Ventilatory responses to brief hypoxic stimuli after simulated altitude exposure in rat

A blunted ventilatory response to acute hypoxia is described in altitude acclimatized small mammals. The peripheral or central origin of this blunting remains controversial. As the O2 sensitivity is always tested by steady-state steps of progressive hypoxia, an enhanced central inhibiting mechanism similar to those created by sustained hypoxia is suspected to be involved in the blunting. To avoid such central effect, we analyzed the ventilatory responses to very short glomic stimuli such as N2 tests and administration of NaCN by intravenous bolus in chronically hypoxic rats. The blunted ventilatory response was present only for hypoxemic hypoxia. A D2-receptor blockade failed to restore this response to those of littermate controls. We concluded that in rats, after altitude acclimatization, the central transduction of glomic afferences was intact because of the unchanged ventilatory response to cyanide. Reduced glomic sensitivity to low PO2 appeared specifically responsible for the blunted response to N2 tests with a very limited contribution from inhibitory dopaminergic mechanisms.