The inhibitory effect of Mn2+ on the ATP-dependent Ca2+ pump in rat brain synaptic plasma membrane vesicles.

Synaptic plasma membrane (SPM) vesicles take up Ca2+ when both ATP and Mg2+ are added to the reaction medium. Maximal ATP-dependent Ca2+ uptake is obtained with between 3 and 5 mM Mg2+. Higher [Mg2+] results in a slight decrease in the ATP-dependent Ca2+ influx, which at 10 mM Mg2+ is equal to 70% of the maximal uptake. [Mn2+] up to 0.3 mM, can support ATP-dependent Ca2+ uptake as efficiently as equimolar [Mg2+]. Maximal ATP-dependent Ca2+ uptake in the presence of Mn2+ is obtained between 1 and 2 mM but its extent is only 70% of the maximal uptake obtained in the presence of Mg2+. As the [Mn2+] increases the ATP-dependent Ca2+ uptake decreases and at 10 mM Mn2+ it is only 40% of that obtained in the presence of Mg2+. The reduction in ATP-dependent Ca2+ uptake in the presence of Mn2+ is obtained also when Mg2+ is present in the reaction medium. The effect is independent of the [Mg2+] used and depends on the [Mn2+] alone. Studying the relationship between [Mn2+] and [Ca2+] revealed three patterns: at 0.3 mM Mn2+ this ion could replace Mg2+ over the entire [Ca2+] range tested, 5-200 microM, supporting ATP-dependent Ca2+ uptake. When the [Mn2+] was increased to 1.8 mM, it exhibited a competitive behaviour with Ca2+ which resulted in an increase in apparent Km to Ca2+ of the SPM Ca2+ pump from 10.69 microM (SD = 3.49) to 28.88 microM (SD = 21.08). Four millimolar Mn2+ inhibited ATP-dependent Ca2+ uptake to 50% of that obtained in equimolar Mg2+ over the entire [Ca2+], 5-200 microM, tested. No transport of 54Mn2+ by the SPM Ca2+ pump could be detected. Inclusion of Mn2+ in the reaction medium led to excessive phosphorylation of SPM proteins. The excessive levels of phosphorylation persisted also when Ca2+ and/or Mg2+ were present in the reaction media.