Arterial plasma K+ and lactate concentrations ([K+]a and [lactate]a), as well as blood oxygenation status, were measured in relation to increasing swimming speeds in rainbow trout Oncorhynchus mykiss. Neither [K+]a nor [lactate]a changed at swimming speeds below 1.5 BL s-1, where BL is total body length. Between 1.5 and 2.0 BL s-1, [K+]a started to increase, and above 2.0 BL s-1 both [K+]a and [lactate]a increased with swimming speed. Training shifted the onset of these increases to higher swimming speeds and increased the critical swimming speed (Ucrit) from 2.4 to 3.0 BL s-1. Blood oxygen content showed no changes in control fish, whereas in trained fish it increased by 22 % at the final swimming speed. From the [K+]a data, we suggest that no loss of K+ occurred from the working muscle at low swimming speeds, allowing an unlimited endurance, whereas moderate and higher speeds were probably associated with a loss of K+ from the working muscles, indicating a limited endurance. <P>

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