Sodium bicarbonate infusion increases discharge frequency of intrapulmonary chemoreceptors only at high CO2

We felt that earlier determinations of independent effects of extracellular pH and PCO2 on intrapulmonary chemoreceptors (IPC) discharge frequency were difficult to analyze because they used perfused lungs, and ventilation-perfusion changes among parabronchi could not be controlled. We decided to repeat these studies in non-perfused lungs. We cannulated both extrapulmonary bronchi of 10 thoracotomized Pekin ducks anesthetized with sodium pentobarbital (25-35 mg/kg) and unidirectionally ventilated each lung. The perfused right lung maintained gas exchange while the non-perfused left lung received 0.6 L/min of CO2 mixed in air. We recorded the discharge frequency of one IPC per duck at various PCO2, re-established circulation, and infused 3.0 mmol/kg of sodium bicarbonate intravenously. After 15 min, discharge frequencies were again measured from the same IPC in the nonperfused lung. The slopes and intercepts of discharge frequencies vs ln PCO2 relationship were depressed in six IPC, increased in two IPC and not significantly affected in two IPC. Arterial pH was increased significantly (0.11 unit) at 38 Torr arterial PCO2. We conclude that acutely increased extracellular sodium bicarbonate affects IPC discharge only by depressing sensitivity of most IPC to PCO2 and does not have an independent effect through pH.