Components to the acid-base related ventilatory drives in the South American lungfish Lepidosiren paradoxa

Lungfish are closely related to terrestrial vertebrates (tetrapoda). Like tetrapods, the South American lungfish (Lepidosiren paradoxa) has central chemoreceptors involved in regulation of acid-base status. However, no data were available on peripheral CO(2)/[H(+)] receptors. Therefore, we tested the hypothesis that such receptors exist by measuring the ventilatory responses during a 5h exposure to combined aquatic/gas phase hypercarbia 7% (approximately 49 mmHg). Normocarbic control ventilation was 22 ml BTPS kg(-1)h(-1), and hypercarbia increased ventilation to 175 ml BTPS kg(-1)h(-1) at 5h. This procedure was repeated with the modification that normocarbic mock CSF (pH 7.45; P(CO2) = 20.7 mmHg) was applied to superfuse the cerebral ventricular system during the last 2h of the experiment. This served to eliminate the hypercarbic stimulus to the central chemoreceptors, while possible responses from peripheral chemoreceptors would remain intact. Peripheral receptors were detected, since ventilation became reduced to 62 ml BTPS kg(-1)h(-1) (P<0.05), which exceeds the initial normocarbic control ventilation (P<0.05). Based on this, the peripheral contribution accounted for 20% of the total response to hypercarbia, similar to the contribution of these receptors in man.