ACCLIMATION TO HARD OR SOFT-WATER AT WEAKLY ALKALINE PH INFLUENCES GILL PERMEABILITY AND GILL SURFACE CALCIUM-BINDING IN RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

Rainbow trout (Oncorhynchus mykiss) were acclimated for 10 d to soft ( 0.1 mM as CaCO3) or hard (1.0 mM as CaCO3) water at weakly alkaline pH (8.06-8.34). Following acclimation, individual gill arches were remov ed for examining the effects of low hardness or high hardness acclimat ion on gill water permeability, gill Ca2+ interactions, and gill alumi num interactions. Isolated gill arches were exposed to water of varyin g Ca2+ (0.0-1.0 mM) and aluminum (3.7-37 mu M) concentration for osmot ic permeability experiments. High hardness acclimated gills had signif icantly greater percent weight gain over time caused by osmotic water entry than low hardness acclimated gills, when exposed to distilled wa ter (32.34 +/- 1.15 and 24.86 +/- 0.62%, respectively, after 60-min in cubations); these differences were absent when Ca2+ (0.1-1.0 mM) was a dded to the incubation medium. Gill arch Ca2+ binding experiments reso lved two gill surface binding site populations, which differed in thei r Ca2+ binding affinity. The higher affinity sites were probably assoc iated with gill membrane permeability, because low hardness acclimated gills had more such sites (binding capacity, 0.322 +/- 0.027 mu mol C a2+. g(-1)) and less permeable gills than high hardness acclimated gil ls (binding capacity, 0.198 +/- 0.004 mu mol Ca2+. g(-1)). Aluminum ha d little influence on gill permeability and gill Ca2+ binding.