Kinetics of waterborne strontium uptake in the common carp,Cyprinus carpio, at different calcium levels

The uptake kinetics of strontium in the commoncarp(Cyprinus carpio) were characterized in vivo, exposing preac-climated fish to a wide range of Sr2+ (0.27 μM–10.0 mM) and Ca2+ (10 μM–10 mM) concentrations in water; 85Sr, 45Ca, and 47Ca were used as tracers in determining the uptake, and the possibility of adsorption of the tracers to the exterior of the fish were verified. The uptake rates were determined in the whole body, gills, and blood of the fish after an exposure period of 3 h and were analyzed as a function of the free-ion activity of strontium and calcium in water. With the increase of Sr2+ concentration in the exposure water, Sr2+ uptake did not increase linearly but displayed Michaelis–Menten saturation kinetics, and with the increase of Ca2+ concentration, Sr2+ uptake decreased significantly in the whole body, gills, and blood. The competitive inhibition model fitted to the pooled data for whole-body uptake explains about 94% of the variation in Sr2+ uptake and 71% in Ca2+ uptake, indicating a competitive type of interaction during the transport of these metal ions across the biological interfaces. The maximum uptake rate of Sr2+ (JmaxSr) was estimated to be 243.0 μmol/kg/h and that of Ca2+ (JmaxCa) 119.4 μmol/kg/h. The apparent Km for Sr2+ uptake increased greatly with the increase of Ca2+ concentration in water (186, 789, and 5,515 μM for 35, 348, and 3,480 μM Ca2+, respectively). Estimation of the true Km for Sr2+ uptake (KmSr) and its inhibitor constant for Ca2+ (KiCa) yielded the values of 96.3 and 28.5 μM, respectively. These values are very close to those obtained for Ca2+ uptake (KmCa = 24.9 and KiSr = 100.9 μM). This model provides a mechanistic description of the effect of calcium on strontium uptake from water and, vice versa, in carp.