Inspiratory facilitation and inhibition from pulmonary stretch receptors in rabbits

In anesthetized rabbits firing rate (FR) of single fibers of diaphragm and of parasternal intercostal muscles (PIM) was determined at 30, 50 and 70% of control inspiratory time. During first inspiratory effort after airway occlusion at end expiration it increased on the average by 12.1 +/- 0.6 and 43.0 +/- 2.2% relative to control. Under pulmonary stretch receptor (PSR) block with SO2 this increase disappeared in diaphragm and fell to 28.3 +/- 1.8% in PIM. During first inspiratory efforts under PSR block FR decreased by 11.3 +/- 2.2% in diaphragm and 18.3 +/- 1.6 in PIM relative to unblocked efforts. In open inspirations FR under block did not decrease significantly either in diaphragm or PIM relative to unblocked inspirations. Moving average electromyography of diaphragm and of PIM showed similar trends. These results suggest that PSR discharge at FRC activates a mechanism facilitating inspiratory activity while during inspiration it also activates a mechanism inhibiting this activity since early inspiration. Both effects are greater on PIM.