Control of postural changes of end expiratory volume (FRC) by airways slowly adapting mechanoreceptors.

We recorded the e.m.g. activity of the diaphragm and of an abdominal muscle (ext. oblique) and the respiratory volume in anesthetized rabbits challenged with head-up tilting and positive pressure breathing (PPB). Both maneuvers determined an inhibition of inspiratory activity and an activation of abdominal muscles, the latter being especially marked with tilting. After cervical vagotomy neither the inspiratory inhibition nor the abdominal recruitment was present during tilting and PPB and the FRC increase was more pronounced. Sulphur dioxide was given in the inspired air (200 ppm) to selectively block the slowly adapting mechanoreceptors. Such blockade was indicated by the absence of the Hering-Breuer inflation reflex. The permanence of other respiratory reflexes was shown by a parodoxical response to inflation and by a still evident response to deflation. With SO2 block, both tilting and PPB did not elicit either the inspiratory inhibition or the abdominal muscles activation, leading to an FRC shift similar to that observed after vagotomy. We conclude that the slowly adapting mechanoreceptors subserve a reflex mechanism relevant in controlling FRC.