Respiratory chemoreceptor function in vertebrates comparative and evolutionary aspects

The sensing of blood gas tensions and/or pH is an evolutionarilyconserved, homeostatic mechanism, observable in almost all speciesstudied from invertebrates to man. In vertebrates, a shift fromthe peripheral O₂-oriented sensing in fish, to the central CO₂/pHsensing in most tetrapods reflects the specific behavioral requirementsof these two groups whereby, in teleost fish, a highly O₂-orientedcontrol of breathing matches the ever-changing and low oxygenlevels in water, whilst the transition to air-breathing increasedthe importance of acid–base regulation and O₂-relateddrive, although retained, became relatively less important.The South American lungfish and tetrapods are probably sistergroups, a conclusion backed up by many similar features of respiratorycontrol. For example, the relative roles of peripheral and centralchemoreceptors are present both in the lungfish and in landvertebrates. In both groups, the central CO₂/pH receptors dominatethe ventilatory response to hypercarbia (60–80%), whilethe peripheral CO₂/pH receptors account for 20–30%. Somebasic components of respiratory control have changed littleduring evolution. This review presents studies that reflectthe current trends in the field of chemoreceptor function, andseveral laboratories are involved. An exhaustive review on theprevious literature, however, is beyond the intended scope ofthe article. Rather, we present examples of current trends inrespiratory function in vertebrates, ranging from fish to humans,and focus on both O₂ sensing and CO₂ sensing. As well, we considerthe impact of chronic levels of hypoxia—a physiologicalcondition in fish and in land vertebrates resident at high elevationsor suffering from one of the many cardiorespiratory diseasestates that predispose an animal to impaired ventilation orcardiac output. This provides a basis for a comparative physiologythat is informative about the evolution of respiratory functionsin vertebrates and about human disease. Currently, most detailis known for mammals, for which molecular biology and respiratoryphysiology have combined in the discovery of the mechanismsunderlying the responses of respiratory chemoreceptors. Ourreview includes new data on nonmammalian vertebrates, whichstresses that some chemoreceptor sites are of ancient origin.