1. Active transepithelial Na+ transport (Ji) and O2 consumption (Jr) were measured simultaneously in rabbit distal colon, under standard (control) incubation conditions and after various manoeuvres, known to inhibit Na+ transport. 2. The determination of Jr was complicated by the presence of fluctuations of the PO2 in the incubation solution and by spontaneous variations of the tissue respiration, which usually declined slowly with time. 3. The control values of Ji and Jr after 2 h incubation were 55 +/- 4 nequiv min-1 cm-2 and 16 +/- 1 nmol O2 min-1 cm-2, respectively (n = 44). The electrical resistance was 386 +/- 23 omega cm2; it was stable over 6 h. 4. Ji was reduced to a very low level with either amiloride, ouabain or Na+ substitution with choline. In all instances, Jr decreased concomitantly by 15-30%. 5. A plot of the change in Jr versus the change in Ji gave a straight line for all situations, i.e. for the spontaneous decline of Na+ transport and respiration and for the effects of the inhibitors. 6. The linearity between Jr and Ji allows for the determination of a stoichiometric ratio. It is of similar magnitude, when calculated either with the data of spontaneous variations or with those obtained by the action of any inhibitor tested. It is 15-20 Na+ ions per O2 molecule, a value close to that reported previously for amphibian epithelia and also close to the maximum theoretical value of 18 Na+ ions per O2 molecule.
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