Abstract
Aspects of Na regulation of Enneacanthus obesus were examined to elucidate the adaptations that allow them to inhabit low-pH environments. We examined Na uptake kinetics in this native of extremely soft waters, and the mechanisms for the prevention of Na loss upon exposure to low pH. Active Na uptake (Jin) at pH 5.8 demonstrated saturation kinetics typical of a soft-water organism with a low Vmax (128 μmol/kg . h). However the relatively high Km (125 μM) compared to that of other, less acid-tolerant fish, indicated a surprisingly low afinity of the uptake mechanism for Na. Jin, which is fully inhibited upon exposure to pH 4.0, showed no recovery after up to 1 wk at this pH. In contrast to Jin, Na eflux (Jout) showed a great resistance to disruption by low pH, and the ability to acclimate. Prior exposure to pH 4. 0 significantly reduced Jout at pH 3.25. It is suggested that an already low Na permeability is reduced further during the acclimation period, and that this change may be hormonally regulated. The external Ca concentration had a large effect on Jout at pH 3.25, but only over a narrow concentration range of 0-50 μM, which indicates an extremely high affinity of the gills for Ca. The Ca concentration of the medium had no effect on Jout at pH 3.0. In conjunction with the literature these results suggest that there are two components to stimulation of Jout by H⁺: (1) leaching of Ca from the gills, and (2) structural damage of gill epithelium. It appears that, as pH declines, Ca leaching becomes less important and gill epithelial damagepredominates as the cause of high Jout.
Published Version
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