PERIPHERAL CHEMORECEPTORS RESPOND TO HYPOXIA IN PONTINE-LESIONED FETAL LAMBS IN-UTERO

Acute hypoxia inhibits, rather than stimulates, fetal breathing moveme nts (FBM), but there has been controversy as to the activity and role of the peripheral arterial chemoreceptors in the regulation of breathi ng movements in the unanesthetized fetus in utero. However, after midc ollicular brain stem transection or lateral pontine lesion, hypoxia ca uses FBM to become continuous and stimulated in rate and depth. To det ermine whether this stimulatory response involves peripheral chemorece ptors, we used a two-stage approach to examine the response to hypoxia after peripheral chemodenervation in lateral pontine-lesioned fetal l ambs. The lateral pons was lesioned at 119-121 days, and the response to hypoxia was tested in the unanesthetized fetus 4 days afterward. Fo urteen fetuses in which hypoxia stimulated FBM underwent either periph eral chemodenervation or sham denervation in a second operation. Hypox ia had no effect when the fetus was tested 4-5 days after peripheral c hemodenervation, and the basal incidence of FBM was significantly lowe r. The stimulatory response was unchanged by sham denervation. We conc lude that the peripheral chemoreceptors are active in fetal life and t hat they mediate the stimulation of FBM seen in response to hypoxia af ter removal of the lateral pontine inhibition. In addition, after pont ine lesion there is evidence of tonic chemoreceptor-mediated influence s on FBM, which are normally overriden in the intact fetus.