The effects of extremely alkaline water (pH 9·5) on rainbow trout gill function and morphology

Rainbow trout that were held under control conditions, at pH8·0, in moderately hard Hamilton tap water, had Cl − and Na + influx rates ( J CL in and J Na , respectively) of 270 and 300 μmol kg −1 h −1 , respectively. Exposure to pH 9·5 water led to an immediate 67% decline in J CL in and a 45% reduction in J Na in at 0–1 h. Influx rates declined further and by 4–5 h the net decreases in both J CL in and J Na in approximated 80%. By 24 h J CL in had recovered to rates not significantly different from those at pH 8·0; while J Na in only partially recovered and remained about 50% lower than control measurements through 72 h. The complete recovery of J CL in and partial recovery of J Na in may have been related to a fourfold greater branchial chloride cell (CC) fractional surface area observed in rainbow trout exposed to pH 9·5 for 72 h. Ammonia excretion ( J Amm ) was about 170 μmol N kg −1 h −1 at pH 8·0 but was initially reduced by 90% over the first hour of high pH exposure. J Amm rapidly recovered and by 24 h it had returned to pre‐exposure levels. This recovery tended to parallel the partial recovery of J Na in . However, subsequent addition of amiloride (10 −4 M) to the water at 75 h led to no change in J Amm , despite a 50% reduction in J Na in . Thus, it does not appear that there is a linkage between Na + influx and the recovery of ammonia excretion under highly alkaline conditions.