Mechanism of Aluminum Inhibition of Net ⁴⁵Ca²⁺ Uptake by Amaranthus Protoplasts

Abstract

Calcium ions serve as a second messenger in signal transduction and metabolic regulation. Effects of Al on calcium homeostasis remain to be elucidated. Short-term net ⁴⁵Ca²⁺ uptake by Amaranthus tricolor protoplasts was monitored from uptake media prepared to test the influence of pH, Al, and various inhibitors. Accumulation of ⁴⁵Ca²⁺ increased during the first 3 to 6 minutes and then leveled off or declined. Al and Ca²⁺ channel blockers (verapamil and bepridil) decreased net ⁴⁵Ca²⁺ uptake. This decrease was more pronounced when Al and bepridil were both present in uptake media, but Al did not aggravate verapamil-induced reduction of net ⁴⁵Ca²⁺ uptake. Erythrosin B and calmidazolium each increased net ⁴⁵Ca²⁺ uptake, probably by interfering with Ca²⁺ efflux. This effect was undetectable in the presence of Al. Mycophenolic acid decreased net ⁴⁵Ca²⁺ uptake; guanosine alleviated this effect. Al-induced reduction of net ⁴⁵Ca²⁺ uptake was not aggravated by mycophenolic acid. Net ⁴⁵Ca²⁺ uptake was generally less at pH 4.5 than at 5.5 for all treatments. It is concluded that Al ions affect net ⁴⁵Ca²⁺ uptake by binding to the verapamil-specific channel site that is different from the bepridil-specific one, as well as by interfering with the action of guanosine 5′-triphosphate-binding proteins.