Hind-limb motor function had recovered normally 48 hours after the operation in all the rabbits (n = 8) treated with insulin-like growth factor 1. In contrast, all the control-treated (n = 8) and all but one of the insulin-treated (n = 6) rabbits had deteriorated to paraplegia by 48 hours after the operation. Histopathologic sections in the involved spinal cord segment showed that a significantly (P <.0001) greater number of motor neuron cells were preserved in the rabbits treated with insulin-like growth factor 1 (17.9 +/- 4.8 per section) than in those treated with the control (8.0 +/- 2.1). Although insulin was equipotent to insulin-like growth factor 1 in preserving the number of motor neuron cells (18.5 +/- 2.7), the percentage of motor neuron cells positive for terminal deoxynucleotidyltransferase-mediated deoxyuridine triphosphate-biotin nick-end labeling were significantly (P <.01) smaller in the rabbits treated with insulin-like growth factor 1 (6.0 +/- 4.6) compared with those treated with the control (54.6 +/- 33.8) and insulin (26.2 +/- 11.7). Immunohistochemical studies revealed that insulin-like growth factor 1 increased expression of the antiapoptotic Bcl-xL protein and inhibited expression of the proapoptotic Bax protein in motor neuron cells 24 and 48 hours after the operation. In contrast, expression of only Bax was increased after the operation in other groups of rabbits subjected to spinal cord ischemia.
CONCLUSIONS: These results suggest that insulin-like growth factor 1, but not insulin with a conventional dose, protects motor neuron cells from ischemic spinal cord injury associated with differential regulation of Bcl-xL and Bax protein.