CHANGES IN RESPIRATORY MUSCLE-ACTIVITY IN PONIES WHEN END-EXPIRATORY LUNG-VOLUME IS INCREASED

The objective of the present study was to determine whether lung and d iaphragm afferents contribute to the changes in respiratory muscle act ivity when end-expiratory lung volume (EELV) is changed in ponies. We studied the responses of the diaphragm and the transversus abdominis ( TA) muscles to passive increases in EELV in awake intact (I), diaphrag m-deafferented (DD), pulmonary vagal- (hilar nerve) denervated (HND), and DD + HND ponies. Negative pressure of -10 or -20 cmH(2)O applied a round the ponies' torsos [positive transrespiratory (TR) pressure] inc reased (P < 0.05) EELV in all ponies; the increases were more (P < 0.0 5) in HND and less (P < 0.05) in DD than in I ponies. In I ponies, pos itive TR pressure increased (P < 0.05) the rate of rise of the integra ted diaphragmatic electromyogram (EMG), reflecting increased drive to the muscle. This increase was less (P < 0.05) in DD and HND than in I ponies. In DD + HND ponies, there was no significant (P > 0.10) change in drive to the diaphragm during positive TR pressure. In I ponies, p ositive TR pressure increased (P < 0.05) the duration and mean activit y of the TA EMG. In HND and DD + HND ponies, the TA EMG was not altere d by positive TR pressure. I and DD ponies decreased (P < 0.05) breath ing frequency but maintained tidal volume (VT) during positive TR pres sure. HND and DD + HND ponies increased breathing frequency (P < 0.05) and decreased (P < 0.05) VT during positive TR pressure. We conclude that, during positive TR pressure when the diaphragm is presumably at a mechanical disadvantage, diaphragm and vagal afferents mediate incre ased drive to the diaphragm to prevent VT from decreasing. In addition , during positive TR pressure, vagal afferents mediate an increase in duration of TA activity, which minimizes the increase in EELV.