Thapsigargin discriminates strongly between Ca2+-ATPase phosphorylated intermediates with different subcellular distributions in bovine adrenal chromaffin cells

We studied the effects of thapsigargin on the formation of the phosphorylated intermediates (E similar to Ps) of endoplasmic reticulum Ca2+-ATPases in microsomes from bovine adrenal medulla, When submicrosomal fractions were separated on a sucrose gradient, two components of 100 kDa Ca2+-ATPase E similar to P displaying distinct subcellular distributions were resolved, The first component was defined by Ca2+-induced protection against thapsigargin inhibition, The second component did not display such protection, with a 3 orders of magnitude difference in thapsigargin inhibitory potency towards the 2 components, In the absence of Ca2+, both E similar to P components were highly sensitive to thapsigargin inhibition, revealing the presence of high-affinity thapsigargin-binding sites characteristic of SERCA ATPases, These data demonstrate a new level of molecular heterogeneity among Ca2+-ATPases of endoplasmic reticulum, and provide the first evidence of differential subcellular localization of individual Ca2+ pump subtypes in cells of neural origin.