Haemoglobin function and respiratory status of the Port Jackson shark, Heterodontus portusjacksoni, in response to lowered salinity

Haemoglobin function and respiratory status of sub-adult sharks, Heterodontus portusjacksoni was investigated for up to 1 week following transfer from 100% to either 75% or 50% seawater. Metabolic rates were unusually low and arterial–venous differences in blood O₂ small. Haemodilution from osmotic inflow lowered haematocrit and reduced blood O₂ content by up to 50%. There was no change in O₂ consumption rate, blood O₂ partial pressure, cardiac output, or the arterial-venous O₂ content difference, and thus O₂ delivery was maintained. Ventilation was acutely elevated but returned to normal within 24 h. The O₂ delivery to the tissues was facilitated by decreased blood O₂-affinity that could not be simply ascribed to changes in the osmolyte concentration. The Hb was unaffected by changes in intra-erythrocyte fluid urea or trimethylamine-N-oxide (TMAO) but was sensitive to changes in NaCl. The Bohr shifts in whole blood were low and there was little role for pH in modulating O₂ transport. Venous Hb saturation remained close to 65%, at the steepest part of the in vivo O₂ equilibrium curve, such that O₂ unloading could be facilitated by small reductions in pressure without increasing cardiac or ventilatory work. H. portusjacksoni tolerated 50% seawater for at least 1 month, but there was little evidence of respiratory responses being adaptive which instead appeared to be consequential on changes in osmotic and ionic status.Abbreviations a–v arterial–venous - CO ₂ CO₂ content - C _a O ₂ content of O₂ in arterial blood - C _v O ₂ content of O₂ in venous blood - %E branchial O₂ extraction efficiency - f _v ventilatory frequency - GTP guanosine triphosphate - Hct haematocrit - [Hb] haemoglobin concentration - ITP inosine triphosphate - met[Hb] methaemoglobin - oxygen consumption - NTP nucleoside triphosphate - OEC oxygen equilibrium curve - P _a O ₂ partial pressure of O₂ in arterial blood - P _e O ₂ partial pressure of expired O₂ - P _i O ₂ partial pressure of inspired O₂ - P _in O ₂ inflow partial pressure of O₂ - PO ₂ partial pressure of O₂ - P _out O ₂ outflow partial pressure of O₂ - pH _a arterial blood pH - pH _pl whole blood pH - PV plasma volume - P _v CO ₂ partial pressure of CO₂in venous blood - P _v O ₂ partial pressure of O₂in venous blood - cardiac output - SW seawater - TMAO trimethylamine-N-oxide - ventilation volume Communicated by G. Heldmaier