Central ventilatory control in the South American lungfish, Lepidosiren paradoxa: contributions of pH and CO(2).

Lungfish represent a probable sister group to the land vertebrates. Lungfish and tetrapods share features of respiratory control, including central, peripheral and intrapulmonary CO(2) receptors. We investigated whether or not central chemoreceptors in the lungfish, L. paradoxa, are stimulated by CO(2) and/or pH. Ventilation was measured by pneumotachography for diving animals. The fourth cerebral ventricle was equipped with two catheters for superfusion. Initially, two control groups were compared: (1) catheterized animals with no superfusion and (2) animals superfused with mock CSF solutions at pH = 7.45; PCO(2) = 21 mmHg. The two groups had virtually the same ventilation of about 40 ml BTPS kg(-1) h(-1) (P > 0.05). Next, PCO(2) was increased from 21 to 42 mmHg, while pH(CSF) was kept at 7.45, which increased ventilation from 40 to 75 ml BTPS kg(-1) h(-1). Conversely, a decrease of pH(CSF) from 7.45 to 7.20 (PCO(2) = 21 mmHg) increased ventilation to 111 ml BTPS kg(-1) h(-1). Further decreases of pH(CSF) had little effect on ventilation, and the combination of pH(CSF) = 7.10 and PCO(2) = 42 mmHg reduced ventilation to 63 ml BTPS kg(-1) h(-1).