THE PATTERN OF CARBON DIOXIDE EXCRE

THE PATTERN OF CARBON DIOXIDE EXCRETION
IN THE RAINBOW TROUT SALMO GAIRDNERI

Plasma bicarbonate is the major form of CO2 in fish blood (Randall & Cameron,
1973) and most of the CO2 excreted across the gills originates from plasma bicarbonate.
Carbonic anhydrase is known to be required for normal patterns of CO4 excretion in
fish because Hoffert & Fromm (1973) have shown that inhibition of carbonic anhydrase
with diamox (acetazolamide) results in a marked rise in P% COi and fall in pHa
in rainbow trout. Haswell & Randall (1976) have shown, in vitro, that intact rainbow
trout erythrocytes, although containing carbonic anhydrase, do not catalyse the
dehydration of plasma bicarbonate. The gill epithelium in fish contains high levels
of carbonic anhydrase (see Table 4) and is a possible site for dehydration of plasma
bicarbonate. The following experiments were carried out to determine if plasma
bicarbonate is dehydrated within the gill epithelium rather than red blood cells in the
rainbow trout.