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.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.