Branchial and renal calcium fluxes in rainbow trout ( Oncorhynchus mykiss) during metabolic alkalosis

Abstract

Rainbow trout ( Oncorhynchus mykiss) were infused with NaHCO 3, inducing a metabolic alkalosis, and renal and branchial Ca 2+ fluxes were monitored. Because such treatment is known to alter gill epithelial morphology by increasing exposed chloride cell surface area, it was hypothesized that Ca 2+ uptake would be increased in these fish. Whole body Ca 2+ influx was significantly increased after 6 hr of NaHCO 3 infusion and remained elevated throughout the duration of the experiment. Branchial and renal Ca 2+ effluxes were largely unaffected by NaHCO 3 infusion. Plasma total Ca 2+ concentrations were significantly decreased after 6 hr of NaHCO 3 infusion and remained so until 48 hr. Such results suggest relocation of Ca 2+ from the plasma to other body compartments, such as bone. Analysis of the kinetics of whole body Ca 2+ uptake revealed that infusion of NaHCO 3 for 48 hr caused a significant increase in the maximal uptake rate of Ca 2+; the affinity constant of Ca 2+ uptake was unchanged. Measurement of various enzymatic activities from gill basolateral membranes revealed that although Na +, K +-ATPase activity was significantly increased in NaHCO 3-infused fish, neither Ca 2+-ATPase activity nor ATP-dependent Ca 2+ transport was affected. These findings suggest that the basolateral membrane Ca 2+ transporter does not alter its capacity to move Ca 2+ under alkalotic conditions. We suggest that the chloride cell apical membrane is the principle regulator of branchial Ca 2+ uptake in rainbow trout under alkalotic conditions. Such a suggestion concurs with the original hypothesis that an increase in apical surface area would lead to increased Ca 2+ uptake.