Chemoreceptors and control of episodic breathing in the bullfrog (Rana catesbeiana).

To test the hypothesis that the episodic breathing pattern of bullfrogs is necessarily caused by fluctuations of PaO2 and PaCO2/pH, the natural oscillations of blood gases associated with periods of ventilation and apnea were experimentally prevented by unidirectional ventilation (UDV) of lungs. UDV with air or a 50% O2 in N2 gas mixture eliminated breathing episodes; only sporadic single breaths were ever observed under these conditions. UDV with hypoxic or hypercarbic gas mixtures, however, produced episodic breathing despite the fact that UDV virtually eliminated fluctuations in pHa, PaCO2 and PaO2. 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 chemoreceptor input were identical. These data indicate that phasic chemoreceptor input plays little or no role in the control of the normal breathing pattern although some tonic level of chemoreceptor input is required for ventilation to occur. Animals on UDV were more sensitive to hypercarbic than hypoxic gases and hypoxemia and hypercapnia affected breathing pattern differently. This indicates that tonic chemoreceptor input also affects the length of the periods of apnea and ventilation but this must be through some mechanism other than an "on" or "off" threshold.