HCO3- dehydration by the blood of an elasmobranch in the absence of a Haldane effect

We measured in vivo arterial PCO2 and CaCO2 in Scyliorhinus canicula and found them to be very low (approximately 1 Torr and 3 mmol l-1 respectively). In vitro, the Haldane effect was functionally absent, and there was no detectable beta-adrenergic Na+/H+ exchange, in contrast to teleosts. The HCO3- dehydration rate of the blood, measured by a radioisotopic assay (Wood and Perry, J. Exp. Biol. 157:349-366, 1991), was independent of steady-state deoxygenation or oxygenation, unaffected by rapid oxygenation, and insensitive to isoprenaline, amiloride, and removal of urea or TMAO. SITS and acetazolamide reduced the rate; HCO3-/Cl- exchange rather than intracellular carbonic anhydrase (CA) was the rate-limiting factor. The rate was not altered by steady-state plasma [HCO3-], but increased linearly with PCO2 and with RBC concentration, saturating at hematocrits > or = 15%. The rate in separated plasma accounted for approximately 50% of the whole blood rate, was higher than in trout plasma or saline, and was inhibited by acetazolamide. The presence of CA in the normally circulating blood plasma of dogfish may contribute to highly efficient CO2 excretion in vivo.