Use of mass spectrometer to measure CO2 and O2 fluxes in voltage-clamped epithelia.

A quadrupole mass spectrometer was coupled to an Ussing chamber in order to evaluate rates of oxidative metabolism in voltage-clamped epithelia. Well-defined mixing characteristics of the continuously perfused chamber allowed CO2 and O2 concentrations to be related to rates of CO2 efflux, JCO2, and oxygen influx, JO2. The use of a model tissue to simulate step changes in JCO2 validated the treatment, with response within a minute. Monitoring of metabolism was facilitated by use of a desk-top computer, which evaluated JCO2 at 6-s intervals. Concurrent measurements of electrical current and JCO2 were made in the toad urinary bladder in order to relate active sodium transport to metabolism; the use of amiloride to eliminate active transport and the associated metabolism then allowed evaluation of the rates of active Na transport (JNa) and suprabasal metabolism (JsbCO2), and their ratio JNa/JsbCO2. We report the ability to resolve a 5 pmol/s change in CO2 efflux or an 11 pmol/s change in O2 influx rates.