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Abstract
Objective: This study was designed to investigate the effects of hypoxia on neural process proliferation by studying its effects on growth cone tubulin and insulin-like growth factor (IGF)-I receptor content.
Methods: Six fetal lambs were catheterized in the brachial artery and vein. Maternal oxygenation was reduced in steps from a fractional inspired oxygen concentration (FiO2) of 20% to 6% by addition of nitrogen to the inhaled gas mixture for a period of 4 h of reduced oxygen intake. Fetal arterial blood was sampled after the maternal FiO2 and oxygen were stable for > 5 min at maternal FiO2 of 20% to 6%. Controls were obtained from normoxic fetuses whose ewes had similar surgery and were kept at an FiO2 of 20% throughout the experiment. Growth cones were isolated from the fetal cerebrum and cerebellum. α-Tubulin and IGF-I receptors were quantified by immunoblotting. Tubulin and IGF-I receptor mRNA expressions were quantified by real-time polymerase chain reaction.
Results: Maternal nitrogen breathing reduced fetal arterial pH from 7.32 ± 0.06 to 6.99 ± 0.02 (p < 0.001). Hypoxia increased IGF-I receptors from 143 ± 10 to 327 ± 14 (p < 0.001) and from 272 ± 26 to 396 ± 34 (p < 0.001) fluorescence units/μg protein in the cerebrum and cerebellum, respectively. It also increased α-tubulin from 713 ± 30 to 1873 ± 126 (p < 0.001) and from 780 ± 34 to 2362 ± 79 (p < 0.001) fluorescence units/μg protein in the cerebrum and cerebellum, respectively. Expression of IGF-I receptor mRNA increased significantly in the hypoxic animals both in the cerebrum and the cerebellum, but there was no change in expression of α-tubulin mRNA.
Conclusions: This increase in IGF-I receptor expression and growth cone content may be an adaptive response to hypoxia to maintain neurite growth by facilitating binding of IGF-I. Hypoxia also increased the growth cone level of α-tubulin but did not increase its mRNA expression, which may indicate an inability to polymerize tubulin and build microtubules.