Influence of lung stiffness on rapidly adapting receptors in rabbits and cats

We examined the response of rapidly adapting receptors (RARs) to changes in dynamic lung compliance (CDYN), recording vagal impulses in open-chest rabbits and cats with lungs ventilated at constant f and VT, and a positive end-expiratory pressure (PEEP) of 3-4 cm H2O. After hyperinflation to produce maximal CDYN, most RARs fired irregularly and sparsely (less than 1 impulse X sec-1). Reducing CDYN in steps by briefly removing PEEP progressively stimulated RARs during inflation, activity increasing sevenfold when CDYN was reduced by 40% (P less than 0.0001). Reducing CDYN also increased RAR responses to static lung inflation. RAR stimulation by decreasing CDYN was virtually unaltered by atropine and hence was largely independent of reflex changes in bronchomotor tone. RARs were also stimulated by increasing VT at constant f, but, expressed as a function of airway pressure, the afferent response was less than that to increased lung stiffness (1/CDYN); expressed as a function of 1/CDYN, however, the responses were similar. We conclude that RARs signal increases in the force required to expand the lung, and speculate that their excitatory influence helps to maintain VT as the lungs become stiffer.