Objectives The purpose of present study was to investigate the effect of CIHH on CSB and the underlying mechanism in developing rat.

Methods Neonatal male Sprague-Dawley rats were randomly divided into four groups: 42-day CIHH treatment group (CIHH 42), 56-day CIHH treatment group (CIHH 56), 42-day control group (Con 42) and 56-day control group (Con 56). CIHH neonatal rats with the maternal rats were exposed to hypoxia mimicking 5000 m altitude (O₂:11.1%) in a hypobaric chamber for 42 and 56 days, 6 h per day, respectively. The control animals lived in the same environment as the CIHH animals with free access to food and water excepting hypoxia. Isolated carotid sinus perfusion technique was used to record the CSB in anaesthetised developing rats. The parameters used to evaluate the CNS include Peak slope (PS), Reflex decrease (RD), Threshold pressure (TP), Equilibrium pressure (EP) and Saturation pressure (SP).

Results (1) CIHH inhibits CSB in developing rats.

Compared with Con 42 and Con 56 rats, the functional curves of CSB in CIHH 42 and CIHH 56 rats shifted upward and rightward. PS in CIHH 42 and CIHH 56 rats were 0.35±0.01 and 0.35±0.02 mm Hg, respectively, markedly decreased (p<0.01)compared with 0.43±0.01 and 0.42±0.03 mm Hg in Con 42 and Con 56 rats. RD in CIHH 42 and Con 56 rats were 36.64±1.48 and 36.36±2.12 mm Hg, respectively, obviously decreased (p<0.01) compared with 43.71±2.59 and 42.66±2.76 mm Hg in Con 42 and Con 56 rat. TP were 72.64±3.04 and 71.64±3.60 mmHg in CIHH42 and CIHH56, respectively, markedly increased (p<0.01) compared with 62.97±1.68 and 63.72±1.61 mm Hg in Con 42 and Con 56 rats. EP were 94.50±1.56 and 93.99±1.60 mm Hg in compared with 92.24±1.65 and 92.36±1.08 mm Hg in Con 42 and respectively, markedly increased (p<0.01) compared with 173.64±2.60 and 173.30±1.86 in Con 42 and Con 56 rats.

(2) Bay K 8644 can cancelled the inhibitory effects of CIHH on CSB in developing rats. Bay K 8644 (500 nmol/l), an agonist of L-type calcium channel, had no effect on functional parameters of CSB in developing rats. Bay K 8644 shifted functional curve of CSB downward and leftward in CIHH 42 and CIHH 56 rats. PS and RD were increased, TP and SP were decreased.

(3) L-NAME (100 µmol/l) had no effect on CSB action of CIHH in developing rats.

L-NAME (100 µmol/l), an inhibitor of NO synthesis, had no effect on functional parameters of CSB in CIHH and Con rats.

Conclusions Taken together, Chronic Intermittent hypobaric Hypoxia inhibits CSB in anesthetised developing rats through blocking of L-type calcium channels in carotid sinus baroreceptor. The effect of CIHH on CSB is not related with NO.