The effects of acid-base disturbances on branchial and renal calcium fluxes in the freshwater rainbow trout (Oncorhynchus mykiss).

Abstract

Whole body calcium influx, branchial calcium efflux, and renal Ca$\sp{2+}$ excretion were measured in rainbow trout (Oncorhynchus mykiss) either exposed to environmental hypercapnia or infused intra-arterially with NaHCO$\sb3$. These experiments were performed to assess the potential impact on Ca$\sp{2+}$ balance of the changes in gill morphology that are known to accompany acid-base disturbances in this species. After 48 hours of environmental hypercapnia, gill filamental chloride cell fractional area was significantly reduced. Despite this reduction, and the presumed involvement of the chloride cell in calcium influx, whole body calcium influx was increased after 12 hours of hypercapnia and remained elevated for 48 hours. Branchial calcium efflux was unaltered during hypercapnia exposure, whereas renal Ca$\sp{2+}$ excretion was elevated over preflux values only at 6 hours of hypercapnia. Measurement of the kinetics of whole body calcium influx after 48 hours of hypercapnia revealed a significant increase in the maximal uptake rate of Ca$\sp{2+}$ yet the affinity constant of Ca$\sp{2+}$ uptake was unaffected. Measurements of high-affinity. Ca$\sp{2+}$-ATPase activities and ATP-dependent Ca$\sp{2+}$ transport of gill basolateral membrane vesicles revealed that the ATP-dependent Ca$\sp{2+}$ extrusion mechanism of the gills was not affected by hypercapnia. The results of this study clearly show that the reduced chloride cell surface area that accompanies hypercapnia in trout does not impair calcium homeostasis. Whole body Ca$\sp{2+}$ influx was significantly increased after 6 hours of NaHCO$\sb3$ infusion and remained elevated throughout the duration of the experiment. Branchial and renal Ca$\sp{2+}$ effluxes were largely unaffected by NaHCO$\sb3$ infusion. Plasma total Ca$\sp{2+}$ concentrations were significantly decreased after 6 hours of NaHCO$\sb3$ infusion and remained so until 48 hours. (Abstract shortened by UMI.)