In vivo blood oxygen binding in hypoxic lesser spear-nosed bats: relationship to control of breathing

Abstract

The hypoxic ventilatory threshold of many mammals correlates with their hemoglobin-oxygen affinity (P 50). Yet, in some small mammals ventilation actually declines, rather than increases, with exposure to decreasing Pa O 2 ; their air convection requirement ( V ̇ E / V ̇ O 2 ) , however, is elevated in hypoxia. We propose that the threshold of the hypoxic V ̇ E / V ̇ O 2 of small mammals coincides with the inflection (‘knee’) of their in vivo O 2 equilibrium curve (O 2EC). In vivo blood gas and pH data were obtained from normoxic and hypoxic lesser-spear nosed bats, Phyllostomus discolor; in vitro blood O 2EC were also generated for normoxic bats at 32 and 37°C and at three P CO 2 ’s. The hypoxic V ̇ E / V ̇ O 2 threshold of P. discolor occurs at Pa O 2 =39 Torr; the corresponding in vivo O 2 saturation is 0.70, approximating the inflection of the O 2EC. This animal has a high blood O 2 affinity (P 50=27.5 Torr at pH 7.40 and 37°C; P 50=30.8 Torr at in vivo pH of 7.31 and T b of 37.4°C). As Pa O 2 is reduced, a pronounced hypoxia-induced respiratory alkalosis and hypothermia help maintain Sa O 2 near the O 2EC inflection (0.64–0.70 S O 2 ).