Respiratory responses to occlusion or hypercapnic blood injection of the anterior inferior cerebellar artery in cats

To examine whether the central chemoreceptors of respiration are located in the perfused area of the anterior inferior cerebellar artery (AICA), we occluded arteries or injected hypercapnic blood into arteries in the ventral surface of the medulla in anesthetized, paralyzed, and peripherally chemodenervated cats. Phrenic nerve activity, as an index of respiratory output, was augmented by an injection of hypercapnic blood into the vertebral artery. This vertebral-injection response decreased during bilateral occlusion of AICA. However, responses of phrenic nerve activity to the occlusion of AICA were complicated; activity increased in 19 cats, did not change in 10, and decreased in 9 during occlusion. In experiments with blood of various PCO2 levels being bilaterally injected into AICA, phrenic discharges increased with increases of PCO2. During the injection of constant PCO2 blood into AICA, phrenic response to alveolar PCO2 decreased by 80% compared with the original response. From these results, the blood flow and blood PCO2 level of AICA seemed to be related to the central chemosensitivity for respiration. To examine the perfused area, the ventral surface pH of the medulla was measured with a micro-combination pH electrode (2 mm diameter). During the injection into AICA, pH in the rostral medulla depended on the PCO2 of injected blood, and pH in other areas depended on the PCO2 of systemic blood. Also, histological study of India ink injection into AICA showed that ink-filled vessels were exclusively observed in the rostral medulla. Thus, we conclude that at least part of the central chemoreceptors of respiration are located in the perfused area of AICA, that is, in the rostral medulla.