Kinetics of Branchial Calcium Uptake in the Rainbow Trout: Effects of Acclimation to Various External Calcium Levels Available to Purchase

Calcium uptake in freshwater rainbow trout (Salmogairdnert) under control conditions (external [Ca²⁺] ≃ 1·8mequivl⁻¹, [NaCl] ≃ 0·8mequiv 1⁻¹) occurred at approximately equal rates (12–15 equiv kg⁻¹ h⁻¹) through the gills and the general body surface in vivo. The gut was not involved. Under the same conditions, in vitro branchial in an isolated, saline-perfused head preparation was equal to that in vivo. The cells involved in are mainly located on lamellae rather than on filaments since ∼ 95 % of occurred across the arterio-arterial circulation of the gill. in vitro, displayed Michaelis-Menten kinetics. Acclimation to low external [Ca²⁺] (50μequivl⁻¹; unchanged [NaCl]) for 1 day caused a five-fold stimulation of characterized by decreased K_m and increased J_max Longer periods of low [Ca²⁺] acclimation resulted in changes of J_max only. J_max gradually returned towards control levels as acclimation time increased, but was still elevated after 30 days. Acclimation to low ambient [Ca²⁺] caused proliferation and increased exposure of lamellar chloride cells which were correlated with increased J_max. Fish exposed to high external [Ca²⁺] (10 μequiv 1⁻¹; unchanged [NaCl]) displayed reduced Similar changes in were observed during in vivo experiments. Plasma Ca²⁺ concentration remained constant regardless of external [Ca²⁺], while plasma Na⁺ and Cl⁻ levels were transiently reduced at 1 day low [Ca²⁺] exposure but had recovered by 7 days. A possible role for cortisol in Ca²⁺ regulation is discussed based on observations of cortisol-stimulated lamellar chloride cell proliferation and and elevated plasma [cortisol] in low-[Ca²⁺] acclimated fish.