Low activation threshold of juxtapulmonary capillary (J) receptors of the lung

Respiratory reflexes arising from stimulating juxtapulmonary capillary (J) receptors by increasing doses of phenyl diguanide (PDG) were examined in 18 spontaneously breathing cats. In 60% an immediate and four-fold increase in breathing frequency (fR) was produced by doses as small as 5.1 ±  μg/kg (range: 3.5-7.5) thus establishing that a significant increase in fR is produced by J receptors by stimulating them with minimal or threshold doses of PDG. In response to similar minimal doses of PDG J receptor afferent activity increased accompanied by acceleration of breathing rate. The response to supra threshold doses was either an apnoea followed by rapid shallow breathing (rsb) or to an apnoea preceded by rsb or only to rsb. Respiratory excursions counted from high-speed run records of intrapleural pressure revealed that the apnoeic response obtained in some cases was a phase of high-frequency breathing and not its suspension. These findings using a chemical stimulus demonstrate that J receptors, with some variability, have a very low threshold for stimulation resulting in notable respiratory acceleration. Thus their afferent output could increase significantly at low intensities of their physiological stimuli such as rise in cardiac output and incipient pulmonary congestion that are generated with mild exercise, to give rise to augmented breathing which is consequently seen.