We used blood vessel myography and a perfused tail preparation from the Chinook salmon (Oncorhynchus tshawytscha) to investigate the effect of 2 commercially used fish anaesthetics, AQUI-S TM and MS222, on vascular tone. Hepatic portal vein rings were exposed to 1 Â 10 )7 M adrenaline in the presence or absence of either AQUI-S TM or MS222 and changes in vessel tension measured. Tail preparations were perfused with saline containing increasing concentrations of either anaesthetic. Exposure to either anaesthetic did not alter the response of vessel rings to adrenaline in vitro. However, both anaesthetics caused a significant (P £ 0.05) decrease in vessel tension when compared to controls. In tail preparations, perfusion with either anaesthetic caused a significant (P £ 0.05) yet reversible, dose dependent decrease in vascular resistance. Further to this, it was found that for both anaesthetics there was a significant effect of fish holding time on the response of preparations, with fish that were acclimated for less than 1 week exhibiting less vasodilation in response to anaesthetic exposure. We conclude that commonly used concentrations of AQUI-S TM and MS222 can have a direct effect on vascular tone of salmon vessels, causing a significant but reversible vasodilation of vessels. This vasodilation may offset increases in haematocrit seen during anaesthesia in the whole animal, and in recovery prevent large rises in peripheral resistance associated with catecholamine release. A reduced vascular resistance may allow the heart to pump greater volumes of blood during recovery.
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