Endogenous opioid effects on abdominal muscle activity during inspiratory loading

In a previous study in unanesthetized goats, we demonstrated that continuous naloxone (NLX) administration during inspiratory flow-resistive loading (IRL) significantly increased tidal volume (VT) but not diaphragm electromyogram (EMGdi). End-expiratory gastric pressure did increase with NLX, implying that increased abdominal muscle activity may have accounted for the NLX effect. In the current study we directly tested the hypothesis that endogenous opioid elaboration depresses the abdominal muscle response to a continuous inspiratory flow-resistive load. In seven unanesthetized goats, VT, arterial blood gases, EMGdi, and EMG activity of external oblique (EMGeo), transversus abdominis (EMGta), and external intercostal (EMGei) muscles were monitored. IRL (50 cmH₂O·l^-1·s) was continued for 3 h, after which NLX (0.1 mg/kg) or saline was given. Our results showed that VT decreased from 323 ± 32 (SE) ml at baseline to 260 ± 16 ml 5 min after the load was imposed (P < 0.05) and further decreased to 229 ± 18 and 217 ± 15 ml by 120 and 180 min, respectively (180 vs. 5 min, P < 0.05). EMGdi increased from 62 ± 5 to 83 ± 4%max at 5 min (P < 0.05) but was unchanged thereafter. In contrast, for this same time period EMGeo increased from 35 ± 5 to 58 ± 11%max but decreased from 67 ± 11%max at 120 min to 37 ± 5%max at 180 min (P < 0.05). NLX administration resulted in significant increases in EMGeo (91% above 180-min value). In contrast, EMGdi increased minimally after NLX (15% above 180-min value). We conclude that the major effect of endogenous opioid elaboration that accompanies IRL is to reduce the abdominal muscle response.