Relative peripheral and central chemosensory responses to metabolic alkalosis

We investigated the relative contribution of peripheral and central chemosensory mechanisms to ventilatory responses to metabolic alkalosis in anesthetized cats by simultaneously measuring steady-state carotid body chemosensory activity and ventilation. The effects of graded steady-state levels of metabolic alkalosis at constant levels of arterial O2 and CO2 partial pressure (PaO2 and PaCO2, respectively) were studied first. Then the responses to isocapnic hypoxia and hyperoxic hypercapnia before and after the induction of a given level of metabolic alkalosis were studied. From the relationship between the carotid chemosensory activity and ventilation, the contribution of the two chemosensory mechanisms was estimated. The depression of ventilation that could not be accounted for by a decrease in the carotid chemosensory activity is attributed to the central effect. We found that metabolic alkalosis decreased both carotid chemosensory activity and ventilation at all levels of PaO2 or PaCO2. The ventilatory effect of alkalosis increased during hypoxia due to suppression of both peripheral chemosensory input and its interaction with the central CO2-H+ drive. During hyperoxia the central effect of alkalosis was predominant, although the peripheral effect increased with hypercapnia. We conclude that acute metabolic alkalosis suppresses both peripheral and central chemosensory drives, and its ventilatory effect grows larger with decreasing PaO2.