Activation of endogenous opioid pathways during intense inspiratory flow-resistive loading (IRL) results in greater inhibition of EMG activity in the external oblique (EMGeo) relative to the diaphragm (EMGdi). Dichloroacetate (DCA) abolishes opioid-mediated inhibitory influences upon these muscles, suggesting a causal relationship between respiratory muscle lactic acidosis and activation of endogenous opioid pathways, during IRL. We tested the hypothesis that a more intense acidosis of the external oblique relative to the diaphragm may be the signal that determines the differential inhibitory opioid-mediated effect upon the respiratory muscles during IRL. Unanesthetized goats were exposed to IRL (50 cm H2O/1/s) for 120 min, before and after intravenous pretreatment with DCA (50 mg/kg) or saline. We measured peak phasic EMGdi and EMGeo, and respective muscle interstitial pH (pHdi, pHeo) using flexible pH probes. After 120 min IRL with saline, pHdi, and pHeo declined by -0.12 +/- 0.03 (mean +/- SEM) and -0.20 +/- 0.04 units, respectively (p < 0.05, pHdi versus pHeo). Naloxone (NLX), 0.3 mg/kg given intravenously at this time, increased EMGdi by 26.5 +/- 6.1%, but EMGeo by 81.9 +/- 13.3% (p < 0.05, EMGdi versus EMGeo). DCA blunted both the change in pHdi and pHeo during IRL (to -0.01 +/- 0.01 and -0.08 +/- 0.03 units, respectively) (p < 0.05, DCA versus saline) and the increase in EMGdi and EMGeo with NLX (to -1.0 +/- 2.6% and 5.7 +/- 5.8%, respectively) (p < 0.05, DCA versus saline).(ABSTRACT TRUNCATED AT 250 WORDS)