P O 2 of the metathoracic ganglion in response to progressive hypoxia in an insect

Mammals regulate their brain tissue P O 2 tightly, and only small changes in brain P O 2 are required to elicit compensatory ventilation. However, unlike the flow-through cardiovascular system of vertebrates, insect tissues exchange gases through blind-ended tracheoles, which may involve a more prominent role for diffusive gas exchange. We tested the effect of progressive hypoxia on ventilation and the P O 2 of the metathoracic ganglion (neural site of control of ventilation) using microelectrodes in the American locust, Schistocerca americana . In normal air (21 kPa), P O 2 of the metathoracic ganglion was 12 kPa. The P O 2 of the ganglion dropped as air P O 2 dropped, with ventilatory responses occurring when ganglion P O 2 reached 3 kPa. Unlike vertebrates, insects tolerate relatively high resting tissue P O 2 levels and allow tissue P O 2 to drop during hypoxia, activity and discontinuous gas exchange before activating convective or spiracular gas exchange. Tracheated animals, and possibly pancrustaceans in general, seem likely to generally experience wide spatial and temporal variation in tissue P O 2 compared with vertebrates, with important implications for physiological function and the evolution of oxygen-using proteins.