CO2/H+ receptors in the lungs of anaesthetized rabbits.

The CO2/H+ sensitivity of 'pulmonary' non-myelinated vagal afferent endings was investigated in anaesthetized rabbits using 'natural' stimuli of lactic (LA) and acetic (AA) acids, for comparison with previous results using sodium dithionite (NaD) and phenyl biguanide (PDG). Right atrial injections of these four chemicals demonstrated a respiratory reflex from 'pulmonary' endings, consisting primarily of a decrease in expiratory duration and consequent increase in respiratory frequency coincident with transient increase in PETCO2. Three of the chemicals (AA, LA and NaD) apparently activated the endings by increased [CO2] and/or [H+], since equimolar doses of sodium lactate and acetate were without effect. PDG additionally elicited respiratory and cardiovascular reflexes from 'cardiac' endings, suggesting that its activation of 'pulmonary' endings may differ from the other three stimuli. An unforeseen complication of these studies arose from using SPF (specific pathogen free) rabbits, but the consistency of different control values in SPF rabbits was established.