Time-dependent effect of hypoxia on carotid body chemosensory function.

The time-dependent effects of hypoxia on the discharge rate carotid chemoreceptors were measured in anesthetized cats. Hypoxic exposure of two different durations were used: a short-term exposure (2-3 h) was used to measure the response of the same carotid chemoreceptors; and a long-term exposure (28 days at inspired PO2 of 70 Torr) to study carotid chemoreceptor properties in one group of cats relative to those of a control group. In the chronically hypoxic and control groups, determinations were made of the 1) steady-state responses to four levels of arterial PO2 (PaO2) at constant levels of arterial PCO2; 2) steady-state responses to acute hypercapnia during hyperoxia; and 3) maximal discharge rates during anoxia. We found that the acute responses of carotid chemoreceptor afferents to a given level of hypoxia (PaO2 = 30-40 Torr) did not significantly change within 2-3 h. After long-term exposure the carotid chemoreceptor responses to hypoxia significantly increased, with no significant changes in the hypercapnic response and in the maximal discharge rate during anoxia. We conclude that isocapnic hypoxia may not elicit a sufficient cellular response within 2-3 h in the cat carotid body to sensitize the O2 responsive mechanism, but hypoxia of longer duration will sensitize such a mechanism, thereby augmenting the chemosensory activity.