Osmotic,sodium, carbon dioxide and acid-base state of the Port Jackson shark,<Emphasis Type="Italic"> Heterodontus portusjacksoni</Emphasis>, in response to lowered salinity |
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Authors: | A?R?Cooper Email author" target="_blank">S?MorrisEmail author |
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Institution: | (1) School of Biological Sciences, University of Sydney, 2006 Sydney, NSW , Australia;(2) Morlab, School of Biological Sciences, University of Bristol, Woodland Road, Bristol , BS8 1UG, UK |
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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 ~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 ~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.Abbreviations
a–v
arterial–venous
-
CA
carbonic anhydrase
-
C
a
CO
2
content of CO2 in arterial blood
-
CCO
2
CO2 content
-
51
Cr-EDTA
51chromium-ethylenediaminetetraactic acid
-
C
v
CO
2
content of CO2 in venous blood
-
FW
freshwater
-
GFR
glomerular filtration rate
-
Hct
haematocrit
-
J
out
Na flux rate
-
MCHC
mean cell haemoglobin concentration
-
OP
osmotic pressure
-
P
a
CO
2
partial pressure of CO2in arterial blood
-
PCO
2
partial pressure of CO2
-
pH
a
arterial blood pH
-
pH
er
intra-erythrocyte fluid
-
pH
pl
whole blood pH
-
pH
v
venous blood pH
-
P
v
CO
2
partial pressure of CO2in venous blood
-
SID
strong ion difference
-
SW
seawater
-
TMAO
trimethylamine-N-oxide
-
UFR
urinary flow rate
Communicated by G. Heldmaier |
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Keywords: | Acid-base Na efflux Shark Hyposaline Heterodontus |
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