Figure 1.

SERCA-mediated ⁴⁵Ca²⁺ uptake into microsomes isolated from PC12 cells: oxalate dep endence and effect of endoplasmic reticulum ⁴⁵Ca²⁺-store depletion by EGTA. In A, numbers of independent experiments (n) were 5,2,3,6 and 5 for oxalate concentrations 0, 6,12,18 and 24 mM, respectively. In B, n = 3 for all data points. Bars represent SEMs. Lines were fitted by linear regression. A. Oxalate selectively enhances ⁴⁵Ca²⁺ uptake to the thapsigargin-sensitive (SERCA-mediated) portion of the total microsomal uptake. Microsomes were prepared from PC12 cells, and ⁴⁵Ca²⁺ uptake was determined as described in Materials and Methods section, with 25 min incubation time and various concentrations of oxalate either in the absence (●) or presence (○) of 1 μM thapsigargin. B. Depletion of Ca²⁺ stores by exposure of cells to EGTA affects specifically the thapsigargin-sensitive (SERCA-mediated) component of microsomal ⁴⁵Ca²⁺ uptake. Microsomes were prepared from cells cultured under control conditions (□,■) or in the presence of 2.5 mM EGTA for 7 h before harvesting (○,●). ⁴⁵Ca²⁺ uptake was determined as a function of time in the presence of 18 mM oxalate either in the absence (○,□) or presence (●,■) of 1 μM thapsigargin.