The influence of venous CO2 on ventilation in garter snakes

Garter snakes were used to study the effects of venous CO2 loading using the skin as an exchanger. The gaseous environment surrounding the snake's body was isolated by placing the body in a plethysmograph with the head out. While the animal breathed room air, the carbon dioxide concentration within the plethysmograph was varied between 0 and 80%. Room air was drawn through a funnel placed over the snake's head, thus collecting the exhaled gases, and this gas was analyzed by O2 and CO2 analyzers. The descending aorta was cannulated to measure blood gases. Expired CO2 flow rose linearly with increasing cutaneous CO2. Ventilation increased 3.5-fold at 80% cutaneous CO2 compared with no cutaneous CO2 load. Neither the mean CO2 concentration in exhaled air nor arterial PCO2 changed when the snake was exposed to high levels of CO2 at the skin. Thus ventilation increased in proportion to the CO2 load, and was not driven by arterial hypercapnia. Bilateral vagotomy eliminated arterial CO2 homeostasis during cutaneous CO2 loading, and ventilation increased with increasing arterial PCO2. Therefore, these snakes respond to extra-arterial elevations in CO2 or to a changing CO2 signal. Furthermore, receptors responsible for the increase in ventilation when venous CO2 is elevated have neurons in the vagus nerves.