The effect of the resistive loading of inspiration and expiration on pulmonary stretch receptor discharge.

Anesthetized, spontaneously breathing cats were used to examine the hypothesized role of slowly adapting pulmonary stretch receptors (PSR) in the control of breath duration. Initially, graded inspiratory and expiratory resistive loads were added to elucidate the inspiratory and expiratory volume-time relationship with both vagi intact. Unilateral vagotomy increased the slope of the VI--TI relationship indicating a reduction of the volume related modulation of TI. PSR frequency (fPSR) at end-inspiration also progressively decreased resulting in a fPSR--TI relationship qualitatively similar to the VI--TI curve. Expiratory resistive loading also produced an increased slope for the VE--TE relationship when the right vagus nerve was severed. The prolongation of TE was associated with a progressive increase in the number of PSR discharges during the loaded expiration. These results support the hypothesized role of PSR in the vagally mediated prolongation of TI and TE during resistive loading. In a subsequent series of experiments, the changes in fPSR were correlated with the tidal volume and transpulmonary pressure (PTP) changes. The fPSR was linearly related to PTP during both eupnic and loaded breathing. When fPSR was plotted against volume, a clockwise hysteresis was observed. These results suggest that in the spontaneously breathing cat, intrathoracic PSR frequency varies as a function of the transmural pressure across the airways.