title: Osmotic, sodium, carbon dioxide and acid-base state of the Port Jackson shark, Heterodontus portusjacksoni, in response to lowered salinity.
abstract: In marine elasmobranch fish the consequences for CO2 and acid-base state of moving into low salinity water are not well described. Sub-adult Port Jackson sharks, Heterodontus portusjacksoni, occasionally enter brackish water and survive in 50% seawater (SW). The unidirectional Na efflux and content, plasma volume, glomerular filtration rate (GFR), body mass, as well as CO2 and acid-base state in H. portusjacksoni were investigated following transfer from 100% SW to 75% SW and then to 50% SW. A rapid water influx resulted in a doubling of the plasma volume within 24 h in sharks in 75% SW and an 11% increase in body weight. Osmotic water influx was only partially offset by a doubling of the GFR. There was a approximately 40% decrease in plasma [Na] through a transiently elevated Na clearance and haemodilution. The result was a decrease in the inward gradient for Na+ together with reductions of nearly 50% in CO2 and buffer capacity. The sharks remained hypo-natric to 50% SW by partially conforming to the decrease in external osmotic pressure and avoided the need for active Na+ uptake. The gradient for Na+ efflux would by extrapolation approach zero at approximately 27% SW which may of itself prove a lethal internal dilution. In sharks transferred to 75% SW, a small transient hypercapnia and a later temporary metabolic alkalosis were all largely explained through anaemia promoting loss of CO2 and buffer capacity. In sharks transferred to 50% SW the metabolic alkalosis persisted until the end of the 1-week trial. Within the erythrocytes, increased pH was consequent on the large decrease in haemoglobin content exhibited by the sharks, which caused a large reduction in intracellular buffer. In water as dilute as 50% SW there was no evidence of specific effects on the mechanisms of management of CO2 or H+ excretion but rather significant and indirect effects of the severe haemodilution.
authors.affiliation: School of Biological Sciences, University of Sydney, 2006 Sydney, NSW, Australia.
authors.forename: A R
authors.initials: AR
authors.lastname: Cooper
authors.forename: S
authors.initials: S
authors.lastname: Morris
mesh_terms: D000064:Acclimatization / Q000187:drug effects / Q000502:physiology*; D000136:Acid-Base Equilibrium / Q000502:physiology*; D000465:Algorithms; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism; D001835:Body Weight / Q000502:physiology; D002245:Carbon Dioxide / Q000378:metabolism / Q000502:physiology*; D004912:Erythrocytes / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism; D005919:Glomerular Filtration Rate / Q000187:drug effects / Q000502:physiology; D006863:Hydrogen-Ion Concentration; D009997:Osmotic Pressure; D010313:Partial Pressure; D010953:Plasma Volume / Q000187:drug effects / Q000502:physiology; D012623:Seawater / Q000737:chemistry*; D012754:Sharks / Q000502:physiology*; D012965:Sodium Chloride / Q000097:blood / Q000378:metabolism* / Q000494:pharmacology; D014882:Water-Electrolyte Balance / Q000502:physiology
chemical_list: D001639:Bicarbonates; D002245:Carbon Dioxide; D012965:Sodium Chloride