Mechanism of aluminum inhibition of net ca uptake by amaranthus protoplasts.

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 (45)Ca(2+) uptake by Amaranthus tricolor protoplasts was monitored from uptake media prepared to test the influence of pH, Al, and various inhibitors. Accumulation of (45)Ca(2+) increased during the first 3 to 6 minutes and then leveled off or declined. Al and Ca(2+) channel blockers (verapamil and bepridil) decreased net (45)Ca(2+) 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 (45)Ca(2+) uptake. Erythrosin B and calmidazolium each increased net (45)Ca(2+) uptake, probably by interfering with Ca(2+) efflux. This effect was undetectable in the presence of Al. Mycophenolic acid decreased net (45)Ca(2+) uptake; guanosine alleviated this effect. Al-induced reduction of net (45)Ca(2+) uptake was not aggravated by mycophenolic acid. Net (45)Ca(2+) uptake was generally less at pH 4.5 than at 5.5 for all treatments. It is concluded that Al ions affect net (45)Ca(2+) 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.