CHEMORECEPTORS AND CONTROL OF EPISODIC BREATHING IN THE BULLFROG (RANA-CATESBEIANA)

To lest the hypothesis that the episodic breathing pattern of bullfrog s is necessarily caused by fluctuations of Pa-o2 and Pa-o2/pH, the nat ural oscillations of brood gases associated with periods of ventilatio n and apnea were experimentally prevented by unidirectional ventilatio n (UDV) of the lungs. UDV with air or a 50% O-2 in N-2 gas mixture eli minated breathing episodes; only sporadic single breaths were ever obs erved under these conditions. UDV with hypoxic or hypercarbic gas mixt ures, however, produced episodic breathing despite the fact that UDV V irtually eliminated fluctuations in pHa, Pa-co2 and Pa-o2. Furthermore , the breathing patterns of animals with the same mean levels of blood gases and acid-base status, with (UDV) and without (non-UDV) phasic c hemoreceptor input were identical. These data indicate that phasic che moreceptor input plays little or no role in the control of the normal breathing pattern although some tonic lever of chemoreceptor input is required for ventilation to occur. Animals on UDV were more sensitive to hypercarbic than hypoxic gases and hypoxemia and hypercapnia affect ed breathing pattern differently. This indicates that tonic chemorecep tor input also affects the length of the periods of apnea and Ventilat ion but this must be through some mechanism other than an ''on'' or '' off' threshold.