Regulation of oxygen consumption in the crayfishAstacus leptodactylus at different levels of oxygenation: role of peripheral O2 chemoreception

Abstract

1. Oxygen consumption\(\dot M_{O_2 } \), ventilation,\(\dot Vw\), and circulatory flow-rate,\(\dot Vb\), were studied in settled crayfishAstacus leptodactylus at different O2 partial pressures in the inspired water,\(PI_{O_2 } \), ranging from 250 to 8 Torr. From\(PI_{O_2 } \)=250 to 25 Torr,\(\dot M_{O_2 } \) was maintained constant while\(\dot Vw\) increased 16 times the hyperoxic value. Since\(P_{O_2 } \) of both arterialized hemolymph and expired water remained in a narrow range independent of\(PI_{O_2 } \), we propose that\(\dot Vw\) is adjusted in order to maintain the level of oxygenation at the gas exchanger. Down to\(PI_{O_2 } \)=40 Torr, the observed arterio-venous concentration difference and\(\dot Vb\) both remained constant. At\(PI_{O_2 } \)=25 Torr, however, a further increase in\(\dot Vw\) could not maintain the (a-v)O2 concentration difference. As\(\dot M_{O_2 } \) did not fall, it is concluded that\(\dot Vb\) increased. 2. The role of the peripheral O2 chemoreceptors in these ventilatory and circulatory adjustments was analysed by a study of responses to short periods of exposure to\(PI_{O_2 } \)≃600 Torr (one-minute ‘O2 test’). Giving O2 tests to animals at\(PI_{O_2 } \) plateaux situated between 150 and 15 Torr induced a rapid change in the respiratory frequency which started before the hyperoxic bolus could have reached the central nervous system (CNS). Response latencies were shorter the higher the reference\(\dot Vw\); they were independent of the time required for the hemolymph to reach the heart and hence the CNS. It is concluded that the responses were initiated by O2 chemoreceptors located in the branchial cavities. The branchial location of O2 reception is consistent with the above proposition of a\(P_{O_2 } \) regulation at the gas exchanger. The importance of a central O2 chemosensitivity is questioned.