Hypertonic saline stimulates vagal afferents that respond to lung deflation

In our present studies, we seek to determine whether increased osmolarity stimulates deflation-activated receptors (DARs). In anesthetized, open-chest, and mechanically ventilated rabbits, we recorded single-unit activities from typical slowly adapting receptors (SARs; responding only to lung inflation) and DAR-containing SARs (DAR-SARs; responding to both lung inflation and deflation) and identified their receptive fields in the lung. We examined responses of these two groups of pulmonary sensory units to direct injection of hypertonic saline (8.1% sodium chloride; 9-fold in tonicity) into the receptive fields. Hypertonic saline decreased the activity in most SAR units from 40.3 ± 5.4 to 34.8 ± 4.7 imp/s (P < 0.05, n = 12). In contrast, it increased the activity in DAR-SAR units quickly and significantly from 15.9 ± 2.2 to 43.4 ± 10.0 imp/s (P < 0.01, n = 10). Many units initially had increased activity, mainly in the deflation phase. DAR-SAR activities largely returned to the control level 30 s after injection. Since hypertonic saline stimulated DAR-SAR units but not SAR units, we conclude that hypertonic saline activates DARs.