BRANCHIAL AND RENAL CALCIUM FLUXES IN RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) DURING METABOLIC ALKALOSIS

Rainbow trout (Oncorhynchus mykiss) were infused with NaHCO3, inducing a metabolic alkalosis, and renal and branchial Ca2+ fluxes were monit ored. Because such treatment is known to alter gill epithelial morphol ogy by increasing exposed chloride cell surface area, it was hypothesi zed that Ca2+ uptake would be increased in these fish. Whole body Ca2 influx was significantly increased after 6 hr of NaHCO3 infusion and remained elevated throughout the duration of the experiment. Branchial and renal Ca2+ effluxes were largely unaffected by NaHCO3 infusion. P lasma total Ca2+ concentrations were significantly decreased after 6 h r of NaHCO3 infusion and remained so until 48 hr. Such results suggest relocation of Ca2+ from the plasma to other body compartments, such a s bone. Analysis of the kinetics of whole body Ca2+ uptake revealed th at infusion of NaHCO3 for 48 hr caused a significant increase in the m aximal uptake rate of Ca2+; the affinity constant of Ca2+ uptake was u nchanged. Measurement of various enzymatic activities from gill basola teral membranes revealed that although Na+,K+-ATPase activity was sign ificantly increased in NaHCO3-infused fish, neither Ca2+-ATPase activi ty nor ATP-dependent Ca2+ transport was affected. These findings sugge st that the basolateral membrane Ca2+ transporter does not alter its c apacity to move Ca2+ under alkalotic conditions. We suggest that the c hloride cell apical membrane is the principle regulator of branchial C a2+ uptake in rainbow trout under alkalotic conditions. Such a suggest ion concurs with the original hypothesis that an increase in apical su rface area would lead to increased Ca2+ uptake. (C) 1997 Elsevier Scie nce Inc.