The effects of etorphine and of morphine on respiration, blood carbon dioxide tension, and carbon dioxide sensitivity in the conscious rabbit

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Abstract
The effects of intravenous doses of morphine and etorphine on respiration, blood pCO2 and sensitivity to carbon dioxide were investigated in the conscious rabbit. Both morphine and etorphine depressed respiratory rate and elevated pCO2. Etorphine was about 5000 times more potent than morphine as a depressant of respiratory rate. With etorphine, depression of respiratory rate was always accompanied by a large increase in tidal volume. In the normal rabbit, inhalation of mixtures of carbon dioxide up to 15% (in oxygen) produced a concentration-dependent increase in tidal volume accompanied by no increase in respiratory rate. After injection of morphine followed by inhalation of carbon dioxide there was a dose-dependent depression of carbon dioxide sensitivity which was concurrent with the depression of respiratory rate; 8 mg/kg completely abolished the respiratory stimulant effects of concentrations of carbon dioxide up to 12%. Whilst high doses of etorphine depressed carbon dioxide sensitivity, a dose of 0·5 μg/kg (which depressed respiratory rate by about 30%) did not depress carbon dioxide sensitivity. Doses of 1 μg/kg of etorphine administered to pregnant animals produced an increase in respiratory rate and a depression of tidal volume. The significance of the differences between the respiratory pharmacology of etorphine and that of morphine is discussed.