Aluminium Inhibits Net 45Ca2+ 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 ⁴⁵Ca²⁺ 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 ⁴⁵Ca²⁺ 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 ⁴⁵Ca²⁺ 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 ⁴⁵Ca²⁺ uptake. The equilibrium constant (Ki) 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.