Aluminum Inhibits Net Ca-45(2+) Uptake By Amaranthus Protoplasts

Calcium ions serve as a second messenger in the transduction of environmental and hormonal signals to the responsive elements of cell metabolism. Effects of Al on the cell Ca2+ homeostasis remain to be elucidated. Short-term net Ca-45(2+) uptake by Amaranthus tricolor protoplasts was measured in media prepared to test the influence of various buffers, pH values, Ca2+ and Al concentrations. A modified GEOCHEM program was used to calculate activities of Ca2+ and all monomeric Al species. Net Ca-45(2+) uptake was smaller at pH 4.5 compared to uptake at pH 5.5 and increased linearly with an increase in Ca2+ activity in the uptake media from 0.23 to 2.0 mM. Aluminium caused a significant decrease in net Ca-45(2+) uptake. This Al-related decrease was less expressed at higher Ca2+ activities. Summed activities of monomeric Al species (Al3+ and hydroxy-Al) were significantly related to the reduction of net Ca-45(2+) uptake. The equilibrium constant (K(i)) of the reversible association of the Ca2+ membrane-transporter and monomeric Al species was calculated from Dixon plots to be equivalent to 4 and 30 micromolar activity of total monomeric Al for uptake media with pH values of 4.5 and 5.5, respectively. It is suggested that the primary Al effects are generally exerted through reducing inward Ca2+ fluxes across the plasma membrane.