Title:
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Arginine vasotocin's role in the adaptive osmoregulatory physiology of euryhaline fish.
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A specific radioimmunoassay was developed and characterised for the
measurement of arginine vasotocin (A VT) in teleost fish. Crossreactivity of the
antibody with the other teleost neurohypophysial peptide, isotocin, was less than 1 %
and the sensitivity of the assay 0.25 pglassay tube. Rapid blood sampling, within 1
minute of capture, by needle puncture of the caudal blood vessels, and collection of
blood through chronic arterial cannula 1 - 4 days after surgery were identified as
procedures which did not influence plasma ion levels, osmolality or hormone
concentrations. These procedures were used in later experimental protocols examining
the effects of chronic and acute osmotic stimuli on plasma A VT concentrations.
Plasma A VT concentrations in a range of euryhaline and stenohaline teleost fish
were measured at 10-12 to low 10-11 M (1-20 pglml). There was no consistent
difference in circulating A VT levels in euryhaline fish (flounder, eel and trout) adapted
to fresh water (FW) or seawater (SW) for at least 2 weeks. The flounder was studied
in greater detail and seasonal influences were identified as having an effected on
osmoregulation. In the winter/spring period plasma osmolality was significantly lower
in FW adapted fish than at other times of the year. In SW adapted fish at this time
plasma osmolality was elevated and this was accompanied by elevated plasma A VT
concentrations. The winter/spring period was associated with the development of the
gonads and this factor may affect osmoregulation in the flounder. In SW adapted
flounder a positive linear relationship between plasma A VT and plasma osmolality and
ion levels was observed, indicating a link between these two parameters but no such
relationship was evident in FW adapted fish.
Chronically cannulated SW adapted flounder were used to examine the effects
of acute osmotic stimuli on plasma A VT concentrations. Intraperitoneal injection of
hypertonic saline acutely increased plasma osmolality and was associated with a
significant increase in plasma A VT concentration, indicating that A VT secretion was
sensitive to increased plasma osmolality. The expansion of extracellar fluid volume
via intravascular infusion of isotonic saline reduced plasma A VT concentrations
independently of any change in plasma osmolality. This indicated that vascular volume
also influences plasma A VT concentrations. Reducing blood volume by haemorrhage
and lowering plasma osmolality by infusion of hypotonic saline did not result in any
change in plasma A VT concentrations.
Plasma A VT concentrations were measured in lamprey at a 10-12 M
concentration, and the transfer of lamprey from FW to Baltic SW caused an initial rise
in plasma A VT concentrations, suggesting that A VT is involved in osmoregulation in
these fish. A VT immunoreactivity was also demonstrated in two species of
elasmobranch fish in a 10-12 to low 10-11 M range.
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